This is libc.info, produced by makeinfo version 4.6 from libc.texinfo. INFO-DIR-SECTION GNU libraries START-INFO-DIR-ENTRY * Libc: (libc). C library. END-INFO-DIR-ENTRY This file documents the GNU C library. This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library Reference Manual', for Version 2.3.x. Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "Free Software Needs Free Documentation" and "GNU Lesser General Public License", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.  File: libc.info, Node: GUI program problems, Next: Using gettextized software, Prev: Charset conversion in gettext, Up: Message catalogs with gettext How to use `gettext' in GUI programs .................................... One place where the `gettext' functions, if used normally, have big problems is within programs with graphical user interfaces (GUIs). The problem is that many of the strings which have to be translated are very short. They have to appear in pull-down menus which restricts the length. But strings which are not containing entire sentences or at least large fragments of a sentence may appear in more than one situation in the program but might have different translations. This is especially true for the one-word strings which are frequently used in GUI programs. As a consequence many people say that the `gettext' approach is wrong and instead `catgets' should be used which indeed does not have this problem. But there is a very simple and powerful method to handle these kind of problems with the `gettext' functions. As as example consider the following fictional situation. A GUI program has a menu bar with the following entries: +------------+------------+--------------------------------------+ | File | Printer | | +------------+------------+--------------------------------------+ | Open | | Select | | New | | Open | +----------+ | Connect | +----------+ To have the strings `File', `Printer', `Open', `New', `Select', and `Connect' translated there has to be at some point in the code a call to a function of the `gettext' family. But in two places the string passed into the function would be `Open'. The translations might not be the same and therefore we are in the dilemma described above. One solution to this problem is to artificially enlengthen the strings to make them unambiguous. But what would the program do if no translation is available? The enlengthened string is not what should be printed. So we should use a little bit modified version of the functions. To enlengthen the strings a uniform method should be used. E.g., in the example above the strings could be chosen as Menu|File Menu|Printer Menu|File|Open Menu|File|New Menu|Printer|Select Menu|Printer|Open Menu|Printer|Connect Now all the strings are different and if now instead of `gettext' the following little wrapper function is used, everything works just fine: char * sgettext (const char *msgid) { char *msgval = gettext (msgid); if (msgval == msgid) msgval = strrchr (msgid, '|') + 1; return msgval; } What this little function does is to recognize the case when no translation is available. This can be done very efficiently by a pointer comparison since the return value is the input value. If there is no translation we know that the input string is in the format we used for the Menu entries and therefore contains a `|' character. We simply search for the last occurrence of this character and return a pointer to the character following it. That's it! If one now consistently uses the enlengthened string form and replaces the `gettext' calls with calls to `sgettext' (this is normally limited to very few places in the GUI implementation) then it is possible to produce a program which can be internationalized. With advanced compilers (such as GNU C) one can write the `sgettext' functions as an inline function or as a macro like this: #define sgettext(msgid) \ ({ const char *__msgid = (msgid); \ char *__msgstr = gettext (__msgid); \ if (__msgval == __msgid) \ __msgval = strrchr (__msgid, '|') + 1; \ __msgval; }) The other `gettext' functions (`dgettext', `dcgettext' and the `ngettext' equivalents) can and should have corresponding functions as well which look almost identical, except for the parameters and the call to the underlying function. Now there is of course the question why such functions do not exist in the GNU C library? There are two parts of the answer to this question. * They are easy to write and therefore can be provided by the project they are used in. This is not an answer by itself and must be seen together with the second part which is: * There is no way the C library can contain a version which can work everywhere. The problem is the selection of the character to separate the prefix from the actual string in the enlenghtened string. The examples above used `|' which is a quite good choice because it resembles a notation frequently used in this context and it also is a character not often used in message strings. But what if the character is used in message strings. Or if the chose character is not available in the character set on the machine one compiles (e.g., `|' is not required to exist for ISO C; this is why the `iso646.h' file exists in ISO C programming environments). There is only one more comment to make left. The wrapper function above require that the translations strings are not enlengthened themselves. This is only logical. There is no need to disambiguate the strings (since they are never used as keys for a search) and one also saves quite some memory and disk space by doing this.  File: libc.info, Node: Using gettextized software, Prev: GUI program problems, Up: Message catalogs with gettext User influence on `gettext' ........................... The last sections described what the programmer can do to internationalize the messages of the program. But it is finally up to the user to select the message s/he wants to see. S/He must understand them. The POSIX locale model uses the environment variables `LC_COLLATE', `LC_CTYPE', `LC_MESSAGES', `LC_MONETARY', `NUMERIC', and `LC_TIME' to select the locale which is to be used. This way the user can influence lots of functions. As we mentioned above the `gettext' functions also take advantage of this. To understand how this happens it is necessary to take a look at the various components of the filename which gets computed to locate a message catalog. It is composed as follows: DIR_NAME/LOCALE/LC_CATEGORY/DOMAIN_NAME.mo The default value for DIR_NAME is system specific. It is computed from the value given as the prefix while configuring the C library. This value normally is `/usr' or `/'. For the former the complete DIR_NAME is: /usr/share/locale We can use `/usr/share' since the `.mo' files containing the message catalogs are system independent, so all systems can use the same files. If the program executed the `bindtextdomain' function for the message domain that is currently handled, the `dir_name' component is exactly the value which was given to the function as the second parameter. I.e., `bindtextdomain' allows overwriting the only system dependent and fixed value to make it possible to address files anywhere in the filesystem. The CATEGORY is the name of the locale category which was selected in the program code. For `gettext' and `dgettext' this is always `LC_MESSAGES', for `dcgettext' this is selected by the value of the third parameter. As said above it should be avoided to ever use a category other than `LC_MESSAGES'. The LOCALE component is computed based on the category used. Just like for the `setlocale' function here comes the user selection into the play. Some environment variables are examined in a fixed order and the first environment variable set determines the return value of the lookup process. In detail, for the category `LC_xxx' the following variables in this order are examined: `LANGUAGE' `LC_ALL' `LC_xxx' `LANG' This looks very familiar. With the exception of the `LANGUAGE' environment variable this is exactly the lookup order the `setlocale' function uses. But why introducing the `LANGUAGE' variable? The reason is that the syntax of the values these variables can have is different to what is expected by the `setlocale' function. If we would set `LC_ALL' to a value following the extended syntax that would mean the `setlocale' function will never be able to use the value of this variable as well. An additional variable removes this problem plus we can select the language independently of the locale setting which sometimes is useful. While for the `LC_xxx' variables the value should consist of exactly one specification of a locale the `LANGUAGE' variable's value can consist of a colon separated list of locale names. The attentive reader will realize that this is the way we manage to implement one of our additional demands above: we want to be able to specify an ordered list of language. Back to the constructed filename we have only one component missing. The DOMAIN_NAME part is the name which was either registered using the `textdomain' function or which was given to `dgettext' or `dcgettext' as the first parameter. Now it becomes obvious that a good choice for the domain name in the program code is a string which is closely related to the program/package name. E.g., for the GNU C Library the domain name is `libc'. A limit piece of example code should show how the programmer is supposed to work: { setlocale (LC_ALL, ""); textdomain ("test-package"); bindtextdomain ("test-package", "/usr/local/share/locale"); puts (gettext ("Hello, world!")); } At the program start the default domain is `messages', and the default locale is "C". The `setlocale' call sets the locale according to the user's environment variables; remember that correct functioning of `gettext' relies on the correct setting of the `LC_MESSAGES' locale (for looking up the message catalog) and of the `LC_CTYPE' locale (for the character set conversion). The `textdomain' call changes the default domain to `test-package'. The `bindtextdomain' call specifies that the message catalogs for the domain `test-package' can be found below the directory `/usr/local/share/locale'. If now the user set in her/his environment the variable `LANGUAGE' to `de' the `gettext' function will try to use the translations from the file /usr/local/share/locale/de/LC_MESSAGES/test-package.mo From the above descriptions it should be clear which component of this filename is determined by which source. In the above example we assumed that the `LANGUAGE' environment variable to `de'. This might be an appropriate selection but what happens if the user wants to use `LC_ALL' because of the wider usability and here the required value is `de_DE.ISO-8859-1'? We already mentioned above that a situation like this is not infrequent. E.g., a person might prefer reading a dialect and if this is not available fall back on the standard language. The `gettext' functions know about situations like this and can handle them gracefully. The functions recognize the format of the value of the environment variable. It can split the value is different pieces and by leaving out the only or the other part it can construct new values. This happens of course in a predictable way. To understand this one must know the format of the environment variable value. There is one more or less standardized form, originally from the X/Open specification: `language[_territory[.codeset]][@modifier]' Less specific locale names will be stripped of in the order of the following list: 1. `codeset' 2. `normalized codeset' 3. `territory' 4. `modifier' The `language' field will never be dropped for obvious reasons. The only new thing is the `normalized codeset' entry. This is another goodie which is introduced to help reducing the chaos which derives from the inability of the people to standardize the names of character sets. Instead of ISO-8859-1 one can often see 8859-1, 88591, iso8859-1, or iso_8859-1. The `normalized codeset' value is generated from the user-provided character set name by applying the following rules: 1. Remove all characters beside numbers and letters. 2. Fold letters to lowercase. 3. If the same only contains digits prepend the string `"iso"'. So all of the above name will be normalized to `iso88591'. This allows the program user much more freely choosing the locale name. Even this extended functionality still does not help to solve the problem that completely different names can be used to denote the same locale (e.g., `de' and `german'). To be of help in this situation the locale implementation and also the `gettext' functions know about aliases. The file `/usr/share/locale/locale.alias' (replace `/usr' with whatever prefix you used for configuring the C library) contains a mapping of alternative names to more regular names. The system manager is free to add new entries to fill her/his own needs. The selected locale from the environment is compared with the entries in the first column of this file ignoring the case. If they match the value of the second column is used instead for the further handling. In the description of the format of the environment variables we already mentioned the character set as a factor in the selection of the message catalog. In fact, only catalogs which contain text written using the character set of the system/program can be used (directly; there will come a solution for this some day). This means for the user that s/he will always have to take care for this. If in the collection of the message catalogs there are files for the same language but coded using different character sets the user has to be careful.  File: libc.info, Node: Helper programs for gettext, Prev: Message catalogs with gettext, Up: The Uniforum approach Programs to handle message catalogs for `gettext' ------------------------------------------------- The GNU C Library does not contain the source code for the programs to handle message catalogs for the `gettext' functions. As part of the GNU project the GNU gettext package contains everything the developer needs. The functionality provided by the tools in this package by far exceeds the abilities of the `gencat' program described above for the `catgets' functions. There is a program `msgfmt' which is the equivalent program to the `gencat' program. It generates from the human-readable and -editable form of the message catalog a binary file which can be used by the `gettext' functions. But there are several more programs available. The `xgettext' program can be used to automatically extract the translatable messages from a source file. I.e., the programmer need not take care for the translations and the list of messages which have to be translated. S/He will simply wrap the translatable string in calls to `gettext' et.al and the rest will be done by `xgettext'. This program has a lot of option which help to customize the output or do help to understand the input better. Other programs help to manage development cycle when new messages appear in the source files or when a new translation of the messages appear. here it should only be noted that using all the tools in GNU gettext it is possible to _completely_ automize the handling of message catalog. Beside marking the translatable string in the source code and generating the translations the developers do not have anything to do themselves.  File: libc.info, Node: Searching and Sorting, Next: Pattern Matching, Prev: Message Translation, Up: Top Searching and Sorting ********************* This chapter describes functions for searching and sorting arrays of arbitrary objects. You pass the appropriate comparison function to be applied as an argument, along with the size of the objects in the array and the total number of elements. * Menu: * Comparison Functions:: Defining how to compare two objects. Since the sort and search facilities are general, you have to specify the ordering. * Array Search Function:: The `bsearch' function. * Array Sort Function:: The `qsort' function. * Search/Sort Example:: An example program. * Hash Search Function:: The `hsearch' function. * Tree Search Function:: The `tsearch' function.  File: libc.info, Node: Comparison Functions, Next: Array Search Function, Up: Searching and Sorting Defining the Comparison Function ================================ In order to use the sorted array library functions, you have to describe how to compare the elements of the array. To do this, you supply a comparison function to compare two elements of the array. The library will call this function, passing as arguments pointers to two array elements to be compared. Your comparison function should return a value the way `strcmp' (*note String/Array Comparison::) does: negative if the first argument is "less" than the second, zero if they are "equal", and positive if the first argument is "greater". Here is an example of a comparison function which works with an array of numbers of type `double': int compare_doubles (const void *a, const void *b) { const double *da = (const double *) a; const double *db = (const double *) b; return (*da > *db) - (*da < *db); } The header file `stdlib.h' defines a name for the data type of comparison functions. This type is a GNU extension. int comparison_fn_t (const void *, const void *);  File: libc.info, Node: Array Search Function, Next: Array Sort Function, Prev: Comparison Functions, Up: Searching and Sorting Array Search Function ===================== Generally searching for a specific element in an array means that potentially all elements must be checked. The GNU C library contains functions to perform linear search. The prototypes for the following two functions can be found in `search.h'. - Function: void * lfind (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR) The `lfind' function searches in the array with `*NMEMB' elements of SIZE bytes pointed to by BASE for an element which matches the one pointed to by KEY. The function pointed to by COMPAR is used decide whether two elements match. The return value is a pointer to the matching element in the array starting at BASE if it is found. If no matching element is available `NULL' is returned. The mean runtime of this function is `*NMEMB'/2. This function should only be used elements often get added to or deleted from the array in which case it might not be useful to sort the array before searching. - Function: void * lsearch (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR) The `lsearch' function is similar to the `lfind' function. It searches the given array for an element and returns it if found. The difference is that if no matching element is found the `lsearch' function adds the object pointed to by KEY (with a size of SIZE bytes) at the end of the array and it increments the value of `*NMEMB' to reflect this addition. This means for the caller that if it is not sure that the array contains the element one is searching for the memory allocated for the array starting at BASE must have room for at least SIZE more bytes. If one is sure the element is in the array it is better to use `lfind' so having more room in the array is always necessary when calling `lsearch'. To search a sorted array for an element matching the key, use the `bsearch' function. The prototype for this function is in the header file `stdlib.h'. - Function: void * bsearch (const void *KEY, const void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE) The `bsearch' function searches the sorted array ARRAY for an object that is equivalent to KEY. The array contains COUNT elements, each of which is of size SIZE bytes. The COMPARE function is used to perform the comparison. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument. The elements of the ARRAY must already be sorted in ascending order according to this comparison function. The return value is a pointer to the matching array element, or a null pointer if no match is found. If the array contains more than one element that matches, the one that is returned is unspecified. This function derives its name from the fact that it is implemented using the binary search algorithm.  File: libc.info, Node: Array Sort Function, Next: Search/Sort Example, Prev: Array Search Function, Up: Searching and Sorting Array Sort Function =================== To sort an array using an arbitrary comparison function, use the `qsort' function. The prototype for this function is in `stdlib.h'. - Function: void qsort (void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE) The QSORT function sorts the array ARRAY. The array contains COUNT elements, each of which is of size SIZE. The COMPARE function is used to perform the comparison on the array elements. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument. *Warning:* If two objects compare as equal, their order after sorting is unpredictable. That is to say, the sorting is not stable. This can make a difference when the comparison considers only part of the elements. Two elements with the same sort key may differ in other respects. If you want the effect of a stable sort, you can get this result by writing the comparison function so that, lacking other reason distinguish between two elements, it compares them by their addresses. Note that doing this may make the sorting algorithm less efficient, so do it only if necessary. Here is a simple example of sorting an array of doubles in numerical order, using the comparison function defined above (*note Comparison Functions::): { double *array; int size; ... qsort (array, size, sizeof (double), compare_doubles); } The `qsort' function derives its name from the fact that it was originally implemented using the "quick sort" algorithm. The implementation of `qsort' in this library might not be an in-place sort and might thereby use an extra amount of memory to store the array.  File: libc.info, Node: Search/Sort Example, Next: Hash Search Function, Prev: Array Sort Function, Up: Searching and Sorting Searching and Sorting Example ============================= Here is an example showing the use of `qsort' and `bsearch' with an array of structures. The objects in the array are sorted by comparing their `name' fields with the `strcmp' function. Then, we can look up individual objects based on their names. #include #include #include /* Define an array of critters to sort. */ struct critter { const char *name; const char *species; }; struct critter muppets[] = { {"Kermit", "frog"}, {"Piggy", "pig"}, {"Gonzo", "whatever"}, {"Fozzie", "bear"}, {"Sam", "eagle"}, {"Robin", "frog"}, {"Animal", "animal"}, {"Camilla", "chicken"}, {"Sweetums", "monster"}, {"Dr. Strangepork", "pig"}, {"Link Hogthrob", "pig"}, {"Zoot", "human"}, {"Dr. Bunsen Honeydew", "human"}, {"Beaker", "human"}, {"Swedish Chef", "human"} }; int count = sizeof (muppets) / sizeof (struct critter); /* This is the comparison function used for sorting and searching. */ int critter_cmp (const struct critter *c1, const struct critter *c2) { return strcmp (c1->name, c2->name); } /* Print information about a critter. */ void print_critter (const struct critter *c) { printf ("%s, the %s\n", c->name, c->species); } /* Do the lookup into the sorted array. */ void find_critter (const char *name) { struct critter target, *result; target.name = name; result = bsearch (&target, muppets, count, sizeof (struct critter), critter_cmp); if (result) print_critter (result); else printf ("Couldn't find %s.\n", name); } /* Main program. */ int main (void) { int i; for (i = 0; i < count; i++) print_critter (&muppets[i]); printf ("\n"); qsort (muppets, count, sizeof (struct critter), critter_cmp); for (i = 0; i < count; i++) print_critter (&muppets[i]); printf ("\n"); find_critter ("Kermit"); find_critter ("Gonzo"); find_critter ("Janice"); return 0; } The output from this program looks like: Kermit, the frog Piggy, the pig Gonzo, the whatever Fozzie, the bear Sam, the eagle Robin, the frog Animal, the animal Camilla, the chicken Sweetums, the monster Dr. Strangepork, the pig Link Hogthrob, the pig Zoot, the human Dr. Bunsen Honeydew, the human Beaker, the human Swedish Chef, the human Animal, the animal Beaker, the human Camilla, the chicken Dr. Bunsen Honeydew, the human Dr. Strangepork, the pig Fozzie, the bear Gonzo, the whatever Kermit, the frog Link Hogthrob, the pig Piggy, the pig Robin, the frog Sam, the eagle Swedish Chef, the human Sweetums, the monster Zoot, the human Kermit, the frog Gonzo, the whatever Couldn't find Janice.  File: libc.info, Node: Hash Search Function, Next: Tree Search Function, Prev: Search/Sort Example, Up: Searching and Sorting The `hsearch' function. ======================= The functions mentioned so far in this chapter are searching in a sorted or unsorted array. There are other methods to organize information which later should be searched. The costs of insert, delete and search differ. One possible implementation is using hashing tables. The following functions are declared in the the header file `search.h'. - Function: int hcreate (size_t NEL) The `hcreate' function creates a hashing table which can contain at least NEL elements. There is no possibility to grow this table so it is necessary to choose the value for NEL wisely. The used methods to implement this function might make it necessary to make the number of elements in the hashing table larger than the expected maximal number of elements. Hashing tables usually work inefficient if they are filled 80% or more. The constant access time guaranteed by hashing can only be achieved if few collisions exist. See Knuth's "The Art of Computer Programming, Part 3: Searching and Sorting" for more information. The weakest aspect of this function is that there can be at most one hashing table used through the whole program. The table is allocated in local memory out of control of the programmer. As an extension the GNU C library provides an additional set of functions with an reentrant interface which provide a similar interface but which allow to keep arbitrarily many hashing tables. It is possible to use more than one hashing table in the program run if the former table is first destroyed by a call to `hdestroy'. The function returns a non-zero value if successful. If it return zero something went wrong. This could either mean there is already a hashing table in use or the program runs out of memory. - Function: void hdestroy (void) The `hdestroy' function can be used to free all the resources allocated in a previous call of `hcreate'. After a call to this function it is again possible to call `hcreate' and allocate a new table with possibly different size. It is important to remember that the elements contained in the hashing table at the time `hdestroy' is called are _not_ freed by this function. It is the responsibility of the program code to free those strings (if necessary at all). Freeing all the element memory is not possible without extra, separately kept information since there is no function to iterate through all available elements in the hashing table. If it is really necessary to free a table and all elements the programmer has to keep a list of all table elements and before calling `hdestroy' s/he has to free all element's data using this list. This is a very unpleasant mechanism and it also shows that this kind of hashing tables is mainly meant for tables which are created once and used until the end of the program run. Entries of the hashing table and keys for the search are defined using this type: - Data type: struct ENTRY Both elements of this structure are pointers to zero-terminated strings. This is a limiting restriction of the functionality of the `hsearch' functions. They can only be used for data sets which use the NUL character always and solely to terminate the records. It is not possible to handle general binary data. `char *key' Pointer to a zero-terminated string of characters describing the key for the search or the element in the hashing table. `char *data' Pointer to a zero-terminated string of characters describing the data. If the functions will be called only for searching an existing entry this element might stay undefined since it is not used. - Function: ENTRY * hsearch (ENTRY ITEM, ACTION ACTION) To search in a hashing table created using `hcreate' the `hsearch' function must be used. This function can perform simple search for an element (if ACTION has the `FIND') or it can alternatively insert the key element into the hashing table. Entries are never replaced. The key is denoted by a pointer to an object of type `ENTRY'. For locating the corresponding position in the hashing table only the `key' element of the structure is used. If an entry with matching key is found the ACTION parameter is irrelevant. The found entry is returned. If no matching entry is found and the ACTION parameter has the value `FIND' the function returns a `NULL' pointer. If no entry is found and the ACTION parameter has the value `ENTER' a new entry is added to the hashing table which is initialized with the parameter ITEM. A pointer to the newly added entry is returned. As mentioned before the hashing table used by the functions described so far is global and there can be at any time at most one hashing table in the program. A solution is to use the following functions which are a GNU extension. All have in common that they operate on a hashing table which is described by the content of an object of the type `struct hsearch_data'. This type should be treated as opaque, none of its members should be changed directly. - Function: int hcreate_r (size_t NEL, struct hsearch_data *HTAB) The `hcreate_r' function initializes the object pointed to by HTAB to contain a hashing table with at least NEL elements. So this function is equivalent to the `hcreate' function except that the initialized data structure is controlled by the user. This allows having more than one hashing table at one time. The memory necessary for the `struct hsearch_data' object can be allocated dynamically. It must be initialized with zero before calling this function. The return value is non-zero if the operation were successful. if the return value is zero something went wrong which probably means the programs runs out of memory. - Function: void hdestroy_r (struct hsearch_data *HTAB) The `hdestroy_r' function frees all resources allocated by the `hcreate_r' function for this very same object HTAB. As for `hdestroy' it is the programs responsibility to free the strings for the elements of the table. - Function: int hsearch_r (ENTRY ITEM, ACTION ACTION, ENTRY **RETVAL, struct hsearch_data *HTAB) The `hsearch_r' function is equivalent to `hsearch'. The meaning of the first two arguments is identical. But instead of operating on a single global hashing table the function works on the table described by the object pointed to by HTAB (which is initialized by a call to `hcreate_r'). Another difference to `hcreate' is that the pointer to the found entry in the table is not the return value of the functions. It is returned by storing it in a pointer variables pointed to by the RETVAL parameter. The return value of the function is an integer value indicating success if it is non-zero and failure if it is zero. In the latter case the global variable ERRNO signals the reason for the failure. `ENOMEM' The table is filled and `hsearch_r' was called with an so far unknown key and ACTION set to `ENTER'. `ESRCH' The ACTION parameter is `FIND' and no corresponding element is found in the table.  