Memory Allocation
*****************

The GNU system provides several methods for allocating memory space under explicit program control.  They vary in generality and in efficiency.

Dynamic Memory Allocation Concepts
==================================

Dynamic memory allocation is a technique in which programs determine as they are running where to store some information.  You need dynamic allocation when the number of memory blocks you need, or how long you continue to need them, depends on the data you are working on.

For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the storage dynamically and make it dynamically larger as you read more of the line.

Or, you may need a block for each record or each definition in the input data; since you can't know in advance how many there will be, you must allocate a new block for each record or definition as you read it.

When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly.  You call a function or macro when you want to allocate space, and specify the size with an argument.  If you want to free the space, you do so by calling another function or macro. You can do these things whenever you want, as often as you want.

Dynamic Allocation and C
========================

The C language supports two kinds of memory allocation through the variables in C programs:

  * Static allocation is what happens when you declare a static or global variable.  Each static or global variable defines one block of space, of a fixed size.  The space is allocated once, when your program is started, and is never freed.

  * Automatic allocation happens when you declare an automatic variable, such as a function argument or a local variable.  The space for an automatic variable is allocated when the compound statement containing the declaration is entered, and is freed when that compound statement is exited.

     In GNU C, the length of the automatic storage can be an expression that varies.  In other C implementations, it must be a constant.

Dynamic allocation is not supported by C variables; there is no storage class ``dynamic'', and there can never be a C variable whose value is stored in dynamically allocated space.  The only way to refer to dynamically allocated space is through a pointer.  Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers generally use dynamic allocation only when neither static nor automatic allocation will serve.

For example, if you want to allocate dynamically some space to hold a struct foobar, you cannot declare a variable of type struct foobar whose contents are the dynamically allocated space.  But you can declare a variable of pointer type struct foobar * and assign it the address of the space.  Then you can use the operators * and -> on this pointer variable to refer to the contents of the space:

     {
       struct foobar *ptr
          = (struct foobar *) malloc (sizeof (struct foobar));
       ptr->name = x;
       ptr->next = current_foobar;
       current_foobar = ptr;
     }

Unconstrained Allocation
========================

The most general dynamic allocation facility is malloc.  It allows you to allocate blocks of memory of any size at any time, make them bigger or smaller at any time, and free the blocks individually at any time (or never).

Basic Storage Allocation
------------------------

To allocate a block of memory, call malloc.  The prototype for this function is in stdlib.h.

 -- Function: void * malloc (size_t SIZE)
     This function returns a pointer to a newly allocated block SIZE bytes long, or a null pointer if the block could not be allocated.

The contents of the block are undefined; you must initialize it yourself (or use calloc instead; see Allocating Cleared Space). Normally you would cast the value as a pointer to the kind of object that you want to store in the block.  Here we show an example of doing so, and of initializing the space with zeros using the library function memset (see Copying and Concatenation):

     struct foo *ptr;
     ...
     ptr = (struct foo *) malloc (sizeof (struct foo));
     if (ptr == 0) abort ();
     memset (ptr, 0, sizeof (struct foo));

You can store the result of malloc into any pointer variable without a cast, because ISO C automatically converts the type void * to another type of pointer when necessary.  But the cast is necessary in contexts other than assignment operators or if you might want your code to run in traditional C.

Remember that when allocating space for a string, the argument to malloc must be one plus the length of the string.  This is because a string is terminated with a null character that doesn't count in the ``length'' of the string but does need space.  For example:

     char *ptr;
     ...
     ptr = (char *) malloc (length + 1);

See Representation of Strings, for more information about this.

Examples of malloc
------------------

If no more space is available, malloc returns a null pointer. You should check the value of every call to malloc.  It is useful to write a subroutine that calls malloc and reports an error if the value is a null pointer, returning only if the value is nonzero.  This function is conventionally called xmalloc.  Here it is:

     void *
     xmalloc (size_t size)
     {
       register void *value = malloc (size);
       if (value == 0)
         fatal ("virtual memory exhausted");
       return value;
     }

Here is a real example of using malloc (by way of xmalloc). The function savestring will copy a sequence of characters into a newly allocated null-terminated string:

     char *
     savestring (const char *ptr, size_t len)
     {
       register char *value = (char *) xmalloc (len + 1);
       value[len] = '\0';
       return (char *) memcpy (value, ptr, len);
     }

The block that malloc gives you is guaranteed to be aligned so that it can hold any type of data.  In the GNU system, the address is always a multiple of eight on most systems, and a multiple of 16 on 64-bit systems.  Only rarely is any higher boundary (such as a page boundary) necessary; for those cases, use memalign or valloc (see Aligned Memory Blocks).

Note that the memory located after the end of the block is likely to be in use for something else; perhaps a block already allocated by another call to malloc.  If you attempt to treat the block as longer than you asked for it to be, you are liable to destroy the data that malloc uses to keep track of its blocks, or you may destroy the contents of another block.  If you have already allocated a block and discover you want it to be bigger, use realloc (see Changing Block Size).

