This is libc.info, produced by makeinfo version 4.2 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: Traditional Scheduling Functions, Prev: Traditional Scheduling Intro, Up: Traditional Scheduling Functions For Traditional Scheduling .................................... This section describes how you can read and set the nice value of a process. All these symbols are declared in `sys/resource.h'. The function and macro names are defined by POSIX, and refer to "priority," but the functions actually have to do with nice values, as the terms are used both in the manual and POSIX. The range of valid nice values depends on the kernel, but typically it runs from `-20' to `20'. A lower nice value corresponds to higher priority for the process. These constants describe the range of priority values: `PRIO_MIN' The lowest valid nice value. `PRIO_MAX' The highest valid nice value. - Function: int getpriority (int CLASS, int ID) Return the nice value of a set of processes; CLASS and ID specify which ones (see below). If the processes specified do not all have the same nice value, this returns the lowest value that any of them has. On success, the return value is `0'. Otherwise, it is `-1' and `ERRNO' is set accordingly. The `errno' values specific to this function are: `ESRCH' The combination of CLASS and ID does not match any existing process. `EINVAL' The value of CLASS is not valid. If the return value is `-1', it could indicate failure, or it could be the nice value. The only way to make certain is to set `errno = 0' before calling `getpriority', then use `errno != 0' afterward as the criterion for failure. - Function: int setpriority (int CLASS, int ID, int NICEVAL) Set the nice value of a set of processes to NICEVAL; CLASS and ID specify which ones (see below). The return value is `0' on success, and `-1' on failure. The following `errno' error condition are possible for this function: `ESRCH' The combination of CLASS and ID does not match any existing process. `EINVAL' The value of CLASS is not valid. `EPERM' The call would set the nice value of a process which is owned by a different user than the calling process (i.e. the target process' real or effective uid does not match the calling process' effective uid) and the calling process does not have `CAP_SYS_NICE' permission. `EACCES' The call would lower the process' nice value and the process does not have `CAP_SYS_NICE' permission. The arguments CLASS and ID together specify a set of processes in which you are interested. These are the possible values of CLASS: `PRIO_PROCESS' One particular process. The argument ID is a process ID (pid). `PRIO_PGRP' All the processes in a particular process group. The argument ID is a process group ID (pgid). `PRIO_USER' All the processes owned by a particular user (i.e. whose real uid indicates the user). The argument ID is a user ID (uid). If the argument ID is 0, it stands for the calling process, its process group, or its owner (real uid), according to CLASS. - Function: int nice (int INCREMENT) Increment the nice value of the calling process by INCREMENT. The return value is the new nice value on success, and `-1' on failure. In the case of failure, `errno' will be set to the same values as for `setpriority'. Here is an equivalent definition of `nice': int nice (int increment) { int result, old = getpriority (PRIO_PROCESS, 0); result = setpriority (PRIO_PROCESS, 0, old + increment); if (result != -1) return old + increment; else return -1; }  File: libc.info, Node: Memory Resources, Next: Processor Resources, Prev: Priority, Up: Resource Usage And Limitation Querying memory available resources =================================== The amount of memory available in the system and the way it is organized determines oftentimes the way programs can and have to work. For functions like `mmap' it is necessary to know about the size of individual memory pages and knowing how much memory is available enables a program to select appropriate sizes for, say, caches. Before we get into these details a few words about memory subsystems in traditional Unix systems will be given. * Menu: * Memory Subsystem:: Overview about traditional Unix memory handling. * Query Memory Parameters:: How to get information about the memory subsystem?  File: libc.info, Node: Memory Subsystem, Next: Query Memory Parameters, Up: Memory Resources Overview about traditional Unix memory handling ----------------------------------------------- Unix systems normally provide processes virtual address spaces. This means that the addresses of the memory regions do not have to correspond directly to the addresses of the actual physical memory which stores the data. An extra level of indirection is introduced which translates virtual addresses into physical addresses. This is normally done by the hardware of the processor. Using a virtual address space has several advantage. The most important is process isolation. The different processes running on the system cannot interfere directly with each other. No process can write into the address space of another process (except when shared memory is used but then it is wanted and controlled). Another advantage of virtual memory is that the address space