File: libc.info, Node: Tree Search Function, Prev: Hash Search Function, Up: Searching and Sorting The `tsearch' function. ======================= Another common form to organize data for efficient search is to use trees. The `tsearch' function family provides a nice interface to functions to organize possibly large amounts of data by providing a mean access time proportional to the logarithm of the number of elements. The GNU C library implementation even guarantees that this bound is never exceeded even for input data which cause problems for simple binary tree implementations. The functions described in the chapter are all described in the System V and X/Open specifications and are therefore quite portable. In contrast to the `hsearch' functions the `tsearch' functions can be used with arbitrary data and not only zero-terminated strings. The `tsearch' functions have the advantage that no function to initialize data structures is necessary. A simple pointer of type `void *' initialized to `NULL' is a valid tree and can be extended or searched. The prototypes for these functions can be found in the header file `search.h'. - Function: void * tsearch (const void *KEY, void **ROOTP, comparison_fn_t COMPAR) The `tsearch' function searches in the tree pointed to by `*ROOTP' for an element matching KEY. The function pointed to by COMPAR is used to determine whether two elements match. *Note Comparison Functions::, for a specification of the functions which can be used for the COMPAR parameter. If the tree does not contain a matching entry the KEY value will be added to the tree. `tsearch' does not make a copy of the object pointed to by KEY (how could it since the size is unknown). Instead it adds a reference to this object which means the object must be available as long as the tree data structure is used. The tree is represented by a pointer to a pointer since it is sometimes necessary to change the root node of the tree. So it must not be assumed that the variable pointed to by ROOTP has the same value after the call. This also shows that it is not safe to call the `tsearch' function more than once at the same time using the same tree. It is no problem to run it more than once at a time on different trees. The return value is a pointer to the matching element in the tree. If a new element was created the pointer points to the new data (which is in fact KEY). If an entry had to be created and the program ran out of space `NULL' is returned. - Function: void * tfind (const void *KEY, void *const *ROOTP, comparison_fn_t COMPAR) The `tfind' function is similar to the `tsearch' function. It locates an element matching the one pointed to by KEY and returns a pointer to this element. But if no matching element is available no new element is entered (note that the ROOTP parameter points to a constant pointer). Instead the function returns `NULL'. Another advantage of the `tsearch' function in contrast to the `hsearch' functions is that there is an easy way to remove elements. - Function: void * tdelete (const void *KEY, void **ROOTP, comparison_fn_t COMPAR) To remove a specific element matching KEY from the tree `tdelete' can be used. It locates the matching element using the same method as `tfind'. The corresponding element is then removed and a pointer to the parent of the deleted node is returned by the function. If there is no matching entry in the tree nothing can be deleted and the function returns `NULL'. If the root of the tree is deleted `tdelete' returns some unspecified value not equal to `NULL'. - Function: void tdestroy (void *VROOT, __free_fn_t FREEFCT) If the complete search tree has to be removed one can use `tdestroy'. It frees all resources allocated by the `tsearch' function to generate the tree pointed to by VROOT. For the data in each tree node the function FREEFCT is called. The pointer to the data is passed as the argument to the function. If no such work is necessary FREEFCT must point to a function doing nothing. It is called in any case. This function is a GNU extension and not covered by the System V or X/Open specifications. In addition to the function to create and destroy the tree data structure, there is another function which allows you to apply a function to all elements of the tree. The function must have this type: void __action_fn_t (const void *nodep, VISIT value, int level); The NODEP is the data value of the current node (once given as the KEY argument to `tsearch'). LEVEL is a numeric value which corresponds to the depth of the current node in the tree. The root node has the depth 0 and its children have a depth of 1 and so on. The `VISIT' type is an enumeration type. - Data Type: VISIT The `VISIT' value indicates the status of the current node in the tree and how the function is called. The status of a node is either `leaf' or `internal node'. For each leaf node the function is called exactly once, for each internal node it is called three times: before the first child is processed, after the first child is processed and after both children are processed. This makes it possible to handle all three methods of tree traversal (or even a combination of them). `preorder' The current node is an internal node and the function is called before the first child was processed. `postorder' The current node is an internal node and the function is called after the first child was processed. `endorder' The current node is an internal node and the function is called after the second child was processed. `leaf' The current node is a leaf. - Function: void twalk (const void *ROOT, __action_fn_t ACTION) For each node in the tree with a node pointed to by ROOT, the `twalk' function calls the function provided by the parameter ACTION. For leaf nodes the function is called exactly once with VALUE set to `leaf'. For internal nodes the function is called three times, setting the VALUE parameter or ACTION to the appropriate value. The LEVEL argument for the ACTION function is computed while descending the tree with increasing the value by one for the descend to a child, starting with the value 0 for the root node. Since the functions used for the ACTION parameter to `twalk' must not modify the tree data, it is safe to run `twalk' in more than one thread at the same time, working on the same tree. It is also safe to call `tfind' in parallel. Functions which modify the tree must not be used, otherwise the behavior is undefined.  File: libc.info, Node: Pattern Matching, Next: I/O Overview, Prev: Searching and Sorting, Up: Top Pattern Matching **************** The GNU C Library provides pattern matching facilities for two kinds of patterns: regular expressions and file-name wildcards. The library also provides a facility for expanding variable and command references and parsing text into words in the way the shell does. * Menu: * Wildcard Matching:: Matching a wildcard pattern against a single string. * Globbing:: Finding the files that match a wildcard pattern. * Regular Expressions:: Matching regular expressions against strings. * Word Expansion:: Expanding shell variables, nested commands, arithmetic, and wildcards. This is what the shell does with shell commands.  File: libc.info, Node: Wildcard Matching, Next: Globbing, Up: Pattern Matching Wildcard Matching ================= This section describes how to match a wildcard pattern against a particular string. The result is a yes or no answer: does the string fit the pattern or not. The symbols described here are all declared in `fnmatch.h'. - Function: int fnmatch (const char *PATTERN, const char *STRING, int FLAGS) This function tests whether the string STRING matches the pattern PATTERN. It returns `0' if they do match; otherwise, it returns the nonzero value `FNM_NOMATCH'. The arguments PATTERN and STRING are both strings. The argument FLAGS is a combination of flag bits that alter the details of matching. See below for a list of the defined flags. In the GNU C Library, `fnmatch' cannot experience an "error"--it always returns an answer for whether the match succeeds. However, other implementations of `fnmatch' might sometimes report "errors". They would do so by returning nonzero values that are not equal to `FNM_NOMATCH'. These are the available flags for the FLAGS argument: `FNM_FILE_NAME' Treat the `/' character specially, for matching file names. If this flag is set, wildcard constructs in PATTERN cannot match `/' in STRING. Thus, the only way to match `/' is with an explicit `/' in PATTERN. `FNM_PATHNAME' This is an alias for `FNM_FILE_NAME'; it comes from POSIX.2. We don't recommend this name because we don't use the term "pathname" for file names. `FNM_PERIOD' Treat the `.' character specially if it appears at the beginning of STRING. If this flag is set, wildcard constructs in PATTERN cannot match `.' as the first character of STRING. If you set both `FNM_PERIOD' and `FNM_FILE_NAME', then the special treatment applies to `.' following `/' as well as to `.' at the beginning of STRING. (The shell uses the `FNM_PERIOD' and `FNM_FILE_NAME' flags together for matching file names.) `FNM_NOESCAPE' Don't treat the `\' character specially in patterns. Normally, `\' quotes the following character, turning off its special meaning (if any) so that it matches only itself. When quoting is enabled, the pattern `\?' matches only the string `?', because the question mark in the pattern acts like an ordinary character. If you use `FNM_NOESCAPE', then `\' is an ordinary character. `FNM_LEADING_DIR' Ignore a trailing sequence of characters starting with a `/' in STRING; that is to say, test whether STRING starts with a directory name that PATTERN matches. If this flag is set, either `foo*' or `foobar' as a pattern would match the string `foobar/frobozz'. `FNM_CASEFOLD' Ignore case in comparing STRING to PATTERN. `FNM_EXTMATCH' Recognize beside the normal patterns also the extended patterns introduced in `ksh'. The patterns are written in the form explained in the following table where PATTERN-LIST is a `|' separated list of patterns. `?(PATTERN-LIST)' The pattern matches if zero or one occurrences of any of the patterns in the PATTERN-LIST allow matching the input string. `*(PATTERN-LIST)' The pattern matches if zero or more occurrences of any of the patterns in the PATTERN-LIST allow matching the input string. `+(PATTERN-LIST)' The pattern matches if one or more occurrences of any of the patterns in the PATTERN-LIST allow matching the input string. `@(PATTERN-LIST)' The pattern matches if exactly one occurrence of any of the patterns in the PATTERN-LIST allows matching the input string. `!(PATTERN-LIST)' The pattern matches if the input string cannot be matched with any of the patterns in the PATTERN-LIST.  File: libc.info, Node: Globbing, Next: Regular Expressions, Prev: Wildcard Matching, Up: Pattern Matching Globbing ======== The archetypal use of wildcards is for matching against the files in a directory, and making a list of all the matches. This is called "globbing". You could do this using `fnmatch', by reading the directory entries one by one and testing each one with `fnmatch'. But that would be slow (and complex, since you would have to handle subdirectories by hand). The library provides a function `glob' to make this particular use of wildcards convenient. `glob' and the other symbols in this section are declared in `glob.h'. * Menu: * Calling Glob:: Basic use of `glob'. * Flags for Globbing:: Flags that enable various options in `glob'. * More Flags for Globbing:: GNU specific extensions to `glob'.  File: libc.info, Node: Calling Glob, Next: Flags for Globbing, Up: Globbing Calling `glob' -------------- The result of globbing is a vector of file names (strings). To return this vector, `glob' uses a special data type, `glob_t', which is a structure. You pass `glob' the address of the structure, and it fills in the structure's fields to tell you about the results. - Data Type: glob_t This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. The GNU implementation contains some more fields which are non-standard extensions. `gl_pathc' The number of elements in the vector, excluding the initial null entries if the GLOB_DOOFFS flag is used (see gl_offs below). `gl_pathv' The address of the vector. This field has type `char **'. `gl_offs' The offset of the first real element of the vector, from its nominal address in the `gl_pathv' field. Unlike the other fields, this is always an input to `glob', rather than an output from it. If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The `glob' function fills them with null pointers.) The `gl_offs' field is meaningful only if you use the `GLOB_DOOFFS' flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector. `gl_closedir' The address of an alternative implementation of the `closedir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void (*) (void *)'. This is a GNU extension. `gl_readdir' The address of an alternative implementation of the `readdir' function used to read the contents of a directory. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `struct dirent *(*) (void *)'. This is a GNU extension. `gl_opendir' The address of an alternative implementation of the `opendir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void *(*) (const char *)'. This is a GNU extension. `gl_stat' The address of an alternative implementation of the `stat' function to get information about an object in the filesystem. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat *)'. This is a GNU extension. `gl_lstat' The address of an alternative implementation of the `lstat' function to get information about an object in the filesystems, not following symbolic links. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat *)'. This is a GNU extension. For use in the `glob64' function `glob.h' contains another definition for a very similar type. `glob64_t' differs from `glob_t' only in the types of the members `gl_readdir', `gl_stat', and `gl_lstat'. - Data Type: glob64_t This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. The GNU implementation contains some more fields which are non-standard extensions. `gl_pathc' The number of elements in the vector, excluding the initial null entries if the GLOB_DOOFFS flag is used (see gl_offs below). `gl_pathv' The address of the vector. This field has type `char **'. `gl_offs' The offset of the first real element of the vector, from its nominal address in the `gl_pathv' field. Unlike the other fields, this is always an input to `glob', rather than an output from it. If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The `glob' function fills them with null pointers.) The `gl_offs' field is meaningful only if you use the `GLOB_DOOFFS' flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector. `gl_closedir' The address of an alternative implementation of the `closedir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void (*) (void *)'. This is a GNU extension. `gl_readdir' The address of an alternative implementation of the `readdir64' function used to read the contents of a directory. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `struct dirent64 *(*) (void *)'. This is a GNU extension. `gl_opendir' The address of an alternative implementation of the `opendir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void *(*) (const char *)'. This is a GNU extension. `gl_stat' The address of an alternative implementation of the `stat64' function to get information about an object in the filesystem. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat64 *)'. This is a GNU extension. `gl_lstat' The address of an alternative implementation of the `lstat64' function to get information about an object in the filesystems, not following symbolic links. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat64 *)'. This is a GNU extension. - Function: int glob (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob_t *VECTOR-PTR) The function `glob' does globbing using the pattern PATTERN in the current directory. It puts the result in a newly allocated vector, and stores the size and address of this vector into `*VECTOR-PTR'. The argument FLAGS is a combination of bit flags; see *Note Flags for Globbing::, for details of the flags. The result of globbing is a sequence of file names. The function `glob' allocates a string for each resulting word, then allocates a vector of type `char **' to store the addresses of these strings. The last element of the vector is a null pointer. This vector is called the "word vector". To return this vector, `glob' stores both its address and its length (number of elements, not counting the terminating null pointer) into `*VECTOR-PTR'. Normally, `glob' sorts the file names alphabetically before returning them. You can turn this off with the flag `GLOB_NOSORT' if you want to get the information as fast as possible. Usually it's a good idea to let `glob' sort them--if you process the files in alphabetical order, the users will have a feel for the rate of progress that your application is making. If `glob' succeeds, it returns 0. Otherwise, it returns one of these error codes: `GLOB_ABORTED' There was an error opening a directory, and you used the flag `GLOB_ERR' or your specified ERRFUNC returned a nonzero value. *Note Flags for Globbing::, for an explanation of the `GLOB_ERR' flag and ERRFUNC. `GLOB_NOMATCH' The pattern didn't match any existing files. If you use the `GLOB_NOCHECK' flag, then you never get this error code, because that flag tells `glob' to _pretend_ that the pattern matched at least one file. `GLOB_NOSPACE' It was impossible to allocate memory to hold the result. In the event of an error, `glob' stores information in `*VECTOR-PTR' about all the matches it has found so far. It is important to notice that the `glob' function will not fail if it encounters directories or files which cannot be handled without the LFS interfaces. The implementation of `glob' is supposed to use these functions internally. This at least is the assumptions made by the Unix standard. The GNU extension of allowing the user to provide own directory handling and `stat' functions complicates things a bit. If these callback functions are used and a large file or directory is encountered `glob' _can_ fail. - Function: int glob64 (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob64_t *VECTOR-PTR) The `glob64' function was added as part of the Large File Summit extensions but is not part of the original LFS proposal. The reason for this is simple: it is not necessary. The necessity for a `glob64' function is added by the extensions of the GNU `glob' implementation which allows the user to provide own directory handling and `stat' functions. The `readdir' and `stat' functions do depend on the choice of `_FILE_OFFSET_BITS' since the definition of the types `struct dirent' and `struct stat' will change depending on the choice. Beside this difference the `glob64' works just like `glob' in all aspects. This function is a GNU extension.  File: libc.info, Node: Flags for Globbing, Next: More Flags for Globbing, Prev: Calling Glob, Up: Globbing Flags for Globbing ------------------ This section describes the flags that you can specify in the FLAGS argument to `glob'. Choose the flags you want, and combine them with the C bitwise OR operator `|'. `GLOB_APPEND' Append the words from this expansion to the vector of words produced by previous calls to `glob'. This way you can effectively expand several words as if they were concatenated with spaces between them. In order for appending to work, you must not modify the contents of the word vector structure between calls to `glob'. And, if you set `GLOB_DOOFFS' in the first call to `glob', you must also set it when you append to the results. Note that the pointer stored in `gl_pathv' may no longer be valid after you call `glob' the second time, because `glob' might have relocated the vector. So always fetch `gl_pathv' from the `glob_t' structure after each `glob' call; *never* save the pointer across calls. `GLOB_DOOFFS' Leave blank slots at the beginning of the vector of words. The `gl_offs' field says how many slots to leave. The blank slots contain null pointers. `GLOB_ERR' Give up right away and report an error if there is any difficulty reading the directories that must be read in order to expand PATTERN fully. Such difficulties might include a directory in which you don't have the requisite access. Normally, `glob' tries its best to keep on going despite any errors, reading whatever directories it can. You can exercise even more control than this by specifying an error-handler function ERRFUNC when you call `glob'. If ERRFUNC is not a null pointer, then `glob' doesn't give up right away when it can't read a directory; instead, it calls ERRFUNC with two arguments, like this: (*ERRFUNC) (FILENAME, ERROR-CODE) The argument FILENAME is the name of the directory that `glob' couldn't open or couldn't read, and ERROR-CODE is the `errno' value that was reported to `glob'. If the error handler function returns nonzero, then `glob' gives up right away. Otherwise, it continues. `GLOB_MARK' If the pattern matches the name of a directory, append `/' to the directory's name when returning it. `GLOB_NOCHECK' If the pattern doesn't match any file names, return the pattern itself as if it were a file name that had been matched. (Normally, when the pattern doesn't match anything, `glob' returns that there were no matches.) `GLOB_NOSORT' Don't sort the file names; return them in no particular order. (In practice, the order will depend on the order of the entries in the directory.) The only reason _not_ to sort is to save time. `GLOB_NOESCAPE' Don't treat the `\' character specially in patterns. Normally, `\' quotes the following character, turning off its special meaning (if any) so that it matches only itself. When quoting is enabled, the pattern `\?' matches only the string `?', because the question mark in the pattern acts like an ordinary character. If you use `GLOB_NOESCAPE', then `\' is an ordinary character. `glob' does its work by calling the function `fnmatch' repeatedly. It handles the flag `GLOB_NOESCAPE' by turning on the `FNM_NOESCAPE' flag in calls to `fnmatch'.  File: libc.info, Node: More Flags for Globbing, Prev: Flags for Globbing, Up: Globbing More Flags for Globbing ----------------------- Beside the flags described in the last section, the GNU implementation of `glob' allows a few more flags which are also defined in the `glob.h' file. Some of the extensions implement functionality which is available in modern shell implementations. `GLOB_PERIOD' The `.' character (period) is treated special. It cannot be matched by wildcards. *Note Wildcard Matching::, `FNM_PERIOD'. `GLOB_MAGCHAR' The `GLOB_MAGCHAR' value is not to be given to `glob' in the FLAGS parameter. Instead, `glob' sets this bit in the GL_FLAGS element of the GLOB_T structure provided as the result if the pattern used for matching contains any wildcard character. `GLOB_ALTDIRFUNC' Instead of the using the using the normal functions for accessing the filesystem the `glob' implementation uses the user-supplied functions specified in the structure pointed to by PGLOB parameter. For more information about the functions refer to the sections about directory handling see *Note Accessing Directories::, and *Note Reading Attributes::. `GLOB_BRACE' If this flag is given the handling of braces in the pattern is changed. It is now required that braces appear correctly grouped. I.e., for each opening brace there must be a closing one. Braces can be used recursively. So it is possible to define one brace expression in another one. It is important to note that the range of each brace expression is completely contained in the outer brace expression (if there is one). The string between the matching braces is separated into single expressions by splitting at `,' (comma) characters. The commas themselves are discarded. Please note what we said above about recursive brace expressions. The commas used to separate the subexpressions must be at the same level. Commas in brace subexpressions are not matched. They are used during expansion of the brace expression of the deeper level. The example below shows this glob ("{foo/{,bar,biz},baz}", GLOB_BRACE, NULL, &result) is equivalent to the sequence glob ("foo/", GLOB_BRACE, NULL, &result) glob ("foo/bar", GLOB_BRACE|GLOB_APPEND, NULL, &result) glob ("foo/biz", GLOB_BRACE|GLOB_APPEND, NULL, &result) glob ("baz", GLOB_BRACE|GLOB_APPEND, NULL, &result) if we leave aside error handling. `GLOB_NOMAGIC' If the pattern contains no wildcard constructs (it is a literal file name), return it as the sole "matching" word, even if no file exists by that name. `GLOB_TILDE' If this flag is used the character `~' (tilde) is handled special if it appears at the beginning of the pattern. Instead of being taken verbatim it is used to represent the home directory of a known user. If `~' is the only character in pattern or it is followed by a `/' (slash), the home directory of the process owner is substituted. Using `getlogin' and `getpwnam' the information is read from the system databases. As an example take user `bart' with his home directory at `/home/bart'. For him a call like glob ("~/bin/*", GLOB_TILDE, NULL, &result) would return the contents of the directory `/home/bart/bin'. Instead of referring to the own home directory it is also possible to name the home directory of other users. To do so one has to append the user name after the tilde character. So the contents of user `homer''s `bin' directory can be retrieved by glob ("~homer/bin/*", GLOB_TILDE, NULL, &result) If the user name is not valid or the home directory cannot be determined for some reason the pattern is left untouched and itself used as the result. I.e., if in the last example `home' is not available the tilde expansion yields to `"~homer/bin/*"' and `glob' is not looking for a directory named `~homer'. This functionality is equivalent to what is available in C-shells if the `nonomatch' flag is set. `GLOB_TILDE_CHECK' If this flag is used `glob' behaves like as if `GLOB_TILDE' is given. The only difference is that if the user name is not available or the home directory cannot be determined for other reasons this leads to an error. `glob' will return `GLOB_NOMATCH' instead of using the pattern itself as the name. This functionality is equivalent to what is available in C-shells if `nonomatch' flag is not set. `GLOB_ONLYDIR' If this flag is used the globbing function takes this as a *hint* that the caller is only interested in directories matching the pattern. If the information about the type of the file is easily available non-directories will be rejected but no extra work will be done to determine the information for each file. I.e., the caller must still be able to filter directories out. This functionality is only available with the GNU `glob' implementation. It is mainly used internally to increase the performance but might be useful for a user as well and therefore is documented here. Calling `glob' will in most cases allocate resources which are used to represent the result of the function call. If the same object of type `glob_t' is used in multiple call to `glob' the resources are freed or reused so that no leaks appear. But this does not include the time when all `glob' calls are done. - Function: void globfree (glob_t *PGLOB) The `globfree' function frees all resources allocated by previous calls to `glob' associated with the object pointed to by PGLOB. This function should be called whenever the currently used `glob_t' typed object isn't used anymore. - Function: void globfree64 (glob64_t *PGLOB) This function is equivalent to `globfree' but it frees records of type `glob64_t' which were allocated by `glob64'.  File: libc.info, Node: Regular Expressions, Next: Word Expansion, Prev: Globbing, Up: Pattern Matching Regular Expression Matching =========================== The GNU C library supports two interfaces for matching regular expressions. One is the standard POSIX.2 interface, and the other is what the GNU system has had for many years. Both interfaces are declared in the header file `regex.h'. If you define `_POSIX_C_SOURCE', then only the POSIX.2 functions, structures, and constants are declared. * Menu: * POSIX Regexp Compilation:: Using `regcomp' to prepare to match. * Flags for POSIX Regexps:: Syntax variations for `regcomp'. * Matching POSIX Regexps:: Using `regexec' to match the compiled pattern that you get from `regcomp'. * Regexp Subexpressions:: Finding which parts of the string were matched. * Subexpression Complications:: Find points of which parts were matched. * Regexp Cleanup:: Freeing storage; reporting errors.  File: libc.info, Node: POSIX Regexp Compilation, Next: Flags for POSIX Regexps, Up: Regular Expressions POSIX Regular Expression Compilation ------------------------------------ Before you can actually match a regular expression, you must "compile" it. This is not true compilation--it produces a special data structure, not machine instructions. But it is like ordinary compilation in that its purpose is to enable you to "execute" the pattern fast. (*Note Matching POSIX Regexps::, for how to use the compiled regular expression for matching.) There is a special data type for compiled regular expressions: - Data Type: regex_t This type of object holds a compiled regular expression. It is actually a structure. It has just one field that your programs should look at: `re_nsub' This field holds the number of parenthetical subexpressions in the regular expression that was compiled. There are several other fields, but we don't describe them here, because only the functions in the library should use them. After you create a `regex_t' object, you can compile a regular expression into it by calling `regcomp'. - Function: int regcomp (regex_t *COMPILED, const char *PATTERN, int CFLAGS) The function `regcomp' "compiles" a regular expression into a data structure that you can use with `regexec' to match against a string. The compiled regular expression format is designed for efficient matching. `regcomp' stores it into `*COMPILED'. It's up to you to allocate an object of type `regex_t' and pass its address to `regcomp'. The argument CFLAGS lets you specify various options that control the syntax and semantics of regular expressions. *Note Flags for POSIX Regexps::. If you use the flag `REG_NOSUB', then `regcomp' omits from the compiled regular expression the information necessary to record how subexpressions actually match. In this case, you might as well pass `0' for the MATCHPTR and NMATCH arguments when you call `regexec'. If you don't use `REG_NOSUB', then the compiled regular expression does have the capacity to record how subexpressions match. Also, `regcomp' tells you how many subexpressions PATTERN has, by storing the number in `COMPILED->re_nsub'. You can use that value to decide how long an array to allocate to hold information about subexpression matches. `regcomp' returns `0' if it succeeds in compiling the regular expression; otherwise, it returns a nonzero error code (see the table below). You can use `regerror' to produce an error message string describing the reason for a nonzero value; see *Note Regexp Cleanup::. Here are the possible nonzero values that `regcomp' can return: `REG_BADBR' There was an invalid `\{...