the processes see can actually be larger than the physical memory available. The physical memory can be extended by storage on an external media where the content of currently unused memory regions is stored. The address translation can then intercept accesses to these memory regions and make memory content available again by loading the data back into memory. This concept makes it necessary that programs which have to use lots of memory know the difference between available virtual address space and available physical memory. If the working set of virtual memory of all the processes is larger than the available physical memory the system will slow down dramatically due to constant swapping of memory content from the memory to the storage media and back. This is called "thrashing". A final aspect of virtual memory which is important and follows from what is said in the last paragraph is the granularity of the virtual address space handling. When we said that the virtual address handling stores memory content externally it cannot do this on a byte-by-byte basis. The administrative overhead does not allow this (leaving alone the processor hardware). Instead several thousand bytes are handled together and form a "page". The size of each page is always a power of two byte. The smallest page size in use today is 4096, with 8192, 16384, and 65536 being other popular sizes.  File: libc.info, Node: Query Memory Parameters, Prev: Memory Subsystem, Up: Memory Resources How to get information about the memory subsystem? -------------------------------------------------- The page size of the virtual memory the process sees is essential to know in several situations. Some programming interface (e.g., `mmap', *note Memory-mapped I/O::) require the user to provide information adjusted to the page size. In the case of `mmap' is it necessary to provide a length argument which is a multiple of the page size. Another place where the knowledge about the page size is useful is in memory allocation. If one allocates pieces of memory in larger chunks which are then subdivided by the application code it is useful to adjust the size of the larger blocks to the page size. If the total memory requirement for the block is close (but not larger) to a multiple of the page size the kernel's memory handling can work more effectively since it only has to allocate memory pages which are fully used. (To do this optimization it is necessary to know a bit about the memory allocator which will require a bit of memory itself for each block and this overhead must not push the total size over the page size multiple. The page size traditionally was a compile time constant. But recent development of processors changed this. Processors now support different page sizes and they can possibly even vary among different processes on the same system. Therefore the system should be queried at runtime about the current page size and no assumptions (except about it being a power of two) should be made. The correct interface to query about the page size is `sysconf' (*note Sysconf Definition::) with the parameter `_SC_PAGESIZE'. There is a much older interface available, too. - Function: int getpagesize (void) The `getpagesize' function returns the page size of the process. This value is fixed for the runtime of the process but can vary in different runs of the application. The function is declared in `unistd.h'. Widely available on System V derived systems is a method to get information about the physical memory the system has. The call sysconf (_SC_PHYS_PAGES) returns the total number of pages of physical the system has. This does not mean all this memory is available. This information can be found using sysconf (_SC_AVPHYS_PAGES) These two values help to optimize applications. The value returned for `_SC_AVPHYS_PAGES' is the amount of memory the application can use without hindering any other process (given that no other process increases its memory usage). The value returned for `_SC_PHYS_PAGES' is more or less a hard limit for the working set. If all applications together constantly use more than that amount of memory the system is in trouble. The GNU C library provides in addition to these already described way to get this information two functions. They are declared in the file `sys/sysinfo.h'. Programmers should prefer to use the `sysconf' method described above. - Function: long int get_phys_pages (void) The `get_phys_pages' function returns the total number of pages of physical the system has. To get the amount of memory this number has to be multiplied by the page size. This function is a GNU extension. - Function: long int get_avphys_pages (void) The `get_phys_pages' function returns the number of available pages of physical the system has. To get the amount of memory this number has to be multiplied by the page size. This function is a GNU extension.  File: libc.info, Node: Processor Resources, Prev: Memory Resources, Up: Resource Usage And Limitation Learn about the processors available ==================================== The use of threads or processes with shared memory allows an application to take advantage of all the processing power a system can provide. If the task can be parallelized the optimal way to write an application is to have at any time as many processes running as there are processors. To determine the number of processors available to the system one can run sysconf (_SC_NPROCESSORS_CONF) which returns the number of processors the operating system configured. But it might be possible for the operating system to disable individual processors and so the call sysconf (_SC_NPROCESSORS_ONLN) returns the number of processors which are currently inline (i.e., available). For these two pieces of information the GNU C library also provides functions to get the information directly. The functions are declared in `sys/sysinfo.h'. - Function: int get_nprocs_conf (void) The `get_nprocs_conf' function returns the number of processors the operating system configured. This function is a GNU extension. - Function: int get_nprocs (void) The `get_nprocs' function returns the number of available processors. This function is a GNU extension. Before starting more threads it should be checked whether the processors are not already overused. Unix systems calculate something called the "load average". This is a number indicating how many processes were running. This number is average over different periods of times (normally 1, 5, and 15 minutes). - Function: int getloadavg (double LOADAVG[], int NELEM) This function gets the 1, 5 and 15 minute load averages of the system. The values are placed in LOADAVG. `getloadavg' will place at most NELEM elements into the array but never more than three elements. The return value is the number of elements written to LOADAVG, or -1 on error. This function is declared in `stdlib.h'.  File: libc.info, Node: Non-Local Exits, Next: Signal Handling, Prev: Resource Usage And Limitation, Up: Top Non-Local Exits *************** Sometimes when your program detects an unusual situation inside a deeply nested set of function calls, you would like to be able to immediately return to an outer level of control. This section describes how you can do such "non-local exits" using the `setjmp' and `longjmp' functions. * Menu: * Intro: Non-Local Intro. When and how to use these facilities. * Details: Non-Local Details. Functions for non-local exits. * Non-Local Exits and Signals:: Portability issues. * System V contexts:: Complete context control a la System V.  File: libc.info, Node: Non-Local Intro, Next: Non-Local Details, Up: Non-Local Exits Introduction to Non-Local Exits =============================== As an example of a situation where a non-local exit can be useful, suppose you have an interactive program that has a "main loop" that prompts for and executes commands. Suppose the "read" command reads input from a file, doing some lexical analysis and parsing of the input while processing it. If a low-level input error is detected, it would be useful to be able to return immediately to the "main loop" instead of having to make each of the lexical analysis, parsing, and processing phases all have to explicitly deal with error situations initially detected by nested calls. (On the other hand, if each of these phases has to do a substantial amount of cleanup when it exits--such as closing files, deallocating buffers or other data structures, and the like--then it can be more appropriate to do a normal return and have each phase do its own cleanup, because a non-local exit would bypass the intervening phases and their associated cleanup code entirely. Alternatively, you could use a non-local exit but do the cleanup explicitly either before or after returning to the "main loop".) In some ways, a non-local exit is similar to using the `return' statement to return from a function. But while `return' abandons only a single function call, transferring control back to the point at which it was called, a non-local exit can potentially abandon many levels of nested function calls. You identify return points for non-local exits by calling the function `setjmp'. This function saves information about the execution environment in which the call to `setjmp' appears in an object of type `jmp_buf'. Execution of the program continues normally after the call to `setjmp', but if an exit is later made to this return point by calling `longjmp' with the corresponding `jmp_buf' object, control is transferred back to the point where `setjmp' was called. The return value from `setjmp' is used to distinguish between an ordinary return and a return made by a call to `longjmp', so calls to `setjmp' usually appear in an `if' statement. Here is how the example program described above might be set up: #include #include #include jmp_buf main_loop; void abort_to_main_loop (int status) { longjmp (main_loop, status); } int main (void) { while (1) if (setjmp (main_loop)) puts ("Back at main loop...."); else do_command (); } void do_command (void) { char buffer[128]; if (fgets (buffer, 128, stdin) == NULL) abort_to_main_loop (-1); else exit (EXIT_SUCCESS); } The function `abort_to_main_loop' causes an immediate transfer of control back to the main loop of the program, no matter where it is called from. The flow of control inside the `main' function may appear a little mysterious at first, but it is actually a common idiom with `setjmp'. A normal call to `setjmp' returns zero, so the "else" clause of the conditional is executed. If `abort_to_main_loop' is called somewhere within the execution of `do_command', then it actually appears as if the _same_ call to `setjmp' in `main' were returning a second time with a value of `-1'. So, the general pattern for using `setjmp' looks something like: if (setjmp (BUFFER)) /* Code to clean up after premature return. */ ... else /* Code to be executed normally after setting up the return point. */ ...  