\}' construct in the regular expression. A valid `\{...\}' construct must contain either a single number, or two numbers in increasing order separated by a comma. `REG_BADPAT' There was a syntax error in the regular expression. `REG_BADRPT' A repetition operator such as `?' or `*' appeared in a bad position (with no preceding subexpression to act on). `REG_ECOLLATE' The regular expression referred to an invalid collating element (one not defined in the current locale for string collation). *Note Locale Categories::. `REG_ECTYPE' The regular expression referred to an invalid character class name. `REG_EESCAPE' The regular expression ended with `\'. `REG_ESUBREG' There was an invalid number in the `\DIGIT' construct. `REG_EBRACK' There were unbalanced square brackets in the regular expression. `REG_EPAREN' An extended regular expression had unbalanced parentheses, or a basic regular expression had unbalanced `\(' and `\)'. `REG_EBRACE' The regular expression had unbalanced `\{' and `\}'. `REG_ERANGE' One of the endpoints in a range expression was invalid. `REG_ESPACE' `regcomp' ran out of memory.  File: libc.info, Node: Flags for POSIX Regexps, Next: Matching POSIX Regexps, Prev: POSIX Regexp Compilation, Up: Regular Expressions Flags for POSIX Regular Expressions ----------------------------------- These are the bit flags that you can use in the CFLAGS operand when compiling a regular expression with `regcomp'. `REG_EXTENDED' Treat the pattern as an extended regular expression, rather than as a basic regular expression. `REG_ICASE' Ignore case when matching letters. `REG_NOSUB' Don't bother storing the contents of the MATCHES-PTR array. `REG_NEWLINE' Treat a newline in STRING as dividing STRING into multiple lines, so that `$' can match before the newline and `^' can match after. Also, don't permit `.' to match a newline, and don't permit `[^...]' to match a newline. Otherwise, newline acts like any other ordinary character.  File: libc.info, Node: Matching POSIX Regexps, Next: Regexp Subexpressions, Prev: Flags for POSIX Regexps, Up: Regular Expressions Matching a Compiled POSIX Regular Expression -------------------------------------------- Once you have compiled a regular expression, as described in *Note POSIX Regexp Compilation::, you can match it against strings using `regexec'. A match anywhere inside the string counts as success, unless the regular expression contains anchor characters (`^' or `$'). - Function: int regexec (regex_t *COMPILED, char *STRING, size_t NMATCH, regmatch_t MATCHPTR [], int EFLAGS) This function tries to match the compiled regular expression `*COMPILED' against STRING. `regexec' returns `0' if the regular expression matches; otherwise, it returns a nonzero value. See the table below for what nonzero values mean. You can use `regerror' to produce an error message string describing the reason for a nonzero value; see *Note Regexp Cleanup::. The argument EFLAGS is a word of bit flags that enable various options. If you want to get information about what part of STRING actually matched the regular expression or its subexpressions, use the arguments MATCHPTR and NMATCH. Otherwise, pass `0' for NMATCH, and `NULL' for MATCHPTR. *Note Regexp Subexpressions::. You must match the regular expression with the same set of current locales that were in effect when you compiled the regular expression. The function `regexec' accepts the following flags in the EFLAGS argument: `REG_NOTBOL' Do not regard the beginning of the specified string as the beginning of a line; more generally, don't make any assumptions about what text might precede it. `REG_NOTEOL' Do not regard the end of the specified string as the end of a line; more generally, don't make any assumptions about what text might follow it. Here are the possible nonzero values that `regexec' can return: `REG_NOMATCH' The pattern didn't match the string. This isn't really an error. `REG_ESPACE' `regexec' ran out of memory.  File: libc.info, Node: Regexp Subexpressions, Next: Subexpression Complications, Prev: Matching POSIX Regexps, Up: Regular Expressions Match Results with Subexpressions --------------------------------- When `regexec' matches parenthetical subexpressions of PATTERN, it records which parts of STRING they match. It returns that information by storing the offsets into an array whose elements are structures of type `regmatch_t'. The first element of the array (index `0') records the part of the string that matched the entire regular expression. Each other element of the array records the beginning and end of the part that matched a single parenthetical subexpression. - Data Type: regmatch_t This is the data type of the MATCHARRAY array that you pass to `regexec'. It contains two structure fields, as follows: `rm_so' The offset in STRING of the beginning of a substring. Add this value to STRING to get the address of that part. `rm_eo' The offset in STRING of the end of the substring. - Data Type: regoff_t `regoff_t' is an alias for another signed integer type. The fields of `regmatch_t' have type `regoff_t'. The `regmatch_t' elements correspond to subexpressions positionally; the first element (index `1') records where the first subexpression matched, the second element records the second subexpression, and so on. The order of the subexpressions is the order in which they begin. When you call `regexec', you specify how long the MATCHPTR array is, with the NMATCH argument. This tells `regexec' how many elements to store. If the actual regular expression has more than NMATCH subexpressions, then you won't get offset information about the rest of them. But this doesn't alter whether the pattern matches a particular string or not. If you don't want `regexec' to return any information about where the subexpressions matched, you can either supply `0' for NMATCH, or use the flag `REG_NOSUB' when you compile the pattern with `regcomp'.  File: libc.info, Node: Subexpression Complications, Next: Regexp Cleanup, Prev: Regexp Subexpressions, Up: Regular Expressions Complications in Subexpression Matching --------------------------------------- Sometimes a subexpression matches a substring of no characters. This happens when `f\(o*\)' matches the string `fum'. (It really matches just the `f'.) In this case, both of the offsets identify the point in the string where the null substring was found. In this example, the offsets are both `1'. Sometimes the entire regular expression can match without using some of its subexpressions at all--for example, when `ba\(na\)*' matches the string `ba', the parenthetical subexpression is not used. When this happens, `regexec' stores `-1' in both fields of the element for that subexpression. Sometimes matching the entire regular expression can match a particular subexpression more than once--for example, when `ba\(na\)*' matches the string `bananana', the parenthetical subexpression matches three times. When this happens, `regexec' usually stores the offsets of the last part of the string that matched the subexpression. In the case of `bananana', these offsets are `6' and `8'. But the last match is not always the one that is chosen. It's more accurate to say that the last _opportunity_ to match is the one that takes precedence. What this means is that when one subexpression appears within another, then the results reported for the inner subexpression reflect whatever happened on the last match of the outer subexpression. For an example, consider `\(ba\(na\)*s \)*' matching the string `bananas bas '. The last time the inner expression actually matches is near the end of the first word. But it is _considered_ again in the second word, and fails to match there. `regexec' reports nonuse of the "na" subexpression. Another place where this rule applies is when the regular expression \(ba\(na\)*s \|nefer\(ti\)* \)* matches `bananas nefertiti'. The "na" subexpression does match in the first word, but it doesn't match in the second word because the other alternative is used there. Once again, the second repetition of the outer subexpression overrides the first, and within that second repetition, the "na" subexpression is not used. So `regexec' reports nonuse of the "na" subexpression.  File: libc.info, Node: Regexp Cleanup, Prev: Subexpression Complications, Up: Regular Expressions POSIX Regexp Matching Cleanup ----------------------------- When you are finished using a compiled regular expression, you can free the storage it uses by calling `regfree'. - Function: void regfree (regex_t *COMPILED) Calling `regfree' frees all the storage that `*COMPILED' points to. This includes various internal fields of the `regex_t' structure that aren't documented in this manual. `regfree' does not free the object `*COMPILED' itself. You should always free the space in a `regex_t' structure with `regfree' before using the structure to compile another regular expression. When `regcomp' or `regexec' reports an error, you can use the function `regerror' to turn it into an error message string. - Function: size_t regerror (int ERRCODE, regex_t *COMPILED, char *BUFFER, size_t LENGTH) This function produces an error message string for the error code ERRCODE, and stores the string in LENGTH bytes of memory starting at BUFFER. For the COMPILED argument, supply the same compiled regular expression structure that `regcomp' or `regexec' was working with when it got the error. Alternatively, you can supply `NULL' for COMPILED; you will still get a meaningful error message, but it might not be as detailed. If the error message can't fit in LENGTH bytes (including a terminating null character), then `regerror' truncates it. The string that `regerror' stores is always null-terminated even if it has been truncated. The return value of `regerror' is the minimum length needed to store the entire error message. If this is less than LENGTH, then the error message was not truncated, and you can use it. Otherwise, you should call `regerror' again with a larger buffer. Here is a function which uses `regerror', but always dynamically allocates a buffer for the error message: char *get_regerror (int errcode, regex_t *compiled) { size_t length = regerror (errcode, compiled, NULL, 0); char *buffer = xmalloc (length); (void) regerror (errcode, compiled, buffer, length); return buffer; }  File: libc.info, Node: Word Expansion, Prev: Regular Expressions, Up: Pattern Matching Shell-Style Word Expansion ========================== "Word expansion" means the process of splitting a string into "words" and substituting for variables, commands, and wildcards just as the shell does. For example, when you write `ls -l foo.c', this string is split into three separate words--`ls', `-l' and `foo.c'. This is the most basic function of word expansion. When you write `ls *.c', this can become many words, because the word `*.c' can be replaced with any number of file names. This is called "wildcard expansion", and it is also a part of word expansion. When you use `echo $PATH' to print your path, you are taking advantage of "variable substitution", which is also part of word expansion. Ordinary programs can perform word expansion just like the shell by calling the library function `wordexp'. * Menu: * Expansion Stages:: What word expansion does to a string. * Calling Wordexp:: How to call `wordexp'. * Flags for Wordexp:: Options you can enable in `wordexp'. * Wordexp Example:: A sample program that does word expansion. * Tilde Expansion:: Details of how tilde expansion works. * Variable Substitution:: Different types of variable substitution.  File: libc.info, Node: Expansion Stages, Next: Calling Wordexp, Up: Word Expansion The Stages of Word Expansion ---------------------------- When word expansion is applied to a sequence of words, it performs the following transformations in the order shown here: 1. "Tilde expansion": Replacement of `~foo' with the name of the home directory of `foo'. 2. Next, three different transformations are applied in the same step, from left to right: * "Variable substitution": Environment variables are substituted for references such as `$foo'. * "Command substitution": Constructs such as ``cat foo`' and the equivalent `$(cat foo)' are replaced with the output from the inner command. * "Arithmetic expansion": Constructs such as `$(($x-1))' are replaced with the result of the arithmetic computation. 3. "Field splitting": subdivision of the text into "words". 4. "Wildcard expansion": The replacement of a construct such as `*.c' with a list of `.c' file names. Wildcard expansion applies to an entire word at a time, and replaces that word with 0 or more file names that are themselves words. 5. "Quote removal": The deletion of string-quotes, now that they have done their job by inhibiting the above transformations when appropriate. For the details of these transformations, and how to write the constructs that use them, see `The BASH Manual' (to appear).  File: libc.info, Node: Calling Wordexp, Next: Flags for Wordexp, Prev: Expansion Stages, Up: Word Expansion Calling `wordexp' ----------------- All the functions, constants and data types for word expansion are declared in the header file `wordexp.h'. Word expansion produces a vector of words (strings). To return this vector, `wordexp' uses a special data type, `wordexp_t', which is a structure. You pass `wordexp' the address of the structure, and it fills in the structure's fields to tell you about the results. - Data Type: wordexp_t This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. `we_wordc' The number of elements in the vector. `we_wordv' The address of the vector. This field has type `char **'. `we_offs' The offset of the first real element of the vector, from its nominal address in the `we_wordv' field. Unlike the other fields, this is always an input to `wordexp', rather than an output from it. If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The `wordexp' function fills them with null pointers.) The `we_offs' field is meaningful only if you use the `WRDE_DOOFFS' flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector. - Function: int wordexp (const char *WORDS, wordexp_t *WORD-VECTOR-PTR, int FLAGS) Perform word expansion on the string WORDS, putting the result in a newly allocated vector, and store the size and address of this vector into `*WORD-VECTOR-PTR'. The argument FLAGS is a combination of bit flags; see *Note Flags for Wordexp::, for details of the flags. You shouldn't use any of the characters `|&;<>' in the string WORDS unless they are quoted; likewise for newline. If you use these characters unquoted, you will get the `WRDE_BADCHAR' error code. Don't use parentheses or braces unless they are quoted or part of a word expansion construct. If you use quotation characters `'"`', they should come in pairs that balance. The results of word expansion are a sequence of words. The function `wordexp' allocates a string for each resulting word, then allocates a vector of type `char **' to store the addresses of these strings. The last element of the vector is a null pointer. This vector is called the "word vector". To return this vector, `wordexp' stores both its address and its length (number of elements, not counting the terminating null pointer) into `*WORD-VECTOR-PTR'. If `wordexp' succeeds, it returns 0. Otherwise, it returns one of these error codes: `WRDE_BADCHAR' The input string WORDS contains an unquoted invalid character such as `|'. `WRDE_BADVAL' The input string refers to an undefined shell variable, and you used the flag `WRDE_UNDEF' to forbid such references. `WRDE_CMDSUB' The input string uses command substitution, and you used the flag `WRDE_NOCMD' to forbid command substitution. `WRDE_NOSPACE' It was impossible to allocate memory to hold the result. In this case, `wordexp' can store part of the results--as much as it could allocate room for. `WRDE_SYNTAX' There was a syntax error in the input string. For example, an unmatched quoting character is a syntax error. - Function: void wordfree (wordexp_t *WORD-VECTOR-PTR) Free the storage used for the word-strings and vector that `*WORD-VECTOR-PTR' points to. This does not free the structure `*WORD-VECTOR-PTR' itself--only the other data it points to.  File: libc.info, Node: Flags for Wordexp, Next: Wordexp Example, Prev: Calling Wordexp, Up: Word Expansion Flags for Word Expansion ------------------------ This section describes the flags that you can specify in the FLAGS argument to `wordexp'. Choose the flags you want, and combine them with the C operator `|'. `WRDE_APPEND' Append the words from this expansion to the vector of words produced by previous calls to `wordexp'. This way you can effectively expand several words as if they were concatenated with spaces between them. In order for appending to work, you must not modify the contents of the word vector structure between calls to `wordexp'. And, if you set `WRDE_DOOFFS' in the first call to `wordexp', you must also set it when you append to the results. `WRDE_DOOFFS' Leave blank slots at the beginning of the vector of words. The `we_offs' field says how many slots to leave. The blank slots contain null pointers. `WRDE_NOCMD' Don't do command substitution; if the input requests command substitution, report an error. `WRDE_REUSE' Reuse a word vector made by a previous call to `wordexp'. Instead of allocating a new vector of words, this call to `wordexp' will use the vector that already exists (making it larger if necessary). Note that the vector may move, so it is not safe to save an old pointer and use it again after calling `wordexp'. You must fetch `we_pathv' anew after each call. `WRDE_SHOWERR' Do show any error messages printed by commands run by command substitution. More precisely, allow these commands to inherit the standard error output stream of the current process. By default, `wordexp' gives these commands a standard error stream that discards all output. `WRDE_UNDEF' If the input refers to a shell variable that is not defined, report an error.  File: libc.info, Node: Wordexp Example, Next: Tilde Expansion, Prev: Flags for Wordexp, Up: Word Expansion `wordexp' Example ----------------- Here is an example of using `wordexp' to expand several strings and use the results to run a shell command. It also shows the use of `WRDE_APPEND' to concatenate the expansions and of `wordfree' to free the space allocated by `wordexp'. int expand_and_execute (const char *program, const char **options) { wordexp_t result; pid_t pid int status, i; /* Expand the string for the program to run. */ switch (wordexp (program, &result, 0)) { case 0: /* Successful. */ break; case WRDE_NOSPACE: /* If the error was `WRDE_NOSPACE', then perhaps part of the result was allocated. */ wordfree (&result); default: /* Some other error. */ return -1; } /* Expand the strings specified for the arguments. */ for (i = 0; options[i] != NULL; i++) { if (wordexp (options[i], &result, WRDE_APPEND)) { wordfree (&result); return -1; } } pid = fork (); if (pid == 0) { /* This is the child process. Execute the command. */ execv (result.we_wordv[0], result.we_wordv); exit (EXIT_FAILURE); } else if (pid < 0) /* The fork failed. Report failure. */ status = -1; else /* This is the parent process. Wait for the child to complete. */ if (waitpid (pid, &status, 0) != pid) status = -1; wordfree (&result); return status; }  File: libc.info, Node: Tilde Expansion, Next: Variable Substitution, Prev: Wordexp Example, Up: Word Expansion Details of Tilde Expansion -------------------------- It's a standard part of shell syntax that you can use `~' at the beginning of a file name to stand for your own home directory. You can use `~USER' to stand for USER's home directory. "Tilde expansion" is the process of converting these abbreviations to the directory names that they stand for. Tilde expansion applies to the `~' plus all following characters up to whitespace or a slash. It takes place only at the beginning of a word, and only if none of the characters to be transformed is quoted in any way. Plain `~' uses the value of the environment variable `HOME' as the proper home directory name. `~' followed by a user name uses `getpwname' to look up that user in the user database, and uses whatever directory is recorded there. Thus, `~' followed by your own name can give different results from plain `~', if the value of `HOME' is not really your home directory.  File: libc.info, Node: Variable Substitution, Prev: Tilde Expansion, Up: Word Expansion Details of Variable Substitution -------------------------------- Part of ordinary shell syntax is the use of `$VARIABLE' to substitute the value of a shell variable into a command. This is called "variable substitution", and it is one part of doing word expansion. There are two basic ways you can write a variable reference for substitution: `${VARIABLE}' If you write braces around the variable name, then it is completely unambiguous where the variable name ends. You can concatenate additional letters onto the end of the variable value by writing them immediately after the close brace. For example, `${foo}s' expands into `tractors'. `$VARIABLE' If you do not put braces around the variable name, then the variable name consists of all the alphanumeric characters and underscores that follow the `$'. The next punctuation character ends the variable name. Thus, `$foo-bar' refers to the variable `foo' and expands into `tractor-bar'. When you use braces, you can also use various constructs to modify the value that is substituted, or test it in various ways. `${VARIABLE:-DEFAULT}' Substitute the value of VARIABLE, but if that is empty or undefined, use DEFAULT instead. `${VARIABLE:=DEFAULT}' Substitute the value of VARIABLE, but if that is empty or undefined, use DEFAULT instead and set the variable to DEFAULT. `${VARIABLE:?MESSAGE}' If VARIABLE is defined and not empty, substitute its value. Otherwise, print MESSAGE as an error message on the standard error stream, and consider word expansion a failure. `${VARIABLE:+REPLACEMENT}' Substitute REPLACEMENT, but only if VARIABLE is defined and nonempty. Otherwise, substitute nothing for this construct. `${#VARIABLE}' Substitute a numeral which expresses in base ten the number of characters in the value of VARIABLE. `${#foo}' stands for `7', because `tractor' is seven characters. These variants of variable substitution let you remove part of the variable's value before substituting it. The PREFIX and SUFFIX are not mere strings; they are wildcard patterns, just like the patterns that you use to match multiple file names. But in this context, they match against parts of the variable value rather than against file names. `${VARIABLE%%SUFFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the end that matches the pattern SUFFIX. If there is more than one alternative for how to match against SUFFIX, this construct uses the longest possible match. Thus, `${foo%%r*}' substitutes `t', because the largest match for `r*' at the end of `tractor' is `ractor'. `${VARIABLE%SUFFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the end that matches the pattern SUFFIX. If there is more than one alternative for how to match against SUFFIX, this construct uses the shortest possible alternative. Thus, `${foo%r*}' substitutes `tracto', because the shortest match for `r*' at the end of `tractor' is just `r'. `${VARIABLE##PREFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the beginning that matches the pattern PREFIX. If there is more than one alternative for how to match against PREFIX, this construct uses the longest possible match. Thus, `${foo##*t}' substitutes `or', because the largest match for `*t' at the beginning of `tractor' is `tract'. `${VARIABLE#PREFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the beginning that matches the pattern PREFIX. If there is more than one alternative for how to match against PREFIX, this construct uses the shortest possible alternative. Thus, `${foo#*t}' substitutes `ractor', because the shortest match for `*t' at the beginning of `tractor' is just `t'.  File: libc.info, Node: I/O Overview, Next: I/O on Streams, Prev: Pattern Matching, Up: Top Input/Output Overview ********************* Most programs need to do either input (reading data) or output (writing data), or most frequently both, in order to do anything useful. The GNU C library provides such a large selection of input and output functions that the hardest part is often deciding which function is most appropriate! This chapter introduces concepts and terminology relating to input and output. Other chapters relating to the GNU I/O facilities are: * *Note I/O on Streams::, which covers the high-level functions that operate on streams, including formatted input and output. * *Note Low-Level I/O::, which covers the basic I/O and control functions on file descriptors. * *Note File System Interface::, which covers functions for operating on directories and for manipulating file attributes such as access modes and ownership. * *Note Pipes and FIFOs::, which includes information on the basic interprocess communication facilities. * *Note Sockets::, which covers a more complicated interprocess communication facility with support for networking. * *Note Low-Level Terminal Interface::, which covers functions for changing how input and output to terminals or other serial devices are processed. * Menu: * I/O Concepts:: Some basic information and terminology. * File Names:: How to refer to a file.  File: libc.info, Node: I/O Concepts, Next: File Names, Up: I/O Overview Input/Output Concepts ===================== Before you can read or write the contents of a file, you must establish a connection or communications channel to the file. This process is called "opening" the file. You can open a file for reading, writing, or both. The connection to an open file is represented either as a stream or as a file descriptor. You pass this as an argument to the functions that do the actual read or write operations, to tell them which file to operate on. Certain functions expect streams, and others are designed to operate on file descriptors. When you have finished reading to or writing from the file, you can terminate the connection by "closing" the file. Once you have closed a stream or file descriptor, you cannot do any more input or output operations on it. * Menu: * Streams and File Descriptors:: The GNU Library provides two ways to access the contents of files. * File Position:: The number of bytes from the beginning of the file.  File: libc.info, Node: Streams and File Descriptors, Next: File Position, Up: I/O Concepts Streams and File Descriptors ---------------------------- When you want to do input or output to a file, you have a choice of two basic mechanisms for representing the connection between your program and the file: file descriptors and streams. File descriptors are represented as objects of type `int', while streams are represented as `FILE *' objects. File descriptors provide a primitive, low-level interface to input and output operations. Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file. But, if you want to do control operations that are specific to a particular kind of device, you must use a file descriptor; there are no facilities to use streams in this way. You must also use file descriptors if your program needs to do input or output in special modes, such as nonblocking (or polled) input (*note File Status Flags::). Streams provide a higher-level interface, layered on top of the primitive file descriptor facilities. The stream interface treats all kinds of files pretty much alike--the sole exception being the three styles of buffering that you can choose (*note Stream Buffering::). The main advantage of using the stream interface is that the set of functions for performing actual input and output operations (as opposed to control operations) on streams is much richer and more powerful than the corresponding facilities for file descriptors. The file descriptor interface provides only simple functions for transferring blocks of characters, but the stream interface also provides powerful formatted input and output functions (`printf' and `scanf') as well as functions for character- and line-oriented input and output. Since streams are implemented in terms of file descriptors, you can extract the file descriptor from a stream and perform low-level operations directly on the file descriptor. You can also initially open a connection as a file descriptor and then make a stream associated with that file descriptor. In general, you should stick with using streams rather than file descriptors, unless there is some specific operation you want to do that can only be done on a file descriptor. If you are a beginning programmer and aren't sure what functions to use, we suggest that you concentrate on the formatted input functions (*note Formatted Input::) and formatted output functions (*note Formatted Output::). If you are concerned about portability of your programs to systems other than GNU, you should also be aware that file descriptors are not as portable as streams. You can expect any system running ISO C to support streams, but non-GNU systems may not support file descriptors at all, or may only implement a subset of the GNU functions that operate on file descriptors. Most of the file descriptor functions in the GNU library are included in the POSIX.1 standard, however.  File: libc.info, Node: File Position, Prev: Streams and File Descriptors, Up: I/O Concepts File Position ------------- One of the attributes of an open file is its "file position" that keeps track of where in the file the next character is to be read or written. In the GNU system, and all POSIX.1 systems, the file position is simply an integer representing the number of bytes from the beginning of the file. The file position is normally set to the beginning of the file when it is opened, and each time a character is read or written, the file position is incremented. In other words, access to the file is normally "sequential". Ordinary files permit read or write operations at any position within the file. Some other kinds of files may also permit this. Files which do permit this are sometimes referred to as "random-access" files. You can change the file position using the `fseek' function on a stream (*note File Positioning::) or the `lseek' function on a file descriptor (*note I/O Primitives::). If you try to change the file position on a file that doesn't support random access, you get the `ESPIPE' error. Streams and descriptors that are opened for "append access" are treated specially for output: output to such files is _always_ appended sequentially to the _end_ of the file, regardless of the file position. However, the file position is still used to control where in the file reading is done. If you think about it, you'll realize that several programs can read a given file at the same time. In order for each program to be able to read the file at its own pace, each program must have its own file pointer, which is not affected by anything the other programs do. In fact, each opening of a file creates a separate file position. Thus, if you open a file twice even in the same program, you get two streams or descriptors with independent file positions. By contrast, if you open a descriptor and then duplicate it to get another descriptor, these two descriptors share the same file position: changing the file position of one descriptor will affect the other.  File: libc.info, Node: File Names, Prev: I/O Concepts, Up: I/O Overview File Names ========== In order to open a connection to a file, or to perform other operations such as deleting a file, you need some way to refer to the file. Nearly all files have names that are strings--even files which are actually devices such as tape drives or terminals. These strings are called "file names". You specify the file name to say which file you want to open or operate on. This section describes the conventions for file names and how the operating system works with them. * Menu: * Directories:: Directories contain entries for files. * File Name Resolution:: A file name specifies how to look up a file. * File Name Errors:: Error conditions relating to file names. * File Name Portability:: File name portability and syntax issues.  File: libc.info, Node: Directories, Next: File Name Resolution, Up: File Names Directories ----------- In order to understand the syntax of file names, you need to understand how the file system is organized into a hierarchy of directories. A "directory" is a file that contains information to associate other files with names; these associations are called "links" or "directory entries". Sometimes, people speak of "files in a directory", but in reality, a directory only contains pointers to files, not the files themselves. The name of a file contained in a directory entry is called a "file name component". In general, a file name consists of a sequence of one or more such components, separated by the slash character (`/'). A file name which is just one component names a file with respect to its directory. A file name with multiple components names a directory, and then a file in that directory, and so on. Some other documents, such as the POSIX standard, use the term "pathname" for what we call a file name, and either "filename" or "pathname component" for what this manual calls a file name component. We don't use this terminology because a "path" is something completely different (a list of directories to search), and we think that "pathname" used for something else will confuse users. We always use "file name" and "file name component" (or sometimes just "component", where the context is obvious) in GNU documentation. Some macros use the POSIX terminology in their names, such as `PATH_MAX'. These macros are defined by the POSIX standard, so we cannot change their names. You can find more detailed information about operations on directories in *Note File System Interface::.  File: libc.info, Node: File Name Resolution, Next: File Name Errors, Prev: Directories, Up: File Names File Name Resolution -------------------- A file name consists of file name components separated by slash (`/') characters. On the systems that the GNU C library supports, multiple successive `/' characters are equivalent to a single `/' character. The process of determining what file a file name refers to is called "file name resolution". This is performed by examining the components that make up a file name in left-to-right order, and locating each successive component in the directory named by the previous component. Of course, each of the files that are referenced as directories must actually exist, be directories instead of regular files, and have the appropriate permissions to be accessible by the process; otherwise the file name resolution fails. If a file name begins with a `/', the first component in the file name is located in the "root directory" of the process (usually all processes on the system have the same root directory). Such a file name is called an "absolute file name". Otherwise, the first component in the file name is located in the current working directory (*note Working Directory::). This kind of file name is called a "relative file name". The file name components `.' ("dot") and `..' ("dot-dot") have special meanings. Every directory has entries for these file name components. The file name component `.' refers to the directory itself, while the file name component `..' refers to its "parent directory" (the directory that contains the link for the directory in question). As a special case, `..' in the root directory refers to the root directory itself, since it has no parent; thus `/..' is the same as `/'. Here are some examples of file names: `/a' The file named `a', in the root directory. `/a/b' The file named `b', in the directory named `a' in the root directory. `a' The file named `a', in the current working directory. `/a/./b' This is the same as `/a/b'. `./a' The file named `a', in the current working directory. `../a' The file named `a', in the parent directory of the current working directory. A file name that names a directory may optionally end in a `/'. You can specify a file name of `/' to refer to the root directory, but the empty string is not a meaningful file name. If you want to refer to the current working directory, use a file name of `.' or `./'. Unlike some other operating systems, the GNU system doesn't have any built-in support for file types (or extensions) or file versions as part of its file name syntax. Many programs and utilities use conventions for file names--for example, files containing C source code usually have names suffixed with `.c'--but there is nothing in the file system itself that enforces this kind of convention.  File: libc.info, Node: File Name Errors, Next: File Name Portability, Prev: File Name Resolution, Up: File Names File Name Errors ---------------- Functions that accept file name arguments usually detect these `errno' error conditions relating to the file name syntax or trouble finding the named file. These errors are referred to throughout this manual as the "usual file name errors". `EACCES' The process does not have search permission for a directory component of the file name. `ENAMETOOLONG' This error is used when either the total length of a file name is greater than `PATH_MAX', or when an individual file name component has a length greater than `NAME_MAX'. *Note Limits for Files::. In the GNU system, there is no imposed limit on overall file name length, but some file systems may place limits on the length of a component. `ENOENT' This error is reported when a file referenced as a directory component in the file name doesn't exist, or when a component is a symbolic link whose target file does not exist. *Note Symbolic Links::. `ENOTDIR' A file that is referenced as a directory component in the file name exists, but it isn't a directory. `ELOOP' Too many symbolic links were resolved while trying to look up the file name. The system has an arbitrary limit on the number of symbolic links that may be resolved in looking up a single file name, as a primitive way to detect loops. *Note Symbolic Links::.  File: libc.info, Node: File Name Portability, Prev: File Name Errors, Up: File Names Portability of File Names ------------------------- The rules for the syntax of file names discussed in *Note File Names::, are the rules normally used by the GNU system and by other POSIX systems. However, other operating systems may use other conventions. There are two reasons why it can be important for you to be aware of file name portability issues: * If your program makes assumptions about file name syntax, or contains embedded literal file name strings, it is more difficult to get it to run under other operating systems that use different syntax conventions. * Even if you are not concerned about running your program on machines that run other operating systems, it may still be possible to access files that use different naming conventions. For example, you may be able to access file systems on another computer running a different operating system over a network, or read and write disks in formats used by other operating systems. The ISO C standard says very little about file name syntax, only that file names are strings. In addition to varying restrictions on the length of file names and what characters can validly appear in a file name, different operating systems use different conventions and syntax for concepts such as structured directories and file types or extensions. Some concepts such as file versions might be supported in some operating systems and not by others. The POSIX.1 standard allows implementations to put additional restrictions on file name syntax, concerning what characters are permitted in file names and on the length of file name and file name component strings. However, in the GNU system, you do not need to worry about these restrictions; any character except the null character is permitted in a file name string, and there are no limits on the length of file name strings.  File: libc.info, Node: I/O on Streams, Next: Low-Level I/O, Prev: I/O Overview, Up: Top Input/Output on Streams *********************** This chapter describes the functions for creating streams and performing input and output operations on them. As discussed in *Note I/O Overview::, a stream is a fairly abstract, high-level concept representing a communications channel to a file, device, or process. * Menu: * Streams:: About the data type representing a stream. * Standard Streams:: Streams to the standard input and output devices are created for you. * Opening Streams:: How to create a stream to talk to a file. * Closing Streams:: Close a stream when you are finished with it. * Streams and Threads:: Issues with streams in threaded programs. * Streams and I18N:: Streams in internationalized applications. * Simple Output:: Unformatted output by characters and lines. * Character Input:: Unformatted input by characters and words. * Line Input:: Reading a line or a record from a stream. * Unreading:: Peeking ahead/pushing back input just read. * Block Input/Output:: Input and output operations on blocks of data. * Formatted Output:: `printf' and related functions. * Customizing Printf:: You can define new conversion specifiers for `printf' and friends. * Formatted Input:: `scanf' and related functions. * EOF and Errors:: How you can tell if an I/O error happens. * Error Recovery:: What you can do about errors. * Binary Streams:: Some systems distinguish between text files and binary files. * File Positioning:: About random-access streams. * Portable Positioning:: Random access on peculiar ISO C systems. * Stream Buffering:: How to control buffering of streams. * Other Kinds of Streams:: Streams that do not necessarily correspond to an open file. * Formatted Messages:: Print strictly formatted messages.  File: libc.info, Node: Streams, Next: Standard Streams, Up: I/O on Streams Streams ======= For historical reasons, the type of the C data structure that represents a stream is called `FILE' rather than "stream". Since most of the library functions deal with objects of type `FILE *', sometimes the term "file pointer" is also used to mean "stream". This leads to unfortunate confusion over terminology in many books on C. This manual, however, is careful to use the terms "file" and "stream" only in the technical sense. The `FILE' type is declared in the header file `stdio.h'. - Data Type: FILE This is the data type used to represent stream objects. A `FILE' object holds all of the internal state information about the connection to the associated file, including such things as the file position indicator and buffering information. Each stream also has error and end-of-file status indicators that can be tested with the `ferror' and `feof' functions; see *Note EOF and Errors::. `FILE' objects are allocated and managed internally by the input/output library functions. Don't try to create your own objects of type `FILE'; let the library do it. Your programs should deal only with pointers to these objects (that is, `FILE *' values) rather than the objects themselves.  File: libc.info, Node: Standard Streams, Next: Opening Streams, Prev: Streams, Up: I/O on Streams Standard Streams ================ When the `main' function of your program is invoked, it already has three predefined streams open and available for use. These represent the "standard" input and output channels that have been established for the process. These streams are declared in the header file `stdio.h'. - Variable: FILE * stdin The "standard input" stream, which is the normal source of input for the program. - Variable: FILE * stdout The "standard output" stream, which is used for normal output from the program. - Variable: FILE * stderr The "standard error" stream, which is used for error messages and diagnostics issued by the program. In the GNU system, you can specify what files or processes correspond to these streams using the pipe and redirection facilities provided by the shell. (The primitives shells use to implement these facilities are described in *Note File System Interface::.) Most other operating systems provide similar mechanisms, but the details of how to use them can vary. In the GNU C library, `stdin', `stdout', and `stderr' are normal variables which you can set just like any others. For example, to redirect the standard output to a file, you could do: fclose (stdout); stdout = fopen ("standard-output-file", "w"); Note however, that in other systems `stdin', `stdout', and `stderr' are macros that you cannot assign to in the normal way. But you can use `freopen' to get the effect of closing one and reopening it. *Note Opening Streams::. The three streams `stdin', `stdout', and `stderr' are not unoriented at program start (*note Streams and I18N::).  File: libc.info, Node: Opening Streams, Next: Closing Streams, Prev: Standard Streams, Up: I/O on Streams Opening Streams =============== Opening a file with the `fopen' function creates a new stream and establishes a connection between the stream and a file. This may involve creating a new file. Everything described in this section is declared in the header file `stdio.h'. - Function: FILE * fopen (const char *FILENAME, const char *OPENTYPE) The `fopen' function opens a stream for I/O to the file FILENAME, and returns a pointer to the stream. The OPENTYPE argument is a string that controls how the file is opened and specifies attributes of the resulting stream. It must begin with one of the following sequences of characters: `r' Open an existing file for reading only. `w' Open the file for writing only. If the file already exists, it is truncated to zero length. Otherwise a new file is created. `a' Open a file for append access; that is, writing at the end of file only. If the file already exists, its initial contents are unchanged and output to the stream is appended to the end of the file. Otherwise, a new, empty file is created. `r+' Open an existing file for both reading and writing. The initial contents of the file are unchanged and the initial file position is at the beginning of the file. `w+' Open a file for both reading and writing. If the file already exists, it is truncated to zero length. Otherwise, a new file is created. `a+' Open or create file for both reading and appending. If the file exists, its initial contents are unchanged. Otherwise, a new file is created. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file. As you can see, `+' requests a stream that can do both input and output. The ISO standard says that when using such a stream, you must call `fflush' (*note Stream Buffering::) or a file positioning function such as `fseek' (*note File Positioning::) when switching from reading to writing or vice versa. Otherwise, internal buffers might not be emptied properly. The GNU C library does not have this limitation; you can do arbitrary reading and writing operations on a stream in whatever order. Additional characters may appear after these to specify flags for the call. Always put the mode (`r', `w+', etc.) first; that is the only part you are guaranteed will be understood by all systems. The GNU C library defines one additional character for use in OPENTYPE: the character `x' insists on creating a new file--if a file FILENAME already exists, `fopen' fails rather than opening it. If you use `x' you are guaranteed that you will not clobber an existing file. This is equivalent to the `O_EXCL' option to the `open' function (*note Opening and Closing Files::). The character `b' in OPENTYPE has a standard meaning; it requests a binary stream rather than a text stream. But this makes no difference in POSIX systems (including the GNU system). If both `+' and `b' are specified, they can appear in either order. *Note Binary Streams::. If the OPENTYPE string contains the sequence `,ccs=STRING' then STRING is taken as the name of a coded character set and `fopen' will mark the stream as wide-oriented which appropriate conversion functions in place to convert from and to the character set STRING is place. Any other stream is opened initially unoriented and the orientation is decided with the first file operation. If the first operation is a wide character operation, the stream is not only marked as wide-oriented, also the conversion functions to convert to the coded character set used for the current locale are loaded. This will not change anymore from this point on even if the locale selected for the `LC_CTYPE' category is changed. Any other characters in OPENTYPE are simply ignored. They may be meaningful in other systems. If the open fails, `fopen' returns a null pointer. When the sources are compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this function is in fact `fopen64' since the LFS interface replaces transparently the old interface. You can have multiple streams (or file descriptors) pointing to the same file open at the same time. If you do only input, this works straightforwardly, but you must be careful if any output streams are included. *Note Stream/Descriptor Precautions::. This is equally true whether the streams are in one program (not usual) or in several programs (which can easily happen). It may be advantageous to use the file locking facilities to avoid simultaneous access. *Note File Locks::. - Function: FILE * fopen64 (const char *FILENAME, const char *OPENTYPE) This function is similar to `fopen' but the stream it returns a pointer for is opened using `open64'. Therefore this stream can be used even on files larger then 2^31 bytes on 32 bit machines. Please note that the return type is still `FILE *'. There is no special `FILE' type for the LFS interface. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fopen' and so transparently replaces the old interface. - Macro: int FOPEN_MAX The value of this macro is an integer constant expression that represents the minimum number of streams that the implementation guarantees can be open simultaneously. You might be able to open more than this many streams, but that is not guaranteed. The value of this constant is at least eight, which includes the three standard streams `stdin', `stdout', and `stderr'. In POSIX.1 systems this value is determined by the `OPEN_MAX' parameter; *note General Limits::. In BSD and GNU, it is controlled by the `RLIMIT_NOFILE' resource limit; *note Limits on Resources::. - Function: FILE * freopen (const char *FILENAME, const char *OPENTYPE, FILE *STREAM) This function is like a combination of `fclose' and `fopen'. It first closes the stream referred to by STREAM, ignoring any errors that are detected in the process. (Because errors are ignored, you should not use `freopen' on an output stream if you have actually done any output using the stream.) Then the file named by FILENAME is opened with mode OPENTYPE as for `fopen', and associated with the same stream object STREAM. If the operation fails, a null pointer is returned; otherwise, `freopen' returns STREAM. `freopen' has traditionally been used to connect a standard stream such as `stdin' with a file of your own choice. This is useful in programs in which use of a standard stream for certain purposes is hard-coded. In the GNU C library, you can simply close the standard streams and open new ones with `fopen'. But other systems lack this ability, so using `freopen' is more portable. When the sources are compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this function is in fact `freopen64' since the LFS interface replaces transparently the old interface. - Function: FILE * freopen64 (const char *FILENAME, const char *OPENTYPE, FILE *STREAM) This function is similar to `freopen'. The only difference is that on 32 bit machine the stream returned is able to read beyond the 2^31 bytes limits imposed by the normal interface. It should be noted that the stream pointed to by STREAM need not be opened using `fopen64' or `freopen64' since its mode is not important for this function. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `freopen' and so transparently replaces the old interface. In some situations it is useful to know whether a given stream is available for reading or writing. This information is normally not available and would have to be remembered separately. Solaris introduced a few functions to get this information from the stream descriptor and these functions are also available in the GNU C library. - Function: int __freadable (FILE *STREAM) The `__freadable' function determines whether the stream STREAM was opened to allow reading. In this case the return value is nonzero. For write-only streams the function returns zero. This function is declared in `stdio_ext.h'. - Function: int __fwritable (FILE *STREAM) The `__fwritable' function determines whether the stream STREAM was opened to allow writing. In this case the return value is nonzero. For read-only streams the function returns zero. This function is declared in `stdio_ext.h'. For slightly different kind of problems there are two more functions. They provide even finer-grained information. - Function: int __freading (FILE *STREAM) The `__freading' function determines whether the stream STREAM was last read from or whether it is opened read-only. In this case the return value is nonzero, otherwise it is zero. Determining whether a stream opened for reading and writing was last used for writing allows to draw conclusions about the content about the buffer, among other things. This function is declared in `stdio_ext.h'. - Function: int __fwriting (FILE *STREAM) The `__fwriting' function determines whether the stream STREAM was last written to or whether it is opened write-only. In this case the return value is nonzero, otherwise it is zero. This function is declared in `stdio_ext.h'.  File: libc.info, Node: Closing Streams, Next: Streams and Threads, Prev: Opening Streams, Up: I/O on Streams Closing Streams =============== When a stream is closed with `fclose', the connection between the stream and the file is canceled. After you have closed a stream, you cannot perform any additional operations on it. - Function: int fclose (FILE *STREAM) This function causes STREAM to be closed and the connection to the corresponding file to be broken. Any buffered output is written and any buffered input is discarded. The `fclose' function returns a value of `0' if the file was closed successfully, and `EOF' if an error was detected. It is important to check for errors when you call `fclose' to close an output stream, because real, everyday errors can be detected at this time. For example, when `fclose' writes the remaining buffered output, it might get an error because the disk is full. Even if you know the buffer is empty, errors can still occur when closing a file if you are using NFS. The function `fclose' is declared in `stdio.h'. To close all streams currently available the GNU C Library provides another function. - Function: int fcloseall (void) This function causes all open streams of the process to be closed and the connection to corresponding files to be broken. All buffered data is written and any buffered input is discarded. The `fcloseall' function returns a value of `0' if all the files were closed successfully, and `EOF' if an error was detected. This function should be used only in special situations, e.g., when an error occurred and the program must be aborted. Normally each single stream should be closed separately so that problems with individual streams can be identified. It is also problematic since the standard streams (*note Standard Streams::) will also be closed. The function `fcloseall' is declared in `stdio.h'. If the `main' function to your program returns, or if you call the `exit' function (*note Normal Termination::), all open streams are automatically closed properly. If your program terminates in any other manner, such as by calling the `abort' function (*note Aborting a Program::) or from a fatal signal (*note Signal Handling::), open streams might not be closed properly. Buffered output might not be flushed and files may be incomplete. For more information on buffering of streams, see *Note Stream Buffering::.  File: libc.info, Node: Streams and Threads, Next: Streams and I18N, Prev: Closing Streams, Up: I/O on Streams Streams and Threads =================== Streams can be used in multi-threaded applications in the same way they are used in single-threaded applications. But the programmer must be aware of a the possible complications. It is important to know about these also if the program one writes never use threads since the design and implementation of many stream functions is heavily influenced by the requirements added by multi-threaded programming. The POSIX standard requires that by default the stream operations are atomic. I.e., issuing two stream operations for the same stream in two threads at the same time will cause the operations to be executed as if they were issued sequentially. The buffer operations performed while reading or writing are protected from other uses of the same stream. To do this each stream has an internal lock object which has to be (implicitly) acquired before any work can be done. But there are situations where this is not enough and there are also situations where this is not wanted. The implicit locking is not enough if the program requires more than one stream function call to happen atomically. One example would be if an output line a program wants to generate is created by several function calls. The functions by themselves would ensure only atomicity of their own operation, but not atomicity over all the function calls. For this it is necessary to perform the stream locking in the application code. - Function: void flockfile (FILE *STREAM) The `flockfile' function acquires the internal locking object associated with the stream STREAM. This ensures that no other thread can explicitly through `flockfile'/`ftrylockfile' or implicit through a call of a stream function lock the stream. The thread will block until the lock is acquired. An explicit call to `funlockfile' has to be used to release the lock. - Function: int ftrylockfile (FILE *STREAM) The `ftrylockfile' function tries to acquire the internal locking object associated with the stream STREAM just like `flockfile'. But unlike `flockfile' this function does not block if the lock is not available. `ftrylockfile' returns zero if the lock was successfully acquired. Otherwise the stream is locked by another thread. - Function: void funlockfile (FILE *STREAM) The `funlockfile' function releases the internal locking object of the stream STREAM. The stream must have been locked before by a call to `flockfile' or a successful call of `ftrylockfile'. The implicit locking performed by the stream operations do not count. The `funlockfile' function does not return an error status and the behavior of a call for a stream which is not locked by the current thread is undefined. The following example shows how the functions above can be used to generate an output line atomically even in multi-threaded applications (yes, the same job could be done with one `fprintf' call but it is sometimes not possible): FILE *fp; { ... flockfile (fp); fputs ("This is test number ", fp); fprintf (fp, "%d\n", test); funlockfile (fp) } Without the explicit locking it would be possible for another thread to use the stream FP after the `fputs' call return and before `fprintf' was called with the result that the number does not follow the word `number'. From this description it might already be clear that the locking objects in streams are no simple mutexes. Since locking the same stream twice in the same thread is allowed the locking objects must be equivalent to recursive mutexes. These mutexes keep track of the owner and the number of times the lock is acquired. The same number of `funlockfile' calls by the same threads is necessary to unlock the stream completely. For instance: void foo (FILE *fp) { ftrylockfile (fp); fputs ("in foo\n", fp); /* This is very wrong!!! */ funlockfile (fp); } It is important here that the `funlockfile' function is only called if the `ftrylockfile' function succeeded in locking the stream. It is therefore always wrong to ignore the result of `ftrylockfile'. And it makes no sense since otherwise one would use `flockfile'. The result of code like that above is that either `funlockfile' tries to free a stream that hasn't been locked by the current thread or it frees the stream prematurely. The code should look like this: void foo (FILE *fp) { if (ftrylockfile (fp) == 0) { fputs ("in foo\n", fp); funlockfile (fp); } } Now that we covered why it is necessary to have these locking it is necessary to talk about situations when locking is unwanted and what can be done. The locking operations (explicit or implicit) don't come for free. Even if a lock is not taken the cost is not zero. The operations which have to be performed require memory operations that are safe in multi-processor environments. With the many local caches involved in such systems this is quite costly. So it is best to avoid the locking completely if it is not needed - because the code in question is never used in a context where two or more threads may use a stream at a time. This can be determined most of the time for application code; for library code which can be used in many contexts one should default to be conservative and use locking. There are two basic mechanisms to avoid locking. The first is to use the `_unlocked' variants of the stream operations. The POSIX standard defines quite a few of those and the GNU library adds a few more. These variants of the functions behave just like the functions with the name without the suffix except that they do not lock the stream. Using these functions is very desirable since they are potentially much faster. This is not only because the locking operation itself is avoided. More importantly, functions like `putc' and `getc' are very simple and traditionally (before the introduction of threads) were implemented as macros which are very fast if the buffer is not empty. With the addition of locking requirements these functions are no longer implemented as macros since they would would expand to too much code. But these macros are still available with the same functionality under the new names `putc_unlocked' and `getc_unlocked'. This possibly huge difference of speed also suggests the use of the `_unlocked' functions even if locking is required. The difference is that the locking then has to be performed in the program: void foo (FILE *fp, char *buf) { flockfile (fp); while (*buf != '/') putc_unlocked (*buf++, fp); funlockfile (fp); } If in this example the `putc' function would be used and the explicit locking would be missing the `putc' function would have to acquire the lock in every call, potentially many times depending on when the loop terminates. Writing it the way illustrated above allows the `putc_unlocked' macro to be used which means no locking and direct manipulation of the buffer of the stream. A second way to avoid locking is by using a non-standard function which was introduced in Solaris and is available in the GNU C library as well. - Function: int __fsetlocking (FILE *STREAM, int TYPE) The `__fsetlocking' function can be used to select whether the stream operations will implicitly acquire the locking object of the stream STREAM. By default this is done but it can be disabled and reinstated using this function. There are three values defined for the TYPE parameter. `FSETLOCKING_INTERNAL' The stream `stream' will from now on use the default internal locking. Every stream operation with exception of the `_unlocked' variants will implicitly lock the stream. `FSETLOCKING_BYCALLER' After the `__fsetlocking' function returns the user is responsible for locking the stream. None of the stream operations will implicitly do this anymore until the state is set back to `FSETLOCKING_INTERNAL'. `FSETLOCKING_QUERY' `__fsetlocking' only queries the current locking state of the stream. The return value will be `FSETLOCKING_INTERNAL' or `FSETLOCKING_BYCALLER' depending on the state. The return value of `__fsetlocking' is either `FSETLOCKING_INTERNAL' or `FSETLOCKING_BYCALLER' depending on the state of the stream before the call. This function and the values for the TYPE parameter are declared in `stdio_ext.h'. This function is especially useful when program code has to be used which is written without knowledge about the `_unlocked' functions (or if the programmer was too lazy to use them).  