File: libc.info, Node: Non-Local Details, Next: Non-Local Exits and Signals, Prev: Non-Local Intro, Up: Non-Local Exits Details of Non-Local Exits ========================== Here are the details on the functions and data structures used for performing non-local exits. These facilities are declared in `setjmp.h'. - Data Type: jmp_buf Objects of type `jmp_buf' hold the state information to be restored by a non-local exit. The contents of a `jmp_buf' identify a specific place to return to. - Macro: int setjmp (jmp_buf STATE) When called normally, `setjmp' stores information about the execution state of the program in STATE and returns zero. If `longjmp' is later used to perform a non-local exit to this STATE, `setjmp' returns a nonzero value. - Function: void longjmp (jmp_buf STATE, int VALUE) This function restores current execution to the state saved in STATE, and continues execution from the call to `setjmp' that established that return point. Returning from `setjmp' by means of `longjmp' returns the VALUE argument that was passed to `longjmp', rather than `0'. (But if VALUE is given as `0', `setjmp' returns `1'). There are a lot of obscure but important restrictions on the use of `setjmp' and `longjmp'. Most of these restrictions are present because non-local exits require a fair amount of magic on the part of the C compiler and can interact with other parts of the language in strange ways. The `setjmp' function is actually a macro without an actual function definition, so you shouldn't try to `#undef' it or take its address. In addition, calls to `setjmp' are safe in only the following contexts: * As the test expression of a selection or iteration statement (such as `if', `switch', or `while'). * As one operand of a equality or comparison operator that appears as the test expression of a selection or iteration statement. The other operand must be an integer constant expression. * As the operand of a unary `!' operator, that appears as the test expression of a selection or iteration statement. * By itself as an expression statement. Return points are valid only during the dynamic extent of the function that called `setjmp' to establish them. If you `longjmp' to a return point that was established in a function that has already returned, unpredictable and disastrous things are likely to happen. You should use a nonzero VALUE argument to `longjmp'. While `longjmp' refuses to pass back a zero argument as the return value from `setjmp', this is intended as a safety net against accidental misuse and is not really good programming style. When you perform a non-local exit, accessible objects generally retain whatever values they had at the time `longjmp' was called. The exception is that the values of automatic variables local to the function containing the `setjmp' call that have been changed since the call to `setjmp' are indeterminate, unless you have declared them `volatile'.  File: libc.info, Node: Non-Local Exits and Signals, Next: System V contexts, Prev: Non-Local Details, Up: Non-Local Exits Non-Local Exits and Signals =========================== In BSD Unix systems, `setjmp' and `longjmp' also save and restore the set of blocked signals; see *Note Blocking Signals::. However, the POSIX.1 standard requires `setjmp' and `longjmp' not to change the set of blocked signals, and provides an additional pair of functions (`sigsetjmp' and `siglongjmp') to get the BSD behavior. The behavior of `setjmp' and `longjmp' in the GNU library is controlled by feature test macros; see *Note Feature Test Macros::. The default in the GNU system is the POSIX.1 behavior rather than the BSD behavior. The facilities in this section are declared in the header file `setjmp.h'. - Data Type: sigjmp_buf This is similar to `jmp_buf', except that it can also store state information about the set of blocked signals. - Function: int sigsetjmp (sigjmp_buf STATE, int SAVESIGS) This is similar to `setjmp'. If SAVESIGS is nonzero, the set of blocked signals is saved in STATE and will be restored if a `siglongjmp' is later performed with this STATE. - Function: void siglongjmp (sigjmp_buf STATE, int VALUE) This is similar to `longjmp' except for the type of its STATE argument. If the `sigsetjmp' call that set this STATE used a nonzero SAVESIGS flag, `siglongjmp' also restores the set of blocked signals.  