File: libc.info, Node: Streams and I18N, Next: Simple Output, Prev: Streams and Threads, Up: I/O on Streams Streams in Internationalized Applications ========================================= ISO C90 introduced the new type `wchar_t' to allow handling larger character sets. What was missing was a possibility to output strings of `wchar_t' directly. One had to convert them into multibyte strings using `mbstowcs' (there was no `mbsrtowcs' yet) and then use the normal stream functions. While this is doable it is very cumbersome since performing the conversions is not trivial and greatly increases program complexity and size. The Unix standard early on (I think in XPG4.2) introduced two additional format specifiers for the `printf' and `scanf' families of functions. Printing and reading of single wide characters was made possible using the `%C' specifier and wide character strings can be handled with `%S'. These modifiers behave just like `%c' and `%s' only that they expect the corresponding argument to have the wide character type and that the wide character and string are transformed into/from multibyte strings before being used. This was a beginning but it is still not good enough. Not always is it desirable to use `printf' and `scanf'. The other, smaller and faster functions cannot handle wide characters. Second, it is not possible to have a format string for `printf' and `scanf' consisting of wide characters. The result is that format strings would have to be generated if they have to contain non-basic characters. In the Amendment 1 to ISO C90 a whole new set of functions was added to solve the problem. Most of the stream functions got a counterpart which take a wide character or wide character string instead of a character or string respectively. The new functions operate on the same streams (like `stdout'). This is different from the model of the C++ runtime library where separate streams for wide and normal I/O are used. Being able to use the same stream for wide and normal operations comes with a restriction: a stream can be used either for wide operations or for normal operations. Once it is decided there is no way back. Only a call to `freopen' or `freopen64' can reset the "orientation". The orientation can be decided in three ways: * If any of the normal character functions is used (this includes the `fread' and `fwrite' functions) the stream is marked as not wide oriented. * If any of the wide character functions is used the stream is marked as wide oriented. * The `fwide' function can be used to set the orientation either way. It is important to never mix the use of wide and not wide operations on a stream. There are no diagnostics issued. The application behavior will simply be strange or the application will simply crash. The `fwide' function can help avoiding this. - Function: int fwide (FILE *STREAM, int MODE) The `fwide' function can be used to set and query the state of the orientation of the stream STREAM. If the MODE parameter has a positive value the streams get wide oriented, for negative values narrow oriented. It is not possible to overwrite previous orientations with `fwide'. I.e., if the stream STREAM was already oriented before the call nothing is done. If MODE is zero the current orientation state is queried and nothing is changed. The `fwide' function returns a negative value, zero, or a positive value if the stream is narrow, not at all, or wide oriented respectively. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. It is generally a good idea to orient a stream as early as possible. This can prevent surprise especially for the standard streams `stdin', `stdout', and `stderr'. If some library function in some situations uses one of these streams and this use orients the stream in a different way the rest of the application expects it one might end up with hard to reproduce errors. Remember that no errors are signal if the streams are used incorrectly. Leaving a stream unoriented after creation is normally only necessary for library functions which create streams which can be used in different contexts. When writing code which uses streams and which can be used in different contexts it is important to query the orientation of the stream before using it (unless the rules of the library interface demand a specific orientation). The following little, silly function illustrates this. void print_f (FILE *fp) { if (fwide (fp, 0) > 0) /* Positive return value means wide orientation. */ fputwc (L'f', fp); else fputc ('f', fp); } Note that in this case the function `print_f' decides about the orientation of the stream if it was unoriented before (will not happen if the advise above is followed). The encoding used for the `wchar_t' values is unspecified and the user must not make any assumptions about it. For I/O of `wchar_t' values this means that it is impossible to write these values directly to the stream. This is not what follows from the ISO C locale model either. What happens instead is that the bytes read from or written to the underlying media are first converted into the internal encoding chosen by the implementation for `wchar_t'. The external encoding is determined by the `LC_CTYPE' category of the current locale or by the `ccs' part of the mode specification given to `fopen', `fopen64', `freopen', or `freopen64'. How and when the conversion happens is unspecified and it happens invisible to the user. Since a stream is created in the unoriented state it has at that point no conversion associated with it. The conversion which will be used is determined by the `LC_CTYPE' category selected at the time the stream is oriented. If the locales are changed at the runtime this might produce surprising results unless one pays attention. This is just another good reason to orient the stream explicitly as soon as possible, perhaps with a call to `fwide'.  File: libc.info, Node: Simple Output, Next: Character Input, Prev: Streams and I18N, Up: I/O on Streams Simple Output by Characters or Lines ==================================== This section describes functions for performing character- and line-oriented output. These narrow streams functions are declared in the header file `stdio.h' and the wide stream functions in `wchar.h'. - Function: int fputc (int C, FILE *STREAM) The `fputc' function converts the character C to type `unsigned char', and writes it to the stream STREAM. `EOF' is returned if a write error occurs; otherwise the character C is returned. - Function: wint_t fputwc (wchar_t WC, FILE *STREAM) The `fputwc' function writes the wide character WC to the stream STREAM. `WEOF' is returned if a write error occurs; otherwise the character WC is returned. - Function: int fputc_unlocked (int C, FILE *STREAM) The `fputc_unlocked' function is equivalent to the `fputc' function except that it does not implicitly lock the stream. - Function: wint_t fputwc_unlocked (wint_t WC, FILE *STREAM) The `fputwc_unlocked' function is equivalent to the `fputwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int putc (int C, FILE *STREAM) This is just like `fputc', except that most systems implement it as a macro, making it faster. One consequence is that it may evaluate the STREAM argument more than once, which is an exception to the general rule for macros. `putc' is usually the best function to use for writing a single character. - Function: wint_t putwc (wchar_t WC, FILE *STREAM) This is just like `fputwc', except that it can be implement as a macro, making it faster. One consequence is that it may evaluate the STREAM argument more than once, which is an exception to the general rule for macros. `putwc' is usually the best function to use for writing a single wide character. - Function: int putc_unlocked (int C, FILE *STREAM) The `putc_unlocked' function is equivalent to the `putc' function except that it does not implicitly lock the stream. - Function: wint_t putwc_unlocked (wchar_t WC, FILE *STREAM) The `putwc_unlocked' function is equivalent to the `putwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int putchar (int C) The `putchar' function is equivalent to `putc' with `stdout' as the value of the STREAM argument. - Function: wint_t putwchar (wchar_t WC) The `putwchar' function is equivalent to `putwc' with `stdout' as the value of the STREAM argument. - Function: int putchar_unlocked (int C) The `putchar_unlocked' function is equivalent to the `putchar' function except that it does not implicitly lock the stream. - Function: wint_t putwchar_unlocked (wchar_t WC) The `putwchar_unlocked' function is equivalent to the `putwchar' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int fputs (const char *S, FILE *STREAM) The function `fputs' writes the string S to the stream STREAM. The terminating null character is not written. This function does _not_ add a newline character, either. It outputs only the characters in the string. This function returns `EOF' if a write error occurs, and otherwise a non-negative value. For example: fputs ("Are ", stdout); fputs ("you ", stdout); fputs ("hungry?\n", stdout); outputs the text `Are you hungry?' followed by a newline. - Function: int fputws (const wchar_t *WS, FILE *STREAM) The function `fputws' writes the wide character string WS to the stream STREAM. The terminating null character is not written. This function does _not_ add a newline character, either. It outputs only the characters in the string. This function returns `WEOF' if a write error occurs, and otherwise a non-negative value. - Function: int fputs_unlocked (const char *S, FILE *STREAM) The `fputs_unlocked' function is equivalent to the `fputs' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int fputws_unlocked (const wchar_t *WS, FILE *STREAM) The `fputws_unlocked' function is equivalent to the `fputws' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int puts (const char *S) The `puts' function writes the string S to the stream `stdout' followed by a newline. The terminating null character of the string is not written. (Note that `fputs' does _not_ write a newline as this function does.) `puts' is the most convenient function for printing simple messages. For example: puts ("This is a message."); outputs the text `This is a message.' followed by a newline. - Function: int putw (int W, FILE *STREAM) This function writes the word W (that is, an `int') to STREAM. It is provided for compatibility with SVID, but we recommend you use `fwrite' instead (*note Block Input/Output::).  File: libc.info, Node: Character Input, Next: Line Input, Prev: Simple Output, Up: I/O on Streams Character Input =============== This section describes functions for performing character-oriented input. These narrow streams functions are declared in the header file `stdio.h' and the wide character functions are declared in `wchar.h'. These functions return an `int' or `wint_t' value (for narrow and wide stream functions respectively) that is either a character of input, or the special value `EOF'/`WEOF' (usually -1). For the narrow stream functions it is important to store the result of these functions in a variable of type `int' instead of `char', even when you plan to use it only as a character. Storing `EOF' in a `char' variable truncates its value to the size of a character, so that it is no longer distinguishable from the valid character `(char) -1'. So always use an `int' for the result of `getc' and friends, and check for `EOF' after the call; once you've verified that the result is not `EOF', you can be sure that it will fit in a `char' variable without loss of information. - Function: int fgetc (FILE *STREAM) This function reads the next character as an `unsigned char' from the stream STREAM and returns its value, converted to an `int'. If an end-of-file condition or read error occurs, `EOF' is returned instead. - Function: wint_t fgetwc (FILE *STREAM) This function reads the next wide character from the stream STREAM and returns its value. If an end-of-file condition or read error occurs, `WEOF' is returned instead. - Function: int fgetc_unlocked (FILE *STREAM) The `fgetc_unlocked' function is equivalent to the `fgetc' function except that it does not implicitly lock the stream. - Function: wint_t fgetwc_unlocked (FILE *STREAM) The `fgetwc_unlocked' function is equivalent to the `fgetwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int getc (FILE *STREAM) This is just like `fgetc', except that it is permissible (and typical) for it to be implemented as a macro that evaluates the STREAM argument more than once. `getc' is often highly optimized, so it is usually the best function to use to read a single character. - Function: wint_t getwc (FILE *STREAM) This is just like `fgetwc', except that it is permissible for it to be implemented as a macro that evaluates the STREAM argument more than once. `getwc' can be highly optimized, so it is usually the best function to use to read a single wide character. - Function: int getc_unlocked (FILE *STREAM) The `getc_unlocked' function is equivalent to the `getc' function except that it does not implicitly lock the stream. - Function: wint_t getwc_unlocked (FILE *STREAM) The `getwc_unlocked' function is equivalent to the `getwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int getchar (void) The `getchar' function is equivalent to `getc' with `stdin' as the value of the STREAM argument. - Function: wint_t getwchar (void) The `getwchar' function is equivalent to `getwc' with `stdin' as the value of the STREAM argument. - Function: int getchar_unlocked (void) The `getchar_unlocked' function is equivalent to the `getchar' function except that it does not implicitly lock the stream. - Function: wint_t getwchar_unlocked (void) The `getwchar_unlocked' function is equivalent to the `getwchar' function except that it does not implicitly lock the stream. This function is a GNU extension. Here is an example of a function that does input using `fgetc'. It would work just as well using `getc' instead, or using `getchar ()' instead of `fgetc (stdin)'. The code would also work the same for the wide character stream functions. int y_or_n_p (const char *question) { fputs (question, stdout); while (1) { int c, answer; /* Write a space to separate answer from question. */ fputc (' ', stdout); /* Read the first character of the line. This should be the answer character, but might not be. */ c = tolower (fgetc (stdin)); answer = c; /* Discard rest of input line. */ while (c != '\n' && c != EOF) c = fgetc (stdin); /* Obey the answer if it was valid. */ if (answer == 'y') return 1; if (answer == 'n') return 0; /* Answer was invalid: ask for valid answer. */ fputs ("Please answer y or n:", stdout); } } - Function: int getw (FILE *STREAM) This function reads a word (that is, an `int') from STREAM. It's provided for compatibility with SVID. We recommend you use `fread' instead (*note Block Input/Output::). Unlike `getc', any `int' value could be a valid result. `getw' returns `EOF' when it encounters end-of-file or an error, but there is no way to distinguish this from an input word with value -1.  File: libc.info, Node: Line Input, Next: Unreading, Prev: Character Input, Up: I/O on Streams Line-Oriented Input =================== Since many programs interpret input on the basis of lines, it is convenient to have functions to read a line of text from a stream. Standard C has functions to do this, but they aren't very safe: null characters and even (for `gets') long lines can confuse them. So the GNU library provides the nonstandard `getline' function that makes it easy to read lines reliably. Another GNU extension, `getdelim', generalizes `getline'. It reads a delimited record, defined as everything through the next occurrence of a specified delimiter character. All these functions are declared in `stdio.h'. - Function: ssize_t getline (char **LINEPTR, size_t *N, FILE *STREAM) This function reads an entire line from STREAM, storing the text (including the newline and a terminating null character) in a buffer and storing the buffer address in `*LINEPTR'. Before calling `getline', you should place in `*LINEPTR' the address of a buffer `*N' bytes long, allocated with `malloc'. If this buffer is long enough to hold the line, `getline' stores the line in this buffer. Otherwise, `getline' makes the buffer bigger using `realloc', storing the new buffer address back in `*LINEPTR' and the increased size back in `*N'. *Note Unconstrained Allocation::. If you set `*LINEPTR' to a null pointer, and `*N' to zero, before the call, then `getline' allocates the initial buffer for you by calling `malloc'. In either case, when `getline' returns, `*LINEPTR' is a `char *' which points to the text of the line. When `getline' is successful, it returns the number of characters read (including the newline, but not including the terminating null). This value enables you to distinguish null characters that are part of the line from the null character inserted as a terminator. This function is a GNU extension, but it is the recommended way to read lines from a stream. The alternative standard functions are unreliable. If an error occurs or end of file is reached without any bytes read, `getline' returns `-1'. - Function: ssize_t getdelim (char **LINEPTR, size_t *N, int DELIMITER, FILE *STREAM) This function is like `getline' except that the character which tells it to stop reading is not necessarily newline. The argument DELIMITER specifies the delimiter character; `getdelim' keeps reading until it sees that character (or end of file). The text is stored in LINEPTR, including the delimiter character and a terminating null. Like `getline', `getdelim' makes LINEPTR bigger if it isn't big enough. `getline' is in fact implemented in terms of `getdelim', just like this: ssize_t getline (char **lineptr, size_t *n, FILE *stream) { return getdelim (lineptr, n, '\n', stream); } - Function: char * fgets (char *S, int COUNT, FILE *STREAM) The `fgets' function reads characters from the stream STREAM up to and including a newline character and stores them in the string S, adding a null character to mark the end of the string. You must supply COUNT characters worth of space in S, but the number of characters read is at most COUNT - 1. The extra character space is used to hold the null character at the end of the string. If the system is already at end of file when you call `fgets', then the contents of the array S are unchanged and a null pointer is returned. A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer S. *Warning:* If the input data has a null character, you can't tell. So don't use `fgets' unless you know the data cannot contain a null. Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message. We recommend using `getline' instead of `fgets'. - Function: wchar_t * fgetws (wchar_t *WS, int COUNT, FILE *STREAM) The `fgetws' function reads wide characters from the stream STREAM up to and including a newline character and stores them in the string WS, adding a null wide character to mark the end of the string. You must supply COUNT wide characters worth of space in WS, but the number of characters read is at most COUNT - 1. The extra character space is used to hold the null wide character at the end of the string. If the system is already at end of file when you call `fgetws', then the contents of the array WS are unchanged and a null pointer is returned. A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer WS. *Warning:* If the input data has a null wide character (which are null bytes in the input stream), you can't tell. So don't use `fgetws' unless you know the data cannot contain a null. Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message. - Function: char * fgets_unlocked (char *S, int COUNT, FILE *STREAM) The `fgets_unlocked' function is equivalent to the `fgets' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: wchar_t * fgetws_unlocked (wchar_t *WS, int COUNT, FILE *STREAM) The `fgetws_unlocked' function is equivalent to the `fgetws' function except that it does not implicitly lock the stream. This function is a GNU extension. - Deprecated function: char * gets (char *S) The function `gets' reads characters from the stream `stdin' up to the next newline character, and stores them in the string S. The newline character is discarded (note that this differs from the behavior of `fgets', which copies the newline character into the string). If `gets' encounters a read error or end-of-file, it returns a null pointer; otherwise it returns S. *Warning:* The `gets' function is *very dangerous* because it provides no protection against overflowing the string S. The GNU library includes it for compatibility only. You should *always* use `fgets' or `getline' instead. To remind you of this, the linker (if using GNU `ld') will issue a warning whenever you use `gets'.  File: libc.info, Node: Unreading, Next: Block Input/Output, Prev: Line Input, Up: I/O on Streams Unreading ========= In parser programs it is often useful to examine the next character in the input stream without removing it from the stream. This is called "peeking ahead" at the input because your program gets a glimpse of the input it will read next. Using stream I/O, you can peek ahead at input by first reading it and then "unreading" it (also called "pushing it back" on the stream). Unreading a character makes it available to be input again from the stream, by the next call to `fgetc' or other input function on that stream. * Menu: * Unreading Idea:: An explanation of unreading with pictures. * How Unread:: How to call `ungetc' to do unreading.  File: libc.info, Node: Unreading Idea, Next: How Unread, Up: Unreading What Unreading Means -------------------- Here is a pictorial explanation of unreading. Suppose you have a stream reading a file that contains just six characters, the letters `foobar'. Suppose you have read three characters so far. The situation looks like this: f o o b a r ^ so the next input character will be `b'. If instead of reading `b' you unread the letter `o', you get a situation like this: f o o b a r | o-- ^ so that the next input characters will be `o' and `b'. If you unread `9' instead of `o', you get this situation: f o o b a r | 9-- ^ so that the next input characters will be `9' and `b'.  File: libc.info, Node: How Unread, Prev: Unreading Idea, Up: Unreading Using `ungetc' To Do Unreading ------------------------------ The function to unread a character is called `ungetc', because it reverses the action of `getc'. - Function: int ungetc (int C, FILE *STREAM) The `ungetc' function pushes back the character C onto the input stream STREAM. So the next input from STREAM will read C before anything else. If C is `EOF', `ungetc' does nothing and just returns `EOF'. This lets you call `ungetc' with the return value of `getc' without needing to check for an error from `getc'. The character that you push back doesn't have to be the same as the last character that was actually read from the stream. In fact, it isn't necessary to actually read any characters from the stream before unreading them with `ungetc'! But that is a strange way to write a program; usually `ungetc' is used only to unread a character that was just read from the same stream. The GNU C library supports this even on files opened in binary mode, but other systems might not. The GNU C library only supports one character of pushback--in other words, it does not work to call `ungetc' twice without doing input in between. Other systems might let you push back multiple characters; then reading from the stream retrieves the characters in the reverse order that they were pushed. Pushing back characters doesn't alter the file; only the internal buffering for the stream is affected. If a file positioning function (such as `fseek', `fseeko' or `rewind'; *note File Positioning::) is called, any pending pushed-back characters are discarded. Unreading a character on a stream that is at end of file clears the end-of-file indicator for the stream, because it makes the character of input available. After you read that character, trying to read again will encounter end of file. - Function: wint_t ungetwc (wint_t WC, FILE *STREAM) The `ungetwc' function behaves just like `ungetc' just that it pushes back a wide character. Here is an example showing the use of `getc' and `ungetc' to skip over whitespace characters. When this function reaches a non-whitespace character, it unreads that character to be seen again on the next read operation on the stream. #include #include void skip_whitespace (FILE *stream) { int c; do /* No need to check for `EOF' because it is not `isspace', and `ungetc' ignores `EOF'. */ c = getc (stream); while (isspace (c)); ungetc (c, stream); }  File: libc.info, Node: Block Input/Output, Next: Formatted Output, Prev: Unreading, Up: I/O on Streams Block Input/Output ================== This section describes how to do input and output operations on blocks of data. You can use these functions to read and write binary data, as well as to read and write text in fixed-size blocks instead of by characters or lines. Binary files are typically used to read and write blocks of data in the same format as is used to represent the data in a running program. In other words, arbitrary blocks of memory--not just character or string objects--can be written to a binary file, and meaningfully read in again by the same program. Storing data in binary form is often considerably more efficient than using the formatted I/O functions. Also, for floating-point numbers, the binary form avoids possible loss of precision in the conversion process. On the other hand, binary files can't be examined or modified easily using many standard file utilities (such as text editors), and are not portable between different implementations of the language, or different kinds of computers. These functions are declared in `stdio.h'. - Function: size_t fread (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) This function reads up to COUNT objects of size SIZE into the array DATA, from the stream STREAM. It returns the number of objects actually read, which might be less than COUNT if a read error occurs or the end of the file is reached. This function returns a value of zero (and doesn't read anything) if either SIZE or COUNT is zero. If `fread' encounters end of file in the middle of an object, it returns the number of complete objects read, and discards the partial object. Therefore, the stream remains at the actual end of the file. - Function: size_t fread_unlocked (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) The `fread_unlocked' function is equivalent to the `fread' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: size_t fwrite (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) This function writes up to COUNT objects of size SIZE from the array DATA, to the stream STREAM. The return value is normally COUNT, if the call succeeds. Any other value indicates some sort of error, such as running out of space. - Function: size_t fwrite_unlocked (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) The `fwrite_unlocked' function is equivalent to the `fwrite' function except that it does not implicitly lock the stream. This function is a GNU extension.  File: libc.info, Node: Formatted Output, Next: Customizing Printf, Prev: Block Input/Output, Up: I/O on Streams Formatted Output ================ The functions described in this section (`printf' and related functions) provide a convenient way to perform formatted output. You call `printf' with a "format string" or "template string" that specifies how to format the values of the remaining arguments. Unless your program is a filter that specifically performs line- or character-oriented processing, using `printf' or one of the other related functions described in this section is usually the easiest and most concise way to perform output. These functions are especially useful for printing error messages, tables of data, and the like. * Menu: * Formatted Output Basics:: Some examples to get you started. * Output Conversion Syntax:: General syntax of conversion specifications. * Table of Output Conversions:: Summary of output conversions and what they do. * Integer Conversions:: Details about formatting of integers. * Floating-Point Conversions:: Details about formatting of floating-point numbers. * Other Output Conversions:: Details about formatting of strings, characters, pointers, and the like. * Formatted Output Functions:: Descriptions of the actual functions. * Dynamic Output:: Functions that allocate memory for the output. * Variable Arguments Output:: `vprintf' and friends. * Parsing a Template String:: What kinds of args does a given template call for? * Example of Parsing:: Sample program using `parse_printf_format'.  File: libc.info, Node: Formatted Output Basics, Next: Output Conversion Syntax, Up: Formatted Output Formatted Output Basics ----------------------- The `printf' function can be used to print any number of arguments. The template string argument you supply in a call provides information not only about the number of additional arguments, but also about their types and what style should be used for printing them. Ordinary characters in the template string are simply written to the output stream as-is, while "conversion specifications" introduced by a `%' character in the template cause subsequent arguments to be formatted and written to the output stream. For example, int pct = 37; char filename[] = "foo.txt"; printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n", filename, pct); produces output like Processing of `foo.txt' is 37% finished. Please be patient. This example shows the use of the `%d' conversion to specify that an `int' argument should be printed in decimal notation, the `%s' conversion to specify printing of a string argument, and the `%%' conversion to print a literal `%' character. There are also conversions for printing an integer argument as an unsigned value in octal, decimal, or hexadecimal radix (`%o', `%u', or `%x', respectively); or as a character value (`%c'). Floating-point numbers can be printed in normal, fixed-point notation using the `%f' conversion or in exponential notation using the `%e' conversion. The `%g' conversion uses either `%e' or `%f' format, depending on what is more appropriate for the magnitude of the particular number. You can control formatting more precisely by writing "modifiers" between the `%' and the character that indicates which conversion to apply. These slightly alter the ordinary behavior of the conversion. For example, most conversion specifications permit you to specify a minimum field width and a flag indicating whether you want the result left- or right-justified within the field. The specific flags and modifiers that are permitted and their interpretation vary depending on the particular conversion. They're all described in more detail in the following sections. Don't worry if this all seems excessively complicated at first; you can almost always get reasonable free-format output without using any of the modifiers at all. The modifiers are mostly used to make the output look "prettier" in tables.  File: libc.info, Node: Output Conversion Syntax, Next: Table of Output Conversions, Prev: Formatted Output Basics, Up: Formatted Output Output Conversion Syntax ------------------------ This section provides details about the precise syntax of conversion specifications that can appear in a `printf' template string. Characters in the template string that are not part of a conversion specification are printed as-is to the output stream. Multibyte character sequences (*note Character Set Handling::) are permitted in a template string. The conversion specifications in a `printf' template string have the general form: % [ PARAM-NO $] FLAGS WIDTH [ . PRECISION ] TYPE CONVERSION or % [ PARAM-NO $] FLAGS WIDTH . * [ PARAM-NO $] TYPE CONVERSION For example, in the conversion specifier `%-10.8ld', the `-' is a flag, `10' specifies the field width, the precision is `8', the letter `l' is a type modifier, and `d' specifies the conversion style. (This particular type specifier says to print a `long int' argument in decimal notation, with a minimum of 8 digits left-justified in a field at least 10 characters wide.) In more detail, output conversion specifications consist of an initial `%' character followed in sequence by: * An optional specification of the parameter used for this format. Normally the parameters to the `printf' function are assigned to the formats in the order of appearance in the format string. But in some situations (such as message translation) this is not desirable and this extension allows an explicit parameter to be specified. The PARAM-NO parts of the format must be integers in the range of 1 to the maximum number of arguments present to the function call. Some implementations limit this number to a certainly upper bound. The exact limit can be retrieved by the following constant. - Macro: NL_ARGMAX The value of `NL_ARGMAX' is the maximum value allowed for the specification of an positional parameter in a `printf' call. The actual value in effect at runtime can be retrieved by using `sysconf' using the `_SC_NL_ARGMAX' parameter *note Sysconf Definition::. Some system have a quite low limit such as 9 for System V systems. The GNU C library has no real limit. If any of the formats has a specification for the parameter position all of them in the format string shall have one. Otherwise the behavior is undefined. * Zero or more "flag characters" that modify the normal behavior of the conversion specification. * An optional decimal integer specifying the "minimum field width". If the normal conversion produces fewer characters than this, the field is padded with spaces to the specified width. This is a _minimum_ value; if the normal conversion produces more characters than this, the field is _not_ truncated. Normally, the output is right-justified within the field. You can also specify a field width of `*'. This means that the next argument in the argument list (before the actual value to be printed) is used as the field width. The value must be an `int'. If the value is negative, this means to set the `-' flag (see below) and to use the absolute value as the field width. * An optional "precision" to specify the number of digits to be written for the numeric conversions. If the precision is specified, it consists of a period (`.') followed optionally by a decimal integer (which defaults to zero if omitted). You can also specify a precision of `*'. This means that the next argument in the argument list (before the actual value to be printed) is used as the precision. The value must be an `int', and is ignored if it is negative. If you specify `*' for both the field width and precision, the field width argument precedes the precision argument. Other C library versions may not recognize this syntax. * An optional "type modifier character", which is used to specify the data type of the corresponding argument if it differs from the default type. (For example, the integer conversions assume a type of `int', but you can specify `h', `l', or `L' for other integer types.) * A character that specifies the conversion to be applied. The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they use. With the `-Wformat' option, the GNU C compiler checks calls to `printf' and related functions. It examines the format string and verifies that the correct number and types of arguments are supplied. There is also a GNU C syntax to tell the compiler that a function you write uses a `printf'-style format string. *Note Declaring Attributes of Functions: (gcc.info)Function Attributes, for more information.  File: libc.info, Node: Table of Output Conversions, Next: Integer Conversions, Prev: Output Conversion Syntax, Up: Formatted Output Table of Output Conversions --------------------------- Here is a table summarizing what all the different conversions do: `%d', `%i' Print an integer as a signed decimal number. *Note Integer Conversions::, for details. `%d' and `%i' are synonymous for output, but are different when used with `scanf' for input (*note Table of Input Conversions::). `%o' Print an integer as an unsigned octal number. *Note Integer Conversions::, for details. `%u' Print an integer as an unsigned decimal number. *Note Integer Conversions::, for details. `%x', `%X' Print an integer as an unsigned hexadecimal number. `%x' uses lower-case letters and `%X' uses upper-case. *Note Integer Conversions::, for details. `%f' Print a floating-point number in normal (fixed-point) notation. *Note Floating-Point Conversions::, for details. `%e', `%E' Print a floating-point number in exponential notation. `%e' uses lower-case letters and `%E' uses upper-case. *Note Floating-Point Conversions::, for details. `%g', `%G' Print a floating-point number in either normal or exponential notation, whichever is more appropriate for its magnitude. `%g' uses lower-case letters and `%G' uses upper-case. *Note Floating-Point Conversions::, for details. `%a', `%A' Print a floating-point number in a hexadecimal fractional notation which the exponent to base 2 represented in decimal digits. `%a' uses lower-case letters and `%A' uses upper-case. *Note Floating-Point Conversions::, for details. `%c' Print a single character. *Note Other Output Conversions::. `%C' This is an alias for `%lc' which is supported for compatibility with the Unix standard. `%s' Print a string. *Note Other Output Conversions::. `%S' This is an alias for `%ls' which is supported for compatibility with the Unix standard. `%p' Print the value of a pointer. *Note Other Output Conversions::. `%n' Get the number of characters printed so far. *Note Other Output Conversions::. Note that this conversion specification never produces any output. `%m' Print the string corresponding to the value of `errno'. (This is a GNU extension.) *Note Other Output Conversions::. `%%' Print a literal `%' character. *Note Other Output Conversions::. If the syntax of a conversion specification is invalid, unpredictable things will happen, so don't do this. If there aren't enough function arguments provided to supply values for all the conversion specifications in the template string, or if the arguments are not of the correct types, the results are unpredictable. If you supply more arguments than conversion specifications, the extra argument values are simply ignored; this is sometimes useful.  File: libc.info, Node: Integer Conversions, Next: Floating-Point Conversions, Prev: Table of Output Conversions, Up: Formatted Output Integer Conversions ------------------- This section describes the options for the `%d', `%i', `%o', `%u', `%x', and `%X' conversion specifications. These conversions print integers in various formats. The `%d' and `%i' conversion specifications both print an `int' argument as a signed decimal number; while `%o', `%u', and `%x' print the argument as an unsigned octal, decimal, or hexadecimal number (respectively). The `%X' conversion specification is just like `%x' except that it uses the characters `ABCDEF' as digits instead of `abcdef'. The following flags are meaningful: `-' Left-justify the result in the field (instead of the normal right-justification). `+' For the signed `%d' and `%i' conversions, print a plus sign if the value is positive. ` ' For the signed `%d' and `%i' conversions, if the result doesn't start with a plus or minus sign, prefix it with a space character instead. Since the `+' flag ensures that the result includes a sign, this flag is ignored if you supply both of them. `#' For the `%o' conversion, this forces the leading digit to be `0', as if by increasing the precision. For `%x' or `%X', this prefixes a leading `0x' or `0X' (respectively) to the result. This doesn't do anything useful for the `%d', `%i', or `%u' conversions. Using this flag produces output which can be parsed by the `strtoul' function (*note Parsing of Integers::) and `scanf' with the `%i' conversion (*note Numeric Input Conversions::). `'' Separate the digits into groups as specified by the locale specified for the `LC_NUMERIC' category; *note General Numeric::. This flag is a GNU extension. `0' Pad the field with zeros instead of spaces. The zeros are placed after any indication of sign or base. This flag is ignored if the `-' flag is also specified, or if a precision is specified. If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary. If you don't specify a precision, the number is printed with as many digits as it needs. If you convert a value of zero with an explicit precision of zero, then no characters at all are produced. Without a type modifier, the corresponding argument is treated as an `int' (for the signed conversions `%i' and `%d') or `unsigned int' (for the unsigned conversions `%o', `%u', `%x', and `%X'). Recall that since `printf' and friends are variadic, any `char' and `short' arguments are automatically converted to `int' by the default argument promotions. For arguments of other integer types, you can use these modifiers: `hh' Specifies that the argument is a `signed char' or `unsigned char', as appropriate. A `char' argument is converted to an `int' or `unsigned int' by the default argument promotions anyway, but the `h' modifier says to convert it back to a `char' again. This modifier was introduced in ISO C99. `h' Specifies that the argument is a `short int' or `unsigned short int', as appropriate. A `short' argument is converted to an `int' or `unsigned int' by the default argument promotions anyway, but the `h' modifier says to convert it back to a `short' again. `j' Specifies that the argument is a `intmax_t' or `uintmax_t', as appropriate. This modifier was introduced in ISO C99. `l' Specifies that the argument is a `long int' or `unsigned long int', as appropriate. Two `l' characters is like the `L' modifier, below. If used with `%c' or `%s' the corresponding parameter is considered as a wide character or wide character string respectively. This use of `l' was introduced in Amendment 1 to ISO C90. `L' `ll' `q' Specifies that the argument is a `long long int'. (This type is an extension supported by the GNU C compiler. On systems that don't support extra-long integers, this is the same as `long int'.) The `q' modifier is another name for the same thing, which comes from 4.4 BSD; a `long long int' is sometimes called a "quad" `int'. `t' Specifies that the argument is a `ptrdiff_t'. This modifier was introduced in ISO C99. `z' `Z' Specifies that the argument is a `size_t'. `z' was introduced in ISO C99. `Z' is a GNU extension predating this addition and should not be used in new code. Here is an example. Using the template string: "|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n" to print numbers using the different options for the `%d' conversion gives results like: | 0|0 | +0|+0 | 0|00000| | 00|0| | 1|1 | +1|+1 | 1|00001| 1| 01|1| | -1|-1 | -1|-1 | -1|-0001| -1| -01|-1| |100000|100000|+100000|+100000| 100000|100000|100000|100000|100000| In particular, notice what happens in the last case where the number is too large to fit in the minimum field width specified. Here are some more examples showing how unsigned integers print under various format options, using the template string: "|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n" | 0| 0| 0| 0| 0| 0| 0| 00000000| | 1| 1| 1| 1| 01| 0x1| 0X1|0x00000001| |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|  File: libc.info, Node: Floating-Point Conversions, Next: Other Output Conversions, Prev: Integer Conversions, Up: Formatted Output Floating-Point Conversions -------------------------- This section discusses the conversion specifications for floating-point numbers: the `%f', `%e', `%E', `%g', and `%G' conversions. The `%f' conversion prints its argument in fixed-point notation, producing output of the form [`-']DDD`.'DDD, where the number of digits following the decimal point is controlled by the precision you specify. The `%e' conversion prints its argument in exponential notation, producing output of the form [`-']D`.'DDD`e'[`+'|`-']DD. Again, the number of digits following the decimal point is controlled by the precision. The exponent always contains at least two digits. The `%E' conversion is similar but the exponent is marked with the letter `E' instead of `e'. The `%g' and `%G' conversions print the argument in the style of `%e' or `%E' (respectively) if the exponent would be less than -4 or greater than or equal to the precision; otherwise they use the `%f' style. A precision of `0', is taken as 1. is Trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit. The `%a' and `%A' conversions are meant for representing floating-point numbers exactly in textual form so that they can be exchanged as texts between different programs and/or machines. The numbers are represented is the form [`-']`0x'H`.'HHH`p'[`+'|`-']DD. At the left of the decimal-point character exactly one digit is print. This character is only `0' if the number is denormalized. Otherwise the value is unspecified; it is implementation dependent how many bits are used. The number of hexadecimal digits on the right side of the decimal-point character is equal to the precision. If the precision is zero it is determined to be large enough to provide an exact representation of the number (or it is large enough to distinguish two adjacent values if the `FLT_RADIX' is not a power of 2, *note Floating Point Parameters::). For the `%a' conversion lower-case characters are used to represent the hexadecimal number and the prefix and exponent sign are printed as `0x' and `p' respectively. Otherwise upper-case characters are used and `0X' and `P' are used for the representation of prefix and exponent string. The exponent to the base of two is printed as a decimal number using at least one digit but at most as many digits as necessary to represent the value exactly. If the value to be printed represents infinity or a NaN, the output is [`-']`inf' or `nan' respectively if the conversion specifier is `%a', `%e', `%f', or `%g' and it is [`-']`INF' or `NAN' respectively if the conversion is `%A', `%E', or `%G'. The following flags can be used to modify the behavior: `-' Left-justify the result in the field. Normally the result is right-justified. `+' Always include a plus or minus sign in the result. ` ' If the result doesn't start with a plus or minus sign, prefix it with a space instead. Since the `+' flag ensures that the result includes a sign, this flag is ignored if you supply both of them. `#' Specifies that the result should always include a decimal point, even if no digits follow it. For the `%g' and `%G' conversions, this also forces trailing zeros after the decimal point to be left in place where they would otherwise be removed. `'' Separate the digits of the integer part of the result into groups as specified by the locale specified for the `LC_NUMERIC' category; *note General Numeric::. This flag is a GNU extension. `0' Pad the field with zeros instead of spaces; the zeros are placed after any sign. This flag is ignored if the `-' flag is also specified. The precision specifies how many digits follow the decimal-point character for the `%f', `%e', and `%E' conversions. For these conversions, the default precision is `6'. If the precision is explicitly `0', this suppresses the decimal point character entirely. For the `%g' and `%G' conversions, the precision specifies how many significant digits to print. Significant digits are the first digit before the decimal point, and all the digits after it. If the precision is `0' or not specified for `%g' or `%G', it is treated like a value of `1'. If the value being printed cannot be expressed accurately in the specified number of digits, the value is rounded to the nearest number that fits. Without a type modifier, the floating-point conversions use an argument of type `double'. (By the default argument promotions, any `float' arguments are automatically converted to `double'.) The following type modifier is supported: `L' An uppercase `L' specifies that the argument is a `long double'. Here are some examples showing how numbers print using the various floating-point conversions. All of the numbers were printed using this template string: "|%13.4a|%13.4f|%13.4e|%13.4g|\n" Here is the output: | 0x0.0000p+0| 0.0000| 0.0000e+00| 0| | 0x1.0000p-1| 0.5000| 5.0000e-01| 0.5| | 0x1.0000p+0| 1.0000| 1.0000e+00| 1| | -0x1.0000p+0| -1.0000| -1.0000e+00| -1| | 0x1.9000p+6| 100.0000| 1.0000e+02| 100| | 0x1.f400p+9| 1000.0000| 1.0000e+03| 1000| | 0x1.3880p+13| 10000.0000| 1.0000e+04| 1e+04| | 0x1.81c8p+13| 12345.0000| 1.2345e+04| 1.234e+04| | 0x1.86a0p+16| 100000.0000| 1.0000e+05| 1e+05| | 0x1.e240p+16| 123456.0000| 1.2346e+05| 1.235e+05| Notice how the `%g' conversion drops trailing zeros.  File: libc.info, Node: Other Output Conversions, Next: Formatted Output Functions, Prev: Floating-Point Conversions, Up: Formatted Output Other Output Conversions ------------------------ This section describes miscellaneous conversions for `printf'. The `%c' conversion prints a single character. In case there is no `l' modifier the `int' argument is first converted to an `unsigned char'. Then, if used in a wide stream function, the character is converted into the corresponding wide character. The `-' flag can be used to specify left-justification in the field, but no other flags are defined, and no precision or type modifier can be given. For example: printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o'); prints `hello'. If there is a `l' modifier present the argument is expected to be of type `wint_t'. If used in a multibyte function the wide character is converted into a multibyte character before being added to the output. In this case more than one output byte can be produced. The `%s' conversion prints a string. If no `l' modifier is present the corresponding argument must be of type `char *' (or `const char *'). If used in a wide stream function the string is first converted in a wide character string. A precision can be specified to indicate the maximum number of characters to write; otherwise characters in the string up to but not including the terminating null character are written to the output stream. The `-' flag can be used to specify left-justification in the field, but no other flags or type modifiers are defined for this conversion. For example: printf ("%3s%-6s", "no", "where"); prints ` nowhere '. If there is a `l' modifier present the argument is expected to be of type `wchar_t' (or `const wchar_t *'). If you accidentally pass a null pointer as the argument for a `%s' conversion, the GNU library prints it as `(null)'. We think this is more useful than crashing. But it's not good practice to pass a null argument intentionally. The `%m' conversion prints the string corresponding to the error code in `errno'. *Note Error Messages::. Thus: fprintf (stderr, "can't open `%s': %m\n", filename); is equivalent to: fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno)); The `%m' conversion is a GNU C library extension. The `%p' conversion prints a pointer value. The corresponding argument must be of type `void *'. In practice, you can use any type of pointer. In the GNU system, non-null pointers are printed as unsigned integers, as if a `%#x' conversion were used. Null pointers print as `(nil)'. (Pointers might print differently in other systems.) For example: printf ("%p", "testing"); prints `0x' followed by a hexadecimal number--the address of the string constant `"testing"'. It does not print the word `testing'. You can supply the `-' flag with the `%p' conversion to specify left-justification, but no other flags, precision, or type modifiers are defined. The `%n' conversion is unlike any of the other output conversions. It uses an argument which must be a pointer to an `int', but instead of printing anything it stores the number of characters printed so far by this call at that location. The `h' and `l' type modifiers are permitted to specify that the argument is of type `short int *' or `long int *' instead of `int *', but no flags, field width, or precision are permitted. For example, int nchar; printf ("%d %s%n\n", 3, "bears", &nchar); prints: 3 bears and sets `nchar' to `7', because `3 bears' is seven characters. The `%%' conversion prints a literal `%' character. This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted.  File: libc.info, Node: Formatted Output Functions, Next: Dynamic Output, Prev: Other Output Conversions, Up: Formatted Output Formatted Output Functions -------------------------- This section describes how to call `printf' and related functions. Prototypes for these functions are in the header file `stdio.h'. Because these functions take a variable number of arguments, you _must_ declare prototypes for them before using them. Of course, the easiest way to make sure you have all the right prototypes is to just include `stdio.h'. - Function: int printf (const char *TEMPLATE, ...) The `printf' function prints the optional arguments under the control of the template string TEMPLATE to the stream `stdout'. It returns the number of characters printed, or a negative value if there was an output error. - Function: int wprintf (const wchar_t *TEMPLATE, ...) The `wprintf' function prints the optional arguments under the control of the wide template string TEMPLATE to the stream `stdout'. It returns the number of wide characters printed, or a negative value if there was an output error. - Function: int fprintf (FILE *STREAM, const char *TEMPLATE, ...) This function is just like `printf', except that the output is written to the stream STREAM instead of `stdout'. - Function: int fwprintf (FILE *STREAM, const wchar_t *TEMPLATE, ...) This function is just like `wprintf', except that the output is written to the stream STREAM instead of `stdout'. - Function: int sprintf (char *S, const char *TEMPLATE, ...) This is like `printf', except that the output is stored in the character array S instead of written to a stream. A null character is written to mark the end of the string. The `sprintf' function returns the number of characters stored in the array S, not including the terminating null character. The behavior of this function is undefined if copying takes place between objects that overlap--for example, if S is also given as an argument to be printed under control of the `%s' conversion. *Note Copying and Concatenation::. *Warning:* The `sprintf' function can be *dangerous* because it can potentially output more characters than can fit in the allocation size of the string S. Remember that the field width given in a conversion specification is only a _minimum_ value. To avoid this problem, you can use `snprintf' or `asprintf', described below. - Function: int swprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, ...) This is like `wprintf', except that the output is stored in the wide character array WS instead of written to a stream. A null wide character is written to mark the end of the string. The SIZE argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least SIZE wide characters for the string WS. The return value is the number of characters generated for the given input, excluding the trailing null. If not all output fits into the provided buffer a negative value is returned. You should try again with a bigger output string. _Note:_ this is different from how `snprintf' handles this situation. Note that the corresponding narrow stream function takes fewer parameters. `swprintf' in fact corresponds to the `snprintf' function. Since the `sprintf' function can be dangerous and should be avoided the ISO C committee refused to make the same mistake again and decided to not define an function exactly corresponding to `sprintf'. - Function: int snprintf (char *S, size_t SIZE, const char *TEMPLATE, ...) The `snprintf' function is similar to `sprintf', except that the SIZE argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least SIZE characters for the string S. The return value is the number of characters which would be generated for the given input, excluding the trailing null. If this value is greater or equal to SIZE, not all characters from the result have been stored in S. You should try again with a bigger output string. Here is an example of doing this: /* Construct a message describing the value of a variable whose name is NAME and whose value is VALUE. */ char * make_message (char *name, char *value) { /* Guess we need no more than 100 chars of space. */ int size = 100; char *buffer = (char *) xmalloc (size); int nchars; if (buffer == NULL) return NULL; /* Try to print in the allocated space. */ nchars = snprintf (buffer, size, "value of %s is %s", name, value); if (nchars >= size) { /* Reallocate buffer now that we know how much space is needed. */ buffer = (char *) xrealloc (buffer, nchars + 1); if (buffer != NULL) /* Try again. */ snprintf (buffer, size, "value of %s is %s", name, value); } /* The last call worked, return the string. */ return buffer; } In practice, it is often easier just to use `asprintf', below. *Attention:* In versions of the GNU C library prior to 2.1 the return value is the number of characters stored, not including the terminating null; unless there was not enough space in S to store the result in which case `-1' is returned. This was changed in order to comply with the ISO C99 standard.  File: libc.info, Node: Dynamic Output, Next: Variable Arguments Output, Prev: Formatted Output Functions, Up: Formatted Output Dynamically Allocating Formatted Output --------------------------------------- The functions in this section do formatted output and place the results in dynamically allocated memory. - Function: int asprintf (char **PTR, const char *TEMPLATE, ...) This function is similar to `sprintf', except that it dynamically allocates a string (as with `malloc'; *note Unconstrained Allocation::) to hold the output, instead of putting the output in a buffer you allocate in advance. The PTR argument should be the address of a `char *' object, and `asprintf' stores a pointer to the newly allocated string at that location. The return value is the number of characters allocated for the buffer, or less than zero if an error occurred. Usually this means that the buffer could not be allocated. Here is how to use `asprintf' to get the same result as the `snprintf' example, but more easily: /* Construct a message describing the value of a variable whose name is NAME and whose value is VALUE. */ char * make_message (char *name, char *value) { char *result; if (asprintf (&result, "value of %s is %s", name, value) < 0) return NULL; return result; } - Function: int obstack_printf (struct obstack *OBSTACK, const char *TEMPLATE, ...) This function is similar to `asprintf', except that it uses the obstack OBSTACK to allocate the space. *Note Obstacks::. The characters are written onto the end of the current object. To get at them, you must finish the object with `obstack_finish' (*note Growing Objects::).  