File: libc.info, Node: System V contexts, Prev: Non-Local Exits and Signals, Up: Non-Local Exits Complete Context Control ======================== The Unix standard one more set of function to control the execution path and these functions are more powerful than those discussed in this chapter so far. These function were part of the original System V API and by this route were added to the Unix API. Beside on branded Unix implementations these interfaces are not widely available. Not all platforms and/or architectures the GNU C Library is available on provide this interface. Use `configure' to detect the availability. Similar to the `jmp_buf' and `sigjmp_buf' types used for the variables to contain the state of the `longjmp' functions the interfaces of interest here have an appropriate type as well. Objects of this type are normally much larger since more information is contained. The type is also used in a few more places as we will see. The types and functions described in this section are all defined and declared respectively in the `ucontext.h' header file. - Data Type: ucontext_t The `ucontext_t' type is defined as a structure with as least the following elements: `ucontext_t *uc_link' This is a pointer to the next context structure which is used if the context described in the current structure returns. `sigset_t uc_sigmask' Set of signals which are blocked when this context is used. `stack_t uc_stack' Stack used for this context. The value need not be (and normally is not) the stack pointer. *Note Signal Stack::. `mcontext_t uc_mcontext' This element contains the actual state of the process. The `mcontext_t' type is also defined in this header but the definition should be treated as opaque. Any use of knowledge of the type makes applications less portable. Objects of this type have to be created by the user. The initialization and modification happens through one of the following functions: - Function: int getcontext (ucontext_t *UCP) The `getcontext' function initializes the variable pointed to by UCP with the context of the calling thread. The context contains the content of the registers, the signal mask, and the current stack. Executing the contents would start at the point where the `getcontext' call just returned. The function returns `0' if successful. Otherwise it returns `-1' and sets ERRNO accordingly. The `getcontext' function is similar to `setjmp' but it does not provide an indication of whether the function returns for the first time or whether the initialized context was used and the execution is resumed at just that point. If this is necessary the user has to take determine this herself. This must be done carefully since the context contains registers which might contain register variables. This is a good situation to define variables with `volatile'. Once the context variable is initialized it can be used as is or it can be modified. The latter is normally done to implement co-routines or similar constructs. The `makecontext' function is what has to be used to do that. - Function: void makecontext (ucontext_t *UCP, void (*FUNC) (void), int ARGC, ...) The UCP parameter passed to the `makecontext' shall be initialized by a call to `getcontext'. The context will be modified to in a way so that if the context is resumed it will start by calling the function `func' which gets ARGC integer arguments passed. The integer arguments which are to be passed should follow the ARGC parameter in the call to `makecontext'. Before the call to this function the `uc_stack' and `uc_link' element of the UCP structure should be initialized. The `uc_stack' element describes the stack which is used for this context. No two contexts which are used at the same time should use the same memory region for a stack. The `uc_link' element of the object pointed to by UCP should be a pointer to the context to be executed when the function FUNC returns or it should be a null pointer. See `setcontext' for more information about the exact use. While allocating the memory for the stack one has to be careful. Most modern processors keep track of whether a certain memory region is allowed to contain code which is executed or not. Data segments and heap memory is normally not tagged to allow this. The result is that programs would fail. Examples for such code include the calling sequences the GNU C compiler generates for calls to nested functions. Safe ways to allocate stacks correctly include using memory on the original threads stack or explicitly allocate memory tagged for execution using (*note Memory-mapped I/O::). *Compatibility note*: The current Unix standard is very imprecise about the way the stack is allocated. All implementations seem to agree that the `uc_stack' element must be used but the values stored in the elements of the `stack_t' value are unclear. The GNU C library and most other Unix implementations require the `ss_sp' value of the `uc_stack' element to point to the base of the memory region allocated for the stack and the size of the memory region is stored in `ss_size'. There are implements out there which require `ss_sp' to be set to the value the stack pointer will have (which can depending on the direction the stack grows be different). This difference makes the `makecontext' function hard to use and it requires detection of the platform at compile time. - Function: int setcontext (const ucontext_t *UCP) The `setcontext' function restores the context described by UCP. The context is not modified and can be reused as often as wanted. If the context was created by `getcontext' execution resumes with the registers