File: libc.info, Node: Variable Arguments Output, Next: Parsing a Template String, Prev: Dynamic Output, Up: Formatted Output Variable Arguments Output Functions ----------------------------------- The functions `vprintf' and friends are provided so that you can define your own variadic `printf'-like functions that make use of the same internals as the built-in formatted output functions. The most natural way to define such functions would be to use a language construct to say, "Call `printf' and pass this template plus all of my arguments after the first five." But there is no way to do this in C, and it would be hard to provide a way, since at the C language level there is no way to tell how many arguments your function received. Since that method is impossible, we provide alternative functions, the `vprintf' series, which lets you pass a `va_list' to describe "all of my arguments after the first five." When it is sufficient to define a macro rather than a real function, the GNU C compiler provides a way to do this much more easily with macros. For example: #define myprintf(a, b, c, d, e, rest...) \ printf (mytemplate , ## rest) *Note Macros with Variable Numbers of Arguments: (gcc.info)Macro Varargs, for details. But this is limited to macros, and does not apply to real functions at all. Before calling `vprintf' or the other functions listed in this section, you _must_ call `va_start' (*note Variadic Functions::) to initialize a pointer to the variable arguments. Then you can call `va_arg' to fetch the arguments that you want to handle yourself. This advances the pointer past those arguments. Once your `va_list' pointer is pointing at the argument of your choice, you are ready to call `vprintf'. That argument and all subsequent arguments that were passed to your function are used by `vprintf' along with the template that you specified separately. In some other systems, the `va_list' pointer may become invalid after the call to `vprintf', so you must not use `va_arg' after you call `vprintf'. Instead, you should call `va_end' to retire the pointer from service. However, you can safely call `va_start' on another pointer variable and begin fetching the arguments again through that pointer. Calling `vprintf' does not destroy the argument list of your function, merely the particular pointer that you passed to it. GNU C does not have such restrictions. You can safely continue to fetch arguments from a `va_list' pointer after passing it to `vprintf', and `va_end' is a no-op. (Note, however, that subsequent `va_arg' calls will fetch the same arguments which `vprintf' previously used.) Prototypes for these functions are declared in `stdio.h'. - Function: int vprintf (const char *TEMPLATE, va_list AP) This function is similar to `printf' except that, instead of taking a variable number of arguments directly, it takes an argument list pointer AP. - Function: int vwprintf (const wchar_t *TEMPLATE, va_list AP) This function is similar to `wprintf' except that, instead of taking a variable number of arguments directly, it takes an argument list pointer AP. - Function: int vfprintf (FILE *STREAM, const char *TEMPLATE, va_list AP) This is the equivalent of `fprintf' with the variable argument list specified directly as for `vprintf'. - Function: int vfwprintf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `fwprintf' with the variable argument list specified directly as for `vwprintf'. - Function: int vsprintf (char *S, const char *TEMPLATE, va_list AP) This is the equivalent of `sprintf' with the variable argument list specified directly as for `vprintf'. - Function: int vswprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `swprintf' with the variable argument list specified directly as for `vwprintf'. - Function: int vsnprintf (char *S, size_t SIZE, const char *TEMPLATE, va_list AP) This is the equivalent of `snprintf' with the variable argument list specified directly as for `vprintf'. - Function: int vasprintf (char **PTR, const char *TEMPLATE, va_list AP) The `vasprintf' function is the equivalent of `asprintf' with the variable argument list specified directly as for `vprintf'. - Function: int obstack_vprintf (struct obstack *OBSTACK, const char *TEMPLATE, va_list AP) The `obstack_vprintf' function is the equivalent of `obstack_printf' with the variable argument list specified directly as for `vprintf'. Here's an example showing how you might use `vfprintf'. This is a function that prints error messages to the stream `stderr', along with a prefix indicating the name of the program (*note Error Messages::, for a description of `program_invocation_short_name'). #include #include void eprintf (const char *template, ...) { va_list ap; extern char *program_invocation_short_name; fprintf (stderr, "%s: ", program_invocation_short_name); va_start (ap, template); vfprintf (stderr, template, ap); va_end (ap); } You could call `eprintf' like this: eprintf ("file `%s' does not exist\n", filename); In GNU C, there is a special construct you can use to let the compiler know that a function uses a `printf'-style format string. Then it can check the number and types of arguments in each call to the function, and warn you when they do not match the format string. For example, take this declaration of `eprintf': void eprintf (const char *template, ...) __attribute__ ((format (printf, 1, 2))); This tells the compiler that `eprintf' uses a format string like `printf' (as opposed to `scanf'; *note Formatted Input::); the format string appears as the first argument; and the arguments to satisfy the format begin with the second. *Note Declaring Attributes of Functions: (gcc.info)Function Attributes, for more information.  File: libc.info, Node: Parsing a Template String, Next: Example of Parsing, Prev: Variable Arguments Output, Up: Formatted Output Parsing a Template String ------------------------- You can use the function `parse_printf_format' to obtain information about the number and types of arguments that are expected by a given template string. This function permits interpreters that provide interfaces to `printf' to avoid passing along invalid arguments from the user's program, which could cause a crash. All the symbols described in this section are declared in the header file `printf.h'. - Function: size_t parse_printf_format (const char *TEMPLATE, size_t N, int *ARGTYPES) This function returns information about the number and types of arguments expected by the `printf' template string TEMPLATE. The information is stored in the array ARGTYPES; each element of this array describes one argument. This information is encoded using the various `PA_' macros, listed below. The argument N specifies the number of elements in the array ARGTYPES. This is the maximum number of elements that `parse_printf_format' will try to write. `parse_printf_format' returns the total number of arguments required by TEMPLATE. If this number is greater than N, then the information returned describes only the first N arguments. If you want information about additional arguments, allocate a bigger array and call `parse_printf_format' again. The argument types are encoded as a combination of a basic type and modifier flag bits. - Macro: int PA_FLAG_MASK This macro is a bitmask for the type modifier flag bits. You can write the expression `(argtypes[i] & PA_FLAG_MASK)' to extract just the flag bits for an argument, or `(argtypes[i] & ~PA_FLAG_MASK)' to extract just the basic type code. Here are symbolic constants that represent the basic types; they stand for integer values. `PA_INT' This specifies that the base type is `int'. `PA_CHAR' This specifies that the base type is `int', cast to `char'. `PA_STRING' This specifies that the base type is `char *', a null-terminated string. `PA_POINTER' This specifies that the base type is `void *', an arbitrary pointer. `PA_FLOAT' This specifies that the base type is `float'. `PA_DOUBLE' This specifies that the base type is `double'. `PA_LAST' You can define additional base types for your own programs as offsets from `PA_LAST'. For example, if you have data types `foo' and `bar' with their own specialized `printf' conversions, you could define encodings for these types as: #define PA_FOO PA_LAST #define PA_BAR (PA_LAST + 1) Here are the flag bits that modify a basic type. They are combined with the code for the basic type using inclusive-or. `PA_FLAG_PTR' If this bit is set, it indicates that the encoded type is a pointer to the base type, rather than an immediate value. For example, `PA_INT|PA_FLAG_PTR' represents the type `int *'. `PA_FLAG_SHORT' If this bit is set, it indicates that the base type is modified with `short'. (This corresponds to the `h' type modifier.) `PA_FLAG_LONG' If this bit is set, it indicates that the base type is modified with `long'. (This corresponds to the `l' type modifier.) `PA_FLAG_LONG_LONG' If this bit is set, it indicates that the base type is modified with `long long'. (This corresponds to the `L' type modifier.) `PA_FLAG_LONG_DOUBLE' This is a synonym for `PA_FLAG_LONG_LONG', used by convention with a base type of `PA_DOUBLE' to indicate a type of `long double'. For an example of using these facilities, see *Note Example of Parsing::.  File: libc.info, Node: Example of Parsing, Prev: Parsing a Template String, Up: Formatted Output Example of Parsing a Template String ------------------------------------ Here is an example of decoding argument types for a format string. We assume this is part of an interpreter which contains arguments of type `NUMBER', `CHAR', `STRING' and `STRUCTURE' (and perhaps others which are not valid here). /* Test whether the NARGS specified objects in the vector ARGS are valid for the format string FORMAT: if so, return 1. If not, return 0 after printing an error message. */ int validate_args (char *format, int nargs, OBJECT *args) { int *argtypes; int nwanted; /* Get the information about the arguments. Each conversion specification must be at least two characters long, so there cannot be more specifications than half the length of the string. */ argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int)); nwanted = parse_printf_format (string, nelts, argtypes); /* Check the number of arguments. */ if (nwanted > nargs) { error ("too few arguments (at least %d required)", nwanted); return 0; } /* Check the C type wanted for each argument and see if the object given is suitable. */ for (i = 0; i < nwanted; i++) { int wanted; if (argtypes[i] & PA_FLAG_PTR) wanted = STRUCTURE; else switch (argtypes[i] & ~PA_FLAG_MASK) { case PA_INT: case PA_FLOAT: case PA_DOUBLE: wanted = NUMBER; break; case PA_CHAR: wanted = CHAR; break; case PA_STRING: wanted = STRING; break; case PA_POINTER: wanted = STRUCTURE; break; } if (TYPE (args[i]) != wanted) { error ("type mismatch for arg number %d", i); return 0; } } return 1; }  File: libc.info, Node: Customizing Printf, Next: Formatted Input, Prev: Formatted Output, Up: I/O on Streams Customizing `printf' ==================== The GNU C library lets you define your own custom conversion specifiers for `printf' template strings, to teach `printf' clever ways to print the important data structures of your program. The way you do this is by registering the conversion with the function `register_printf_function'; see *Note Registering New Conversions::. One of the arguments you pass to this function is a pointer to a handler function that produces the actual output; see *Note Defining the Output Handler::, for information on how to write this function. You can also install a function that just returns information about the number and type of arguments expected by the conversion specifier. *Note Parsing a Template String::, for information about this. The facilities of this section are declared in the header file `printf.h'. * Menu: * Registering New Conversions:: Using `register_printf_function' to register a new output conversion. * Conversion Specifier Options:: The handler must be able to get the options specified in the template when it is called. * Defining the Output Handler:: Defining the handler and arginfo functions that are passed as arguments to `register_printf_function'. * Printf Extension Example:: How to define a `printf' handler function. * Predefined Printf Handlers:: Predefined `printf' handlers. *Portability Note:* The ability to extend the syntax of `printf' template strings is a GNU extension. ISO standard C has nothing similar.  File: libc.info, Node: Registering New Conversions, Next: Conversion Specifier Options, Up: Customizing Printf Registering New Conversions --------------------------- The function to register a new output conversion is `register_printf_function', declared in `printf.h'. - Function: int register_printf_function (int SPEC, printf_function HANDLER-FUNCTION, printf_arginfo_function ARGINFO-FUNCTION) This function defines the conversion specifier character SPEC. Thus, if SPEC is `'Y'', it defines the conversion `%Y'. You can redefine the built-in conversions like `%s', but flag characters like `#' and type modifiers like `l' can never be used as conversions; calling `register_printf_function' for those characters has no effect. It is advisable not to use lowercase letters, since the ISO C standard warns that additional lowercase letters may be standardized in future editions of the standard. The HANDLER-FUNCTION is the function called by `printf' and friends when this conversion appears in a template string. *Note Defining the Output Handler::, for information about how to define a function to pass as this argument. If you specify a null pointer, any existing handler function for SPEC is removed. The ARGINFO-FUNCTION is the function called by `parse_printf_format' when this conversion appears in a template string. *Note Parsing a Template String::, for information about this. *Attention:* In the GNU C library versions before 2.0 the ARGINFO-FUNCTION function did not need to be installed unless the user used the `parse_printf_format' function. This has changed. Now a call to any of the `printf' functions will call this function when this format specifier appears in the format string. The return value is `0' on success, and `-1' on failure (which occurs if SPEC is out of range). You can redefine the standard output conversions, but this is probably not a good idea because of the potential for confusion. Library routines written by other people could break if you do this.  File: libc.info, Node: Conversion Specifier Options, Next: Defining the Output Handler, Prev: Registering New Conversions, Up: Customizing Printf Conversion Specifier Options ---------------------------- If you define a meaning for `%A', what if the template contains `%+23A' or `%-#A'? To implement a sensible meaning for these, the handler when called needs to be able to get the options specified in the template. Both the HANDLER-FUNCTION and ARGINFO-FUNCTION accept an argument that points to a `struct printf_info', which contains information about the options appearing in an instance of the conversion specifier. This data type is declared in the header file `printf.h'. - Type: struct printf_info This structure is used to pass information about the options appearing in an instance of a conversion specifier in a `printf' template string to the handler and arginfo functions for that specifier. It contains the following members: `int prec' This is the precision specified. The value is `-1' if no precision was specified. If the precision was given as `*', the `printf_info' structure passed to the handler function contains the actual value retrieved from the argument list. But the structure passed to the arginfo function contains a value of `INT_MIN', since the actual value is not known. `int width' This is the minimum field width specified. The value is `0' if no width was specified. If the field width was given as `*', the `printf_info' structure passed to the handler function contains the actual value retrieved from the argument list. But the structure passed to the arginfo function contains a value of `INT_MIN', since the actual value is not known. `wchar_t spec' This is the conversion specifier character specified. It's stored in the structure so that you can register the same handler function for multiple characters, but still have a way to tell them apart when the handler function is called. `unsigned int is_long_double' This is a boolean that is true if the `L', `ll', or `q' type modifier was specified. For integer conversions, this indicates `long long int', as opposed to `long double' for floating point conversions. `unsigned int is_char' This is a boolean that is true if the `hh' type modifier was specified. `unsigned int is_short' This is a boolean that is true if the `h' type modifier was specified. `unsigned int is_long' This is a boolean that is true if the `l' type modifier was specified. `unsigned int alt' This is a boolean that is true if the `#' flag was specified. `unsigned int space' This is a boolean that is true if the ` ' flag was specified. `unsigned int left' This is a boolean that is true if the `-' flag was specified. `unsigned int showsign' This is a boolean that is true if the `+' flag was specified. `unsigned int group' This is a boolean that is true if the `'' flag was specified. `unsigned int extra' This flag has a special meaning depending on the context. It could be used freely by the user-defined handlers but when called from the `printf' function this variable always contains the value `0'. `unsigned int wide' This flag is set if the stream is wide oriented. `wchar_t pad' This is the character to use for padding the output to the minimum field width. The value is `'0'' if the `0' flag was specified, and `' '' otherwise.  File: libc.info, Node: Defining the Output Handler, Next: Printf Extension Example, Prev: Conversion Specifier Options, Up: Customizing Printf Defining the Output Handler --------------------------- Now let's look at how to define the handler and arginfo functions which are passed as arguments to `register_printf_function'. *Compatibility Note:* The interface changed in GNU libc version 2.0. Previously the third argument was of type `va_list *'. You should define your handler functions with a prototype like: int FUNCTION (FILE *stream, const struct printf_info *info, const void *const *args) The STREAM argument passed to the handler function is the stream to which it should write output. The INFO argument is a pointer to a structure that contains information about the various options that were included with the conversion in the template string. You should not modify this structure inside your handler function. *Note Conversion Specifier Options::, for a description of this data structure. The ARGS is a vector of pointers to the arguments data. The number of arguments was determined by calling the argument information function provided by the user. Your handler function should return a value just like `printf' does: it should return the number of characters it has written, or a negative value to indicate an error. - Data Type: printf_function This is the data type that a handler function should have. If you are going to use `parse_printf_format' in your application, you must also define a function to pass as the ARGINFO-FUNCTION argument for each new conversion you install with `register_printf_function'. You have to define these functions with a prototype like: int FUNCTION (const struct printf_info *info, size_t n, int *argtypes) The return value from the function should be the number of arguments the conversion expects. The function should also fill in no more than N elements of the ARGTYPES array with information about the types of each of these arguments. This information is encoded using the various `PA_' macros. (You will notice that this is the same calling convention `parse_printf_format' itself uses.) - Data Type: printf_arginfo_function This type is used to describe functions that return information about the number and type of arguments used by a conversion specifier.  File: libc.info, Node: Printf Extension Example, Next: Predefined Printf Handlers, Prev: Defining the Output Handler, Up: Customizing Printf `printf' Extension Example -------------------------- Here is an example showing how to define a `printf' handler function. This program defines a data structure called a `Widget' and defines the `%W' conversion to print information about `Widget *' arguments, including the pointer value and the name stored in the data structure. The `%W' conversion supports the minimum field width and left-justification options, but ignores everything else. #include #include #include typedef struct { char *name; } Widget; int print_widget (FILE *stream, const struct printf_info *info, const void *const *args) { const Widget *w; char *buffer; int len; /* Format the output into a string. */ w = *((const Widget **) (args[0])); len = asprintf (&buffer, "", w, w->name); if (len == -1) return -1; /* Pad to the minimum field width and print to the stream. */ len = fprintf (stream, "%*s", (info->left ? -info->width : info->width), buffer); /* Clean up and return. */ free (buffer); return len; } int print_widget_arginfo (const struct printf_info *info, size_t n, int *argtypes) { /* We always take exactly one argument and this is a pointer to the structure.. */ if (n > 0) argtypes[0] = PA_POINTER; return 1; } int main (void) { /* Make a widget to print. */ Widget mywidget; mywidget.name = "mywidget"; /* Register the print function for widgets. */ register_printf_function ('W', print_widget, print_widget_arginfo); /* Now print the widget. */ printf ("|%W|\n", &mywidget); printf ("|%35W|\n", &mywidget); printf ("|%-35W|\n", &mywidget); return 0; } The output produced by this program looks like: || | | | |  File: libc.info, Node: Predefined Printf Handlers, Prev: Printf Extension Example, Up: Customizing Printf Predefined `printf' Handlers ---------------------------- The GNU libc also contains a concrete and useful application of the `printf' handler extension. There are two functions available which implement a special way to print floating-point numbers. - Function: int printf_size (FILE *FP, const struct printf_info *INFO, const void *const *ARGS) Print a given floating point number as for the format `%f' except that there is a postfix character indicating the divisor for the number to make this less than 1000. There are two possible divisors: powers of 1024 or powers of 1000. Which one is used depends on the format character specified while registered this handler. If the character is of lower case, 1024 is used. For upper case characters, 1000 is used. The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, etc. The full table is: +------+--------------+--------+--------+---------------+ |low|Multiplier|From|Upper|Multiplier| +------+--------------+--------+--------+---------------+ |' '|1||' '|1| +------+--------------+--------+--------+---------------+ |k|2^10 (1024)|kilo|K|10^3 (1000)| +------+--------------+--------+--------+---------------+ |m|2^20|mega|M|10^6| +------+--------------+--------+--------+---------------+ |g|2^30|giga|G|10^9| +------+--------------+--------+--------+---------------+ |t|2^40|tera|T|10^12| +------+--------------+--------+--------+---------------+ |p|2^50|peta|P|10^15| +------+--------------+--------+--------+---------------+ |e|2^60|exa|E|10^18| +------+--------------+--------+--------+---------------+ |z|2^70|zetta|Z|10^21| +------+--------------+--------+--------+---------------+ |y|2^80|yotta|Y|10^24| +------+--------------+--------+--------+---------------+ The default precision is 3, i.e., 1024 is printed with a lower-case format character as if it were `%.3fk' and will yield `1.000k'. Due to the requirements of `register_printf_function' we must also provide the function which returns information about the arguments. - Function: int printf_size_info (const struct printf_info *INFO, size_t N, int *ARGTYPES) This function will return in ARGTYPES the information about the used parameters in the way the `vfprintf' implementation expects it. The format always takes one argument. To use these functions both functions must be registered with a call like register_printf_function ('B', printf_size, printf_size_info); Here we register the functions to print numbers as powers of 1000 since the format character `'B'' is an upper-case character. If we would additionally use `'b'' in a line like register_printf_function ('b', printf_size, printf_size_info); we could also print using a power of 1024. Please note that all that is different in these two lines is the format specifier. The `printf_size' function knows about the difference between lower and upper case format specifiers. The use of `'B'' and `'b'' is no coincidence. Rather it is the preferred way to use this functionality since it is available on some other systems which also use format specifiers.  File: libc.info, Node: Formatted Input, Next: EOF and Errors, Prev: Customizing Printf, Up: I/O on Streams Formatted Input =============== The functions described in this section (`scanf' and related functions) provide facilities for formatted input analogous to the formatted output facilities. These functions provide a mechanism for reading arbitrary values under the control of a "format string" or "template string". * Menu: * Formatted Input Basics:: Some basics to get you started. * Input Conversion Syntax:: Syntax of conversion specifications. * Table of Input Conversions:: Summary of input conversions and what they do. * Numeric Input Conversions:: Details of conversions for reading numbers. * String Input Conversions:: Details of conversions for reading strings. * Dynamic String Input:: String conversions that `malloc' the buffer. * Other Input Conversions:: Details of miscellaneous other conversions. * Formatted Input Functions:: Descriptions of the actual functions. * Variable Arguments Input:: `vscanf' and friends.  File: libc.info, Node: Formatted Input Basics, Next: Input Conversion Syntax, Up: Formatted Input Formatted Input Basics ---------------------- Calls to `scanf' are superficially similar to calls to `printf' in that arbitrary arguments are read under the control of a template string. While the syntax of the conversion specifications in the template is very similar to that for `printf', the interpretation of the template is oriented more towards free-format input and simple pattern matching, rather than fixed-field formatting. For example, most `scanf' conversions skip over any amount of "white space" (including spaces, tabs, and newlines) in the input file, and there is no concept of precision for the numeric input conversions as there is for the corresponding output conversions. Ordinarily, non-whitespace characters in the template are expected to match characters in the input stream exactly, but a matching failure is distinct from an input error on the stream. Another area of difference between `scanf' and `printf' is that you must remember to supply pointers rather than immediate values as the optional arguments to `scanf'; the values that are read are stored in the objects that the pointers point to. Even experienced programmers tend to forget this occasionally, so if your program is getting strange errors that seem to be related to `scanf', you might want to double-check this. When a "matching failure" occurs, `scanf' returns immediately, leaving the first non-matching character as the next character to be read from the stream. The normal return value from `scanf' is the number of values that were assigned, so you can use this to determine if a matching error happened before all the expected values were read. The `scanf' function is typically used for things like reading in the contents of tables. For example, here is a function that uses `scanf' to initialize an array of `double': void readarray (double *array, int n) { int i; for (i=0; i scanf ("%a[a-zA-Z0-9] = %a[^\n]\n", &variable, &value)) { invalid_input_error (); return 0; } ... }  File: libc.info, Node: Other Input Conversions, Next: Formatted Input Functions, Prev: Dynamic String Input, Up: Formatted Input Other Input Conversions ----------------------- This section describes the miscellaneous input conversions. The `%p' conversion is used to read a pointer value. It recognizes the same syntax used by the `%p' output conversion for `printf' (*note Other Output Conversions::); that is, a hexadecimal number just as the `%x' conversion accepts. The corresponding argument should be of type `void **'; that is, the address of a place to store a pointer. The resulting pointer value is not guaranteed to be valid if it was not originally written during the same program execution that reads it in. The `%n' conversion produces the number of characters read so far by this call. The corresponding argument should be of type `int *'. This conversion works in the same way as the `%n' conversion for `printf'; see *Note Other Output Conversions::, for an example. The `%n' conversion is the only mechanism for determining the success of literal matches or conversions with suppressed assignments. If the `%n' follows the locus of a matching failure, then no value is stored for it since `scanf' returns before processing the `%n'. If you store `-1' in that argument slot before calling `scanf', the presence of `-1' after `scanf' indicates an error occurred before the `%n' was reached. Finally, the `%%' conversion matches a literal `%' character in the input stream, without using an argument. This conversion does not permit any flags, field width, or type modifier to be specified.  File: libc.info, Node: Formatted Input Functions, Next: Variable Arguments Input, Prev: Other Input Conversions, Up: Formatted Input Formatted Input Functions ------------------------- Here are the descriptions of the functions for performing formatted input. Prototypes for these functions are in the header file `stdio.h'. - Function: int scanf (const char *TEMPLATE, ...) The `scanf' function reads formatted input from the stream `stdin' under the control of the template string TEMPLATE. The optional arguments are pointers to the places which receive the resulting values. The return value is normally the number of successful assignments. If an end-of-file condition is detected before any matches are performed, including matches against whitespace and literal characters in the template, then `EOF' is returned. - Function: int wscanf (const wchar_t *TEMPLATE, ...) The `wscanf' function reads formatted input from the stream `stdin' under the control of the template string TEMPLATE. The optional arguments are pointers to the places which receive the resulting values. The return value is normally the number of successful assignments. If an end-of-file condition is detected before any matches are performed, including matches against whitespace and literal characters in the template, then `WEOF' is returned. - Function: int fscanf (FILE *STREAM, const char *TEMPLATE, ...) This function is just like `scanf', except that the input is read from the stream STREAM instead of `stdin'. - Function: int fwscanf (FILE *STREAM, const wchar_t *TEMPLATE, ...) This function is just like `wscanf', except that the input is read from the stream STREAM instead of `stdin'. - Function: int sscanf (const char *S, const char *TEMPLATE, ...) This is like `scanf', except that the characters are taken from the null-terminated string S instead of from a stream. Reaching the end of the string is treated as an end-of-file condition. The behavior of this function is undefined if copying takes place between objects that overlap--for example, if S is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion. - Function: int swscanf (const wchar_t *WS, const char *TEMPLATE, ...) This is like `wscanf', except that the characters are taken from the null-terminated string WS instead of from a stream. Reaching the end of the string is treated as an end-of-file condition. The behavior of this function is undefined if copying takes place between objects that overlap--for example, if WS is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion.  