filled with the same values and the same stack as if the `getcontext' call just returned. If the context was modified with a call to `makecontext' execution continues with the function passed to `makecontext' which gets the specified parameters passed. If this function returns execution is resumed in the context which was referenced by the `uc_link' element of the context structure passed to `makecontext' at the time of the call. If `uc_link' was a null pointer the application terminates in this case. Since the context contains information about the stack no two threads should use the same context at the same time. The result in most cases would be disastrous. The `setcontext' function does not return unless an error occurred in which case it returns `-1'. The `setcontext' function simply replaces the current context with the one described by the UCP parameter. This is often useful but there are situations where the current context has to be preserved. - Function: int swapcontext (ucontext_t *restrict OUCP, const ucontext_t *restrict UCP) The `swapcontext' function is similar to `setcontext' but instead of just replacing the current context the latter is first saved in the object pointed to by OUCP as if this was a call to `getcontext'. The saved context would resume after the call to `swapcontext'. Once the current context is saved the context described in UCP is installed and execution continues as described in this context. If `swapcontext' succeeds the function does not return unless the context OUCP is used without prior modification by `makecontext'. The return value in this case is `0'. If the function fails it returns `-1' and set ERRNO accordingly. Example for SVID Context Handling ================================= The easiest way to use the context handling functions is as a replacement for `setjmp' and `longjmp'. The context contains on most platforms more information which might lead to less surprises but this also means using these functions is more expensive (beside being less portable). int random_search (int n, int (*fp) (int, ucontext_t *)) { volatile int cnt = 0; ucontext_t uc; /* Safe current context. */ if (getcontext (&uc) < 0) return -1; /* If we have not tried N times try again. */ if (cnt++ < n) /* Call the function with a new random number and the context. */ if (fp (rand (), &uc) != 0) /* We found what we were looking for. */ return 1; /* Not found. */ return 0; } Using contexts in such a way enables emulating exception handling. The search functions passed in the FP parameter could be very large, nested, and complex which would make it complicated (or at least would require a lot of code) to leave the function with an error value which has to be passed down to the caller. By using the context it is possible to leave the search function in one step and allow restarting the search which also has the nice side effect that it can be significantly faster. Something which is harder to implement with `setjmp' and `longjmp' is to switch temporarily to a different execution path and then resume where execution was stopped. #include #include #include #include #include /* Set by the signal handler. */ static volatile int expired; /* The contexts. */ static ucontext_t uc[3]; /* We do only a certain number of switches. */ static int switches; /* This is the function doing the work. It is just a skeleton, real code has to be filled in. */ static void f (int n) { int m = 0; while (1) { /* This is where the work would be done. */ if (++m % 100 == 0) { putchar ('.'); fflush (stdout); } /* Regularly the EXPIRE variable must be checked. */ if (expired) { /* We do not want the program to run forever. */ if (++switches == 20) return; printf ("\nswitching from %d to %d\n", n, 3 - n); expired = 0; /* Switch to the other context, saving the current one. */ swapcontext (&uc[n], &uc[3 - n]); } } } /* This is the signal handler which simply set the variable. */ void handler (int signal) { expired = 1; } int main (void) { struct sigaction sa; struct itimerval it; char st1[8192]; char st2[8192]; /* Initialize the data structures for the interval timer. */ sa.sa_flags = SA_RESTART; sigfillset (&sa.sa_mask); sa.sa_handler = handler; it.it_interval.tv_sec = 0; it.it_interval.tv_usec = 1; it.it_value = it.it_interval; /* Install the timer and get the context we can manipulate. */ if (sigaction (SIGPROF, &sa, NULL) < 0 || setitimer (ITIMER_PROF, &it, NULL) < 0 || getcontext (&uc[1]) == -1 || getcontext (&uc[2]) == -1) abort (); /* Create a context with a separate stack which causes the function `f' to be call with the parameter `1'. Note that the `uc_link' points to the main context which will cause the program to terminate once the function return. */ uc[1].uc_link = &uc[0]; uc[1].uc_stack.ss_sp = st1; uc[1].uc_stack.ss_size = sizeof st1; makecontext (&uc[1], (void (*) (void)) f, 1, 1); /* Similarly, but `2' is passed as the parameter to `f'. */ uc[2].uc_link = &uc[0]; uc[2].uc_stack.ss_sp = st2; uc[2].uc_stack.ss_size = sizeof st2; makecontext (&uc[2], (void (*) (void)) f, 1, 2); /* Start running. */ swapcontext (&uc[0], &uc[1]); putchar ('\n'); return 0; } This an example how the context functions can be used to implement co-routines or cooperative multi-threading. All that has to be done is to call every once in a while `swapcontext' to continue running a different context. It is not allowed to do the context switching from the signal handler directly since neither `setcontext' nor `swapcontext' are functions which can be called from a signal handler. But setting a variable in the signal handler and checking it in the body of the functions which are executed. Since `swapcontext' is saving the current context it is possible to have multiple different scheduling points in the code. Execution will always resume where it was left.  File: libc.info, Node: Signal Handling, Next: Program Basics, Prev: Non-Local Exits, Up: Top Signal Handling *************** A "signal" is a software interrupt delivered to a process. The operating system uses signals to report exceptional situations to an executing program. Some signals report errors such as references to invalid memory addresses; others report asynchronous events, such as disconnection of a phone line. The GNU C library defines a variety of signal types, each for a particular kind of event. Some kinds of events make it inadvisable or impossible for the program to proceed as usual, and the corresponding signals normally abort the program. Other kinds of signals that report harmless events are ignored by default. If you anticipate an event that causes signals, you can define a handler function and tell the operating system to run it when that particular type of signal arrives. Finally, one process can send a signal to another process; this allows a parent process to abort a child, or two related processes to communicate and synchronize. * Menu: * Concepts of Signals:: Introduction to the signal facilities. * Standard Signals:: Particular kinds of signals with standard names and meanings. * Signal Actions:: Specifying what happens when a particular signal is delivered. * Defining Handlers:: How to write a signal handler function. * Interrupted Primitives:: Signal handlers affect use of `open', `read', `write' and other functions. * Generating Signals:: How to send a signal to a process. * Blocking Signals:: Making the system hold signals temporarily. * Waiting for a Signal:: Suspending your program until a signal arrives. * Signal Stack:: Using a Separate Signal Stack. * BSD Signal Handling:: Additional functions for backward compatibility with BSD.  File: libc.info, Node: Concepts of Signals, Next: Standard Signals, Up: Signal Handling Basic Concepts of Signals ========================= This section explains basic concepts of how signals are generated, what happens after a signal is delivered, and how programs can handle signals. * Menu: * Kinds of Signals:: Some examples of what can cause a signal. * Signal Generation:: Concepts of why and how signals occur. * Delivery of Signal:: Concepts of what a signal does to the process.  File: libc.info, Node: Kinds of Signals, Next: Signal Generation, Up: Concepts of Signals Some Kinds of Signals --------------------- A signal reports the occurrence of an exceptional event. These are some of the events that can cause (or "generate", or "raise") a signal: * A program error such as dividing by zero or issuing an address outside the valid range. * A user request to interrupt or terminate the program. Most environments are set up to let a user suspend the program by typing `C-z', or terminate it with `C-c'. Whatever key sequence is used, the operating system sends the proper signal to interrupt the process. * The termination of a child process. * Expiration of a timer or alarm. * A call to `kill' or `raise' by the same process. * A call to `kill' from another process. Signals are a limited but useful form of interprocess communication. * An attempt to perform an I/O operation that cannot be done. Examples are reading from a pipe that has no writer (*note Pipes and FIFOs::), and reading or writing to a terminal in certain situations (*note Job Control::). Each of these kinds of events (excepting explicit calls to `kill' and `raise') generates its own particular kind of signal. The various kinds of signals are listed and described in detail in *Note Standard Signals::.  File: libc.info, Node: Signal Generation, Next: Delivery of Signal, Prev: Kinds of Signals, Up: Concepts of Signals Concepts of Signal Generation ----------------------------- In general, the events that generate signals fall into three major categories: errors, external events, and explicit requests. An error means that a program has done something invalid and cannot continue execution. But not all kinds of errors generate signals--in fact, most do not. For example, opening a nonexistent file is an error, but it does not raise a signal; instead, `open' returns `-1'. In general, errors that are necessarily associated with certain library functions are reported by returning a value that indicates an error. The errors which raise signals are those which can happen anywhere in the program, not just in library calls. These include division by zero and invalid memory addresses. An external event generally has to do with I/O or other processes. These include the arrival of input, the expiration of a timer, and the termination of a child process. An