File: libc.info, Node: Variable Arguments Input, Prev: Formatted Input Functions, Up: Formatted Input Variable Arguments Input Functions ---------------------------------- The functions `vscanf' and friends are provided so that you can define your own variadic `scanf'-like functions that make use of the same internals as the built-in formatted output functions. These functions are analogous to the `vprintf' series of output functions. *Note Variable Arguments Output::, for important information on how to use them. *Portability Note:* The functions listed in this section were introduced in ISO C99 and were before available as GNU extensions. - Function: int vscanf (const char *TEMPLATE, va_list AP) This function is similar to `scanf', but instead of taking a variable number of arguments directly, it takes an argument list pointer AP of type `va_list' (*note Variadic Functions::). - Function: int vwscanf (const wchar_t *TEMPLATE, va_list AP) This function is similar to `wscanf', but instead of taking a variable number of arguments directly, it takes an argument list pointer AP of type `va_list' (*note Variadic Functions::). - Function: int vfscanf (FILE *STREAM, const char *TEMPLATE, va_list AP) This is the equivalent of `fscanf' with the variable argument list specified directly as for `vscanf'. - Function: int vfwscanf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `fwscanf' with the variable argument list specified directly as for `vwscanf'. - Function: int vsscanf (const char *S, const char *TEMPLATE, va_list AP) This is the equivalent of `sscanf' with the variable argument list specified directly as for `vscanf'. - Function: int vswscanf (const wchar_t *S, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `swscanf' with the variable argument list specified directly as for `vwscanf'. In GNU C, there is a special construct you can use to let the compiler know that a function uses a `scanf'-style format string. Then it can check the number and types of arguments in each call to the function, and warn you when they do not match the format string. For details, *Note Declaring Attributes of Functions: (gcc.info)Function Attributes.  File: libc.info, Node: EOF and Errors, Next: Error Recovery, Prev: Formatted Input, Up: I/O on Streams End-Of-File and Errors ====================== Many of the functions described in this chapter return the value of the macro `EOF' to indicate unsuccessful completion of the operation. Since `EOF' is used to report both end of file and random errors, it's often better to use the `feof' function to check explicitly for end of file and `ferror' to check for errors. These functions check indicators that are part of the internal state of the stream object, indicators set if the appropriate condition was detected by a previous I/O operation on that stream. - Macro: int EOF This macro is an integer value that is returned by a number of narrow stream functions to indicate an end-of-file condition, or some other error situation. With the GNU library, `EOF' is `-1'. In other libraries, its value may be some other negative number. This symbol is declared in `stdio.h'. - Macro: int WEOF This macro is an integer value that is returned by a number of wide stream functions to indicate an end-of-file condition, or some other error situation. With the GNU library, `WEOF' is `-1'. In other libraries, its value may be some other negative number. This symbol is declared in `wchar.h'. - Function: int feof (FILE *STREAM) The `feof' function returns nonzero if and only if the end-of-file indicator for the stream STREAM is set. This symbol is declared in `stdio.h'. - Function: int feof_unlocked (FILE *STREAM) The `feof_unlocked' function is equivalent to the `feof' function except that it does not implicitly lock the stream. This function is a GNU extension. This symbol is declared in `stdio.h'. - Function: int ferror (FILE *STREAM) The `ferror' function returns nonzero if and only if the error indicator for the stream STREAM is set, indicating that an error has occurred on a previous operation on the stream. This symbol is declared in `stdio.h'. - Function: int ferror_unlocked (FILE *STREAM) The `ferror_unlocked' function is equivalent to the `ferror' function except that it does not implicitly lock the stream. This function is a GNU extension. This symbol is declared in `stdio.h'. In addition to setting the error indicator associated with the stream, the functions that operate on streams also set `errno' in the same way as the corresponding low-level functions that operate on file descriptors. For example, all of the functions that perform output to a stream--such as `fputc', `printf', and `fflush'--are implemented in terms of `write', and all of the `errno' error conditions defined for `write' are meaningful for these functions. For more information about the descriptor-level I/O functions, see *Note Low-Level I/O::.  File: libc.info, Node: Error Recovery, Next: Binary Streams, Prev: EOF and Errors, Up: I/O on Streams Recovering from errors ====================== You may explicitly clear the error and EOF flags with the `clearerr' function. - Function: void clearerr (FILE *STREAM) This function clears the end-of-file and error indicators for the stream STREAM. The file positioning functions (*note File Positioning::) also clear the end-of-file indicator for the stream. - Function: void clearerr_unlocked (FILE *STREAM) The `clearerr_unlocked' function is equivalent to the `clearerr' function except that it does not implicitly lock the stream. This function is a GNU extension. Note that it is _not_ correct to just clear the error flag and retry a failed stream operation. After a failed write, any number of characters since the last buffer flush may have been committed to the file, while some buffered data may have been discarded. Merely retrying can thus cause lost or repeated data. A failed read may leave the file pointer in an inappropriate position for a second try. In both cases, you should seek to a known position before retrying. Most errors that can happen are not recoverable -- a second try will always fail again in the same way. So usually it is best to give up and report the error to the user, rather than install complicated recovery logic. One important exception is `EINTR' (*note Interrupted Primitives::). Many stream I/O implementations will treat it as an ordinary error, which can be quite inconvenient. You can avoid this hassle by installing all signals with the `SA_RESTART' flag. For similar reasons, setting nonblocking I/O on a stream's file descriptor is not usually advisable.  File: libc.info, Node: Binary Streams, Next: File Positioning, Prev: Error Recovery, Up: I/O on Streams Text and Binary Streams ======================= The GNU system and other POSIX-compatible operating systems organize all files as uniform sequences of characters. However, some other systems make a distinction between files containing text and files containing binary data, and the input and output facilities of ISO C provide for this distinction. This section tells you how to write programs portable to such systems. When you open a stream, you can specify either a "text stream" or a "binary stream". You indicate that you want a binary stream by specifying the `b' modifier in the OPENTYPE argument to `fopen'; see *Note Opening Streams::. Without this option, `fopen' opens the file as a text stream. Text and binary streams differ in several ways: * The data read from a text stream is divided into "lines" which are terminated by newline (`'\n'') characters, while a binary stream is simply a long series of characters. A text stream might on some systems fail to handle lines more than 254 characters long (including the terminating newline character). * On some systems, text files can contain only printing characters, horizontal tab characters, and newlines, and so text streams may not support other characters. However, binary streams can handle any character value. * Space characters that are written immediately preceding a newline character in a text stream may disappear when the file is read in again. * More generally, there need not be a one-to-one mapping between characters that are read from or written to a text stream, and the characters in the actual file. Since a binary stream is always more capable and more predictable than a text stream, you might wonder what purpose text streams serve. Why not simply always use binary streams? The answer is that on these operating systems, text and binary streams use different file formats, and the only way to read or write "an ordinary file of text" that can work with other text-oriented programs is through a text stream. In the GNU library, and on all POSIX systems, there is no difference between text streams and binary streams. When you open a stream, you get the same kind of stream regardless of whether you ask for binary. This stream can handle any file content, and has none of the restrictions that text streams sometimes have.  File: libc.info, Node: File Positioning, Next: Portable Positioning, Prev: Binary Streams, Up: I/O on Streams File Positioning ================ The "file position" of a stream describes where in the file the stream is currently reading or writing. I/O on the stream advances the file position through the file. In the GNU system, the file position is represented as an integer, which counts the number of bytes from the beginning of the file. *Note File Position::. During I/O to an ordinary disk file, you can change the file position whenever you wish, so as to read or write any portion of the file. Some other kinds of files may also permit this. Files which support changing the file position are sometimes referred to as "random-access" files. You can use the functions in this section to examine or modify the file position indicator associated with a stream. The symbols listed below are declared in the header file `stdio.h'. - Function: long int ftell (FILE *STREAM) This function returns the current file position of the stream STREAM. This function can fail if the stream doesn't support file positioning, or if the file position can't be represented in a `long int', and possibly for other reasons as well. If a failure occurs, a value of `-1' is returned. - Function: off_t ftello (FILE *STREAM) The `ftello' function is similar to `ftell', except that it returns a value of type `off_t'. Systems which support this type use it to describe all file positions, unlike the POSIX specification which uses a long int. The two are not necessarily the same size. Therefore, using ftell can lead to problems if the implementation is written on top of a POSIX compliant low-level I/O implementation, and using `ftello' is preferable whenever it is available. If this function fails it returns `(off_t) -1'. This can happen due to missing support for file positioning or internal errors. Otherwise the return value is the current file position. The function is an extension defined in the Unix Single Specification version 2. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system this function is in fact `ftello64'. I.e., the LFS interface transparently replaces the old interface. - Function: off64_t ftello64 (FILE *STREAM) This function is similar to `ftello' with the only difference that the return value is of type `off64_t'. This also requires that the stream STREAM was opened using either `fopen64', `freopen64', or `tmpfile64' since otherwise the underlying file operations to position the file pointer beyond the 2^31 bytes limit might fail. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `ftello' and so transparently replaces the old interface. - Function: int fseek (FILE *STREAM, long int OFFSET, int WHENCE) The `fseek' function is used to change the file position of the stream STREAM. The value of WHENCE must be one of the constants `SEEK_SET', `SEEK_CUR', or `SEEK_END', to indicate whether the OFFSET is relative to the beginning of the file, the current file position, or the end of the file, respectively. This function returns a value of zero if the operation was successful, and a nonzero value to indicate failure. A successful call also clears the end-of-file indicator of STREAM and discards any characters that were "pushed back" by the use of `ungetc'. `fseek' either flushes any buffered output before setting the file position or else remembers it so it will be written later in its proper place in the file. - Function: int fseeko (FILE *STREAM, off_t OFFSET, int WHENCE) This function is similar to `fseek' but it corrects a problem with `fseek' in a system with POSIX types. Using a value of type `long int' for the offset is not compatible with POSIX. `fseeko' uses the correct type `off_t' for the OFFSET parameter. For this reason it is a good idea to prefer `ftello' whenever it is available since its functionality is (if different at all) closer the underlying definition. The functionality and return value is the same as for `fseek'. The function is an extension defined in the Unix Single Specification version 2. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system this function is in fact `fseeko64'. I.e., the LFS interface transparently replaces the old interface. - Function: int fseeko64 (FILE *STREAM, off64_t OFFSET, int WHENCE) This function is similar to `fseeko' with the only difference that the OFFSET parameter is of type `off64_t'. This also requires that the stream STREAM was opened using either `fopen64', `freopen64', or `tmpfile64' since otherwise the underlying file operations to position the file pointer beyond the 2^31 bytes limit might fail. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fseeko' and so transparently replaces the old interface. *Portability Note:* In non-POSIX systems, `ftell', `ftello', `fseek' and `fseeko' might work reliably only on binary streams. *Note Binary Streams::. The following symbolic constants are defined for use as the WHENCE argument to `fseek'. They are also used with the `lseek' function (*note I/O Primitives::) and to specify offsets for file locks (*note Control Operations::). - Macro: int SEEK_SET This is an integer constant which, when used as the WHENCE argument to the `fseek' or `fseeko' function, specifies that the offset provided is relative to the beginning of the file. - Macro: int SEEK_CUR This is an integer constant which, when used as the WHENCE argument to the `fseek' or `fseeko' function, specifies that the offset provided is relative to the current file position. - Macro: int SEEK_END This is an integer constant which, when used as the WHENCE argument to the `fseek' or `fseeko' function, specifies that the offset provided is relative to the end of the file. - Function: void rewind (FILE *STREAM) The `rewind' function positions the stream STREAM at the beginning of the file. It is equivalent to calling `fseek' or `fseeko' on the STREAM with an OFFSET argument of `0L' and a WHENCE argument of `SEEK_SET', except that the return value is discarded and the error indicator for the stream is reset. These three aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'. `L_SET' An alias for `SEEK_SET'. `L_INCR' An alias for `SEEK_CUR'. `L_XTND' An alias for `SEEK_END'.  File: libc.info, Node: Portable Positioning, Next: Stream Buffering, Prev: File Positioning, Up: I/O on Streams Portable File-Position Functions ================================ On the GNU system, the file position is truly a character count. You can specify any character count value as an argument to `fseek' or `fseeko' and get reliable results for any random access file. However, some ISO C systems do not represent file positions in this way. On some systems where text streams truly differ from binary streams, it is impossible to represent the file position of a text stream as a count of characters from the beginning of the file. For example, the file position on some systems must encode both a record offset within the file, and a character offset within the record. As a consequence, if you want your programs to be portable to these systems, you must observe certain rules: * The value returned from `ftell' on a text stream has no predictable relationship to the number of characters you have read so far. The only thing you can rely on is that you can use it subsequently as the OFFSET argument to `fseek' or `fseeko' to move back to the same file position. * In a call to `fseek' or `fseeko' on a text stream, either the OFFSET must be zero, or WHENCE must be `SEEK_SET' and and the OFFSET must be the result of an earlier call to `ftell' on the same stream. * The value of the file position indicator of a text stream is undefined while there are characters that have been pushed back with `ungetc' that haven't been read or discarded. *Note Unreading::. But even if you observe these rules, you may still have trouble for long files, because `ftell' and `fseek' use a `long int' value to represent the file position. This type may not have room to encode all the file positions in a large file. Using the `ftello' and `fseeko' functions might help here since the `off_t' type is expected to be able to hold all file position values but this still does not help to handle additional information which must be associated with a file position. So if you do want to support systems with peculiar encodings for the file positions, it is better to use the functions `fgetpos' and `fsetpos' instead. These functions represent the file position using the data type `fpos_t', whose internal representation varies from system to system. These symbols are declared in the header file `stdio.h'. - Data Type: fpos_t This is the type of an object that can encode information about the file position of a stream, for use by the functions `fgetpos' and `fsetpos'. In the GNU system, `fpos_t' is an opaque data structure that contains internal data to represent file offset and conversion state information. In other systems, it might have a different internal representation. When compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this type is in fact equivalent to `fpos64_t' since the LFS interface transparently replaces the old interface. - Data Type: fpos64_t This is the type of an object that can encode information about the file position of a stream, for use by the functions `fgetpos64' and `fsetpos64'. In the GNU system, `fpos64_t' is an opaque data structure that contains internal data to represent file offset and conversion state information. In other systems, it might have a different internal representation. - Function: int fgetpos (FILE *STREAM, fpos_t *POSITION) This function stores the value of the file position indicator for the stream STREAM in the `fpos_t' object pointed to by POSITION. If successful, `fgetpos' returns zero; otherwise it returns a nonzero value and stores an implementation-defined positive value in `errno'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system the function is in fact `fgetpos64'. I.e., the LFS interface transparently replaces the old interface. - Function: int fgetpos64 (FILE *STREAM, fpos64_t *POSITION) This function is similar to `fgetpos' but the file position is returned in a variable of type `fpos64_t' to which POSITION points. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fgetpos' and so transparently replaces the old interface. - Function: int fsetpos (FILE *STREAM, const fpos_t *POSITION) This function sets the file position indicator for the stream STREAM to the position POSITION, which must have been set by a previous call to `fgetpos' on the same stream. If successful, `fsetpos' clears the end-of-file indicator on the stream, discards any characters that were "pushed back" by the use of `ungetc', and returns a value of zero. Otherwise, `fsetpos' returns a nonzero value and stores an implementation-defined positive value in `errno'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system the function is in fact `fsetpos64'. I.e., the LFS interface transparently replaces the old interface. - Function: int fsetpos64 (FILE *STREAM, const fpos64_t *POSITION) This function is similar to `fsetpos' but the file position used for positioning is provided in a variable of type `fpos64_t' to which POSITION points. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fsetpos' and so transparently replaces the old interface.  File: libc.info, Node: Stream Buffering, Next: Other Kinds of Streams, Prev: Portable Positioning, Up: I/O on Streams Stream Buffering ================ Characters that are written to a stream are normally accumulated and transmitted asynchronously to the file in a block, instead of appearing as soon as they are output by the application program. Similarly, streams often retrieve input from the host environment in blocks rather than on a character-by-character basis. This is called "buffering". If you are writing programs that do interactive input and output using streams, you need to understand how buffering works when you design the user interface to your program. Otherwise, you might find that output (such as progress or prompt messages) doesn't appear when you intended it to, or displays some other unexpected behavior. This section deals only with controlling when characters are transmitted between the stream and the file or device, and _not_ with how things like echoing, flow control, and the like are handled on specific classes of devices. For information on common control operations on terminal devices, see *Note Low-Level Terminal Interface::. You can bypass the stream buffering facilities altogether by using the low-level input and output functions that operate on file descriptors instead. *Note Low-Level I/O::. * Menu: * Buffering Concepts:: Terminology is defined here. * Flushing Buffers:: How to ensure that output buffers are flushed. * Controlling Buffering:: How to specify what kind of buffering to use.  File: libc.info, Node: Buffering Concepts, Next: Flushing Buffers, Up: Stream Buffering Buffering Concepts ------------------ There are three different kinds of buffering strategies: * Characters written to or read from an "unbuffered" stream are transmitted individually to or from the file as soon as possible. * Characters written to a "line buffered" stream are transmitted to the file in blocks when a newline character is encountered. * Characters written to or read from a "fully buffered" stream are transmitted to or from the file in blocks of arbitrary size. Newly opened streams are normally fully buffered, with one exception: a stream connected to an interactive device such as a terminal is initially line buffered. *Note Controlling Buffering::, for information on how to select a different kind of buffering. Usually the automatic selection gives you the most convenient kind of buffering for the file or device you open. The use of line buffering for interactive devices implies that output messages ending in a newline will appear immediately--which is usually what you want. Output that doesn't end in a newline might or might not show up immediately, so if you want them to appear immediately, you should flush buffered output explicitly with `fflush', as described in *Note Flushing Buffers::.  File: libc.info, Node: Flushing Buffers, Next: Controlling Buffering, Prev: Buffering Concepts, Up: Stream Buffering Flushing Buffers ---------------- "Flushing" output on a buffered stream means transmitting all accumulated characters to the file. There are many circumstances when buffered output on a stream is flushed automatically: * When you try to do output and the output buffer is full. * When the stream is closed. *Note Closing Streams::. * When the program terminates by calling `exit'. *Note Normal Termination::. * When a newline is written, if the stream is line buffered. * Whenever an input operation on _any_ stream actually reads data from its file. If you want to flush the buffered output at another time, call `fflush', which is declared in the header file `stdio.h'. - Function: int fflush (FILE *STREAM) This function causes any buffered output on STREAM to be delivered to the file. If STREAM is a null pointer, then `fflush' causes buffered output on _all_ open output streams to be flushed. This function returns `EOF' if a write error occurs, or zero otherwise. - Function: int fflush_unlocked (FILE *STREAM) The `fflush_unlocked' function is equivalent to the `fflush' function except that it does not implicitly lock the stream. The `fflush' function can be used to flush all streams currently opened. While this is useful in some situations it does often more than necessary since it might be done in situations when terminal input is required and the program wants to be sure that all output is visible on the terminal. But this means that only line buffered streams have to be flushed. Solaris introduced a function especially for this. It was always available in the GNU C library in some form but never officially exported. - Function: void _flushlbf (void) The `_flushlbf' function flushes all line buffered streams currently opened. This function is declared in the `stdio_ext.h' header. *Compatibility Note:* Some brain-damaged operating systems have been known to be so thoroughly fixated on line-oriented input and output that flushing a line buffered stream causes a newline to be written! Fortunately, this "feature" seems to be becoming less common. You do not need to worry about this in the GNU system. In some situations it might be useful to not flush the output pending for a stream but instead simply forget it. If transmission is costly and the output is not needed anymore this is valid reasoning. In this situation a non-standard function introduced in Solaris and available in the GNU C library can be used. - Function: void __fpurge (FILE *STREAM) The `__fpurge' function causes the buffer of the stream STREAM to be emptied. If the stream is currently in read mode all input in the buffer is lost. If the stream is in output mode the buffered output is not written to the device (or whatever other underlying storage) and the buffer the cleared. This function is declared in `stdio_ext.h'.  File: libc.info, Node: Controlling Buffering, Prev: Flushing Buffers, Up: Stream Buffering Controlling Which Kind of Buffering ----------------------------------- After opening a stream (but before any other operations have been performed on it), you can explicitly specify what kind of buffering you want it to have using the `setvbuf' function. The facilities listed in this section are declared in the header file `stdio.h'. - Function: int setvbuf (FILE *STREAM, char *BUF, int MODE, size_t SIZE) This function is used to specify that the stream STREAM should have the buffering mode MODE, which can be either `_IOFBF' (for full buffering), `_IOLBF' (for line buffering), or `_IONBF' (for unbuffered input/output). If you specify a null pointer as the BUF argument, then `setvbuf' allocates a buffer itself using `malloc'. This buffer will be freed when you close the stream. Otherwise, BUF should be a character array that can hold at least SIZE characters. You should not free the space for this array as long as the stream remains open and this array remains its buffer. You should usually either allocate it statically, or `malloc' (*note Unconstrained Allocation::) the buffer. Using an automatic array is not a good idea unless you close the file before exiting the block that declares the array. While the array remains a stream buffer, the stream I/O functions will use the buffer for their internal purposes. You shouldn't try to access the values in the array directly while the stream is using it for buffering. The `setvbuf' function returns zero on success, or a nonzero value if the value of MODE is not valid or if the request could not be honored. - Macro: int _IOFBF The value of this macro is an integer constant expression that can be used as the MODE argument to the `setvbuf' function to specify that the stream should be fully buffered. - Macro: int _IOLBF The value of this macro is an integer constant expression that can be used as the MODE argument to the `setvbuf' function to specify that the stream should be line buffered. - Macro: int _IONBF The value of this macro is an integer constant expression that can be used as the MODE argument to the `setvbuf' function to specify that the stream should be unbuffered. - Macro: int BUFSIZ The value of this macro is an integer constant expression that is good to use for the SIZE argument to `setvbuf'. This value is guaranteed to be at least `256'. The value of `BUFSIZ' is chosen on each system so as to make stream I/O efficient. So it is a good idea to use `BUFSIZ' as the size for the buffer when you call `setvbuf'. Actually, you can get an even better value to use for the buffer size by means of the `fstat' system call: it is found in the `st_blksize' field of the file attributes. *Note Attribute Meanings::. Sometimes people also use `BUFSIZ' as the allocation size of buffers used for related purposes, such as strings used to receive a line of input with `fgets' (*note Character Input::). There is no particular reason to use `BUFSIZ' for this instead of any other integer, except that it might lead to doing I/O in chunks of an efficient size. - Function: void setbuf (FILE *STREAM, char *BUF) If BUF is a null pointer, the effect of this function is equivalent to calling `setvbuf' with a MODE argument of `_IONBF'. Otherwise, it is equivalent to calling `setvbuf' with BUF, and a MODE of `_IOFBF' and a SIZE argument of `BUFSIZ'. The `setbuf' function is provided for compatibility with old code; use `setvbuf' in all new programs. - Function: void setbuffer (FILE *STREAM, char *BUF, size_t SIZE) If BUF is a null pointer, this function makes STREAM unbuffered. Otherwise, it makes STREAM fully buffered using BUF as the buffer. The SIZE argument specifies the length of BUF. This function is provided for compatibility with old BSD code. Use `setvbuf' instead. - Function: void setlinebuf (FILE *STREAM) This function makes STREAM be line buffered, and allocates the buffer for you. This function is provided for compatibility with old BSD code. Use `setvbuf' instead. It is possible to query whether a given stream is line buffered or not using a non-standard function introduced in Solaris and available in the GNU C library. - Function: int __flbf (FILE *STREAM) The `__flbf' function will return a nonzero value in case the stream STREAM is line buffered. Otherwise the return value is zero. This function is declared in the `stdio_ext.h' header. Two more extensions allow to determine the size of the buffer and how much of it is used. These functions were also introduced in Solaris. - Function: size_t __fbufsize (FILE *STREAM) The `__fbufsize' function return the size of the buffer in the stream STREAM. This value can be used to optimize the use of the stream. This function is declared in the `stdio_ext.h' header. - Function: size_t __fpending (FILE *STREAM) The `__fpending' function returns the number of bytes currently in the output buffer. For wide-oriented stream the measuring unit is wide characters. This function should not be used on buffers in read mode or opened read-only. This function is declared in the `stdio_ext.h' header.  File: libc.info, Node: Other Kinds of Streams, Next: Formatted Messages, Prev: Stream Buffering, Up: I/O on Streams Other Kinds of Streams ====================== The GNU library provides ways for you to define additional kinds of streams that do not necessarily correspond to an open file. One such type of stream takes input from or writes output to a string. These kinds of streams are used internally to implement the `sprintf' and `sscanf' functions. You can also create such a stream explicitly, using the functions described in *Note String Streams::. More generally, you can define streams that do input/output to arbitrary objects using functions supplied by your program. This protocol is discussed in *Note Custom Streams::. *Portability Note:* The facilities described in this section are specific to GNU. Other systems or C implementations might or might not provide equivalent functionality. * Menu: * String Streams:: Streams that get data from or put data in a string or memory buffer. * Obstack Streams:: Streams that store data in an obstack. * Custom Streams:: Defining your own streams with an arbitrary input data source and/or output data sink.