explicit request means the use of a library function such as `kill' whose purpose is specifically to generate a signal. Signals may be generated "synchronously" or "asynchronously". A synchronous signal pertains to a specific action in the program, and is delivered (unless blocked) during that action. Most errors generate signals synchronously, and so do explicit requests by a process to generate a signal for that same process. On some machines, certain kinds of hardware errors (usually floating-point exceptions) are not reported completely synchronously, but may arrive a few instructions later. Asynchronous signals are generated by events outside the control of the process that receives them. These signals arrive at unpredictable times during execution. External events generate signals asynchronously, and so do explicit requests that apply to some other process. A given type of signal is either typically synchronous or typically asynchronous. For example, signals for errors are typically synchronous because errors generate signals synchronously. But any type of signal can be generated synchronously or asynchronously with an explicit request.  File: libc.info, Node: Delivery of Signal, Prev: Signal Generation, Up: Concepts of Signals How Signals Are Delivered ------------------------- When a signal is generated, it becomes "pending". Normally it remains pending for just a short period of time and then is "delivered" to the process that was signaled. However, if that kind of signal is currently "blocked", it may remain pending indefinitely--until signals of that kind are "unblocked". Once unblocked, it will be delivered immediately. *Note Blocking Signals::. When the signal is delivered, whether right away or after a long delay, the "specified action" for that signal is taken. For certain signals, such as `SIGKILL' and `SIGSTOP', the action is fixed, but for most signals, the program has a choice: ignore the signal, specify a "handler function", or accept the "default action" for that kind of signal. The program specifies its choice using functions such as `signal' or `sigaction' (*note Signal Actions::). We sometimes say that a handler "catches" the signal. While the handler is running, that particular signal is normally blocked. If the specified action for a kind of signal is to ignore it, then any such signal which is generated is discarded immediately. This happens even if the signal is also blocked at the time. A signal discarded in this way will never be delivered, not even if the program subsequently specifies a different action for that kind of signal and then unblocks it. If a signal arrives which the program has neither handled nor ignored, its "default action" takes place. Each kind of signal has its own default action, documented below (*note Standard Signals::). For most kinds of signals, the default action is to terminate the process. For certain kinds of signals that represent "harmless" events, the default action is to do nothing. When a signal terminates a process, its parent process can determine the cause of termination by examining the termination status code reported by the `wait' or `waitpid' functions. (This is discussed in more detail in *Note Process Completion::.) The information it can get includes the fact that termination was due to a signal and the kind of signal involved. If a program you run from a shell is terminated by a signal, the shell typically prints some kind of error message. The signals that normally represent program errors have a special property: when one of these signals terminates the process, it also writes a "core dump file" which records the state of the process at the time of termination. You can examine the core dump with a debugger to investigate what caused the error. If you raise a "program error" signal by explicit request, and this terminates the process, it makes a core dump file just as if the signal had been due directly to an error.  File: libc.info, Node: Standard Signals, Next: Signal Actions, Prev: Concepts of Signals, Up: Signal Handling Standard Signals ================ This section lists the names for various standard kinds of signals and describes what kind of event they mean. Each signal name is a macro which stands for a positive integer--the "signal number" for that kind of signal. Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined here. This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform. The signal names are defined in the header file `signal.h'. - Macro: int NSIG The value of this symbolic constant is the total number of signals defined. Since the signal numbers are allocated consecutively, `NSIG' is also one greater than the largest defined signal number. * Menu: * Program Error Signals:: Used to report serious program errors. * Termination Signals:: Used to interrupt and/or terminate the program. * Alarm Signals:: Used to indicate expiration of timers. * Asynchronous I/O Signals:: Used to indicate input is available. * Job Control Signals:: Signals used to support job control. * Operation Error Signals:: Used to report operational system errors. * Miscellaneous Signals:: Miscellaneous Signals. * Signal Messages:: Printing a message describing a signal.