Freeing Memory Allocated with malloc
------------------------------------

When you no longer need a block that you got with malloc, use the function free to make the block available to be allocated again. The prototype for this function is in stdlib.h.

 -- Function: void free (void *PTR)
     The free function deallocates the block of storage pointed at by PTR.

 -- Function: void cfree (void *PTR)
     This function does the same thing as free.  It's provided for backward compatibility with SunOS; you should use free instead.

Freeing a block alters the contents of the block.  Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it.  Copy whatever you need out of the block before freeing it!  Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to:

     struct chain
       {
         struct chain *next;
         char *name;
       }

     void
     free_chain (struct chain *chain)
     {
       while (chain != 0)
         {
           struct chain *next = chain->next;
           free (chain->name);
           free (chain);
           chain = next;
         }
     }

Occasionally, free can actually return memory to the operating system and make the process smaller.  Usually, all it can do is allow a later call to malloc to reuse the space.  In the meantime, the space remains in your program as part of a free-list used internally by malloc.

There is no point in freeing blocks at the end of a program, because all of the program's space is given back to the system when the process terminates.

Changing the Size of a Block
----------------------------

Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block.  For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer.

You can make the block longer by calling realloc.  This function is declared in stdlib.h.

 -- Function: void * realloc (void *PTR, size_t NEWSIZE)
     The realloc function changes the size of the block whose address is PTR to be NEWSIZE.

     Since the space after the end of the block may be in use, realloc may find it necessary to copy the block to a new address where more free space is available.  The value of realloc is the new address of the block.  If the block needs to be moved, realloc copies the old contents.

     If you pass a null pointer for PTR, realloc behaves just like malloc (NEWSIZE).  This can be convenient, but beware that older implementations (before ISO C) may not support this behavior, and will probably crash when realloc is passed a null pointer.

Like malloc, realloc may return a null pointer if no memory space is available to make the block bigger.  When this happens, the original block is untouched; it has not been modified or relocated.

In most cases it makes no difference what happens to the original block when realloc fails, because the application program cannot continue when it is out of memory, and the only thing to do is to give a fatal error message.  Often it is convenient to write and use a subroutine, conventionally called xrealloc, that takes care of the error message as xmalloc does for malloc:

     void *
     xrealloc (void *ptr, size_t size)
     {
       register void *value = realloc (ptr, size);
       if (value == 0)
         fatal ("Virtual memory exhausted");
       return value;
     }

You can also use realloc to make a block smaller.  The reason you would do this is to avoid tying up a lot of memory space when only a little is needed. In several allocation implementations, making a block smaller sometimes necessitates copying it, so it can fail if no other space is available.

If the new size you specify is the same as the old size, realloc is guaranteed to change nothing and return the same address that you gave.

Allocating Cleared Space
------------------------

The function calloc allocates memory and clears it to zero.  It is declared in stdlib.h.

 -- Function: void * calloc (size_t COUNT, size_t ELTSIZE)
     This function allocates a block long enough to contain a vector of COUNT elements, each of size ELTSIZE.  Its contents are cleared to zero before calloc returns.

You could define calloc as follows:

     void *
     calloc (size_t count, size_t eltsize)
     {
       size_t size = count * eltsize;
       void *value = malloc (size);
       if (value != 0)
         memset (value, 0, size);
       return value;
     }

But in general, it is not guaranteed that calloc calls malloc internally.  Therefore, if an application provides its own malloc/realloc/free outside the C library, it should always define calloc, too.

Efficiency Considerations for malloc
------------------------------------

As opposed to other versions, the malloc in GNU libc does not round up block sizes to powers of two, neither for large nor for small sizes.  Neighboring chunks can be coalesced on a free no matter what their size is.  This makes the implementation suitable for all kinds of allocation patterns without generally incurring high memory waste through fragmentation.

Very large blocks (much larger than a page) are allocated with mmap (anonymous or via /dev/zero) by this implementation. This has the great advantage that these chunks are returned to the system immediately when they are freed.  Therefore, it cannot happen that a large chunk becomes ``locked'' in between smaller ones and even after calling free wastes memory.  The size threshold for mmap to be used can be adjusted with mallopt.  The use of mmap can also be disabled completely.

Allocating Aligned Memory Blocks
--------------------------------

The address of a block returned by malloc or realloc in the GNU system is always a multiple of eight (or sixteen on 64-bit systems).  If you need a block whose address is a multiple of a higher power of two than that, use memalign or valloc.  These functions are declared in stdlib.h.

With the GNU library, you can use free to free the blocks that memalign and valloc return.  That does not work in BSD, however---BSD does not provide any way to free such blocks.

 -- Function: void * memalign (size_t BOUNDARY, size_t SIZE)
     The memalign function allocates a block of SIZE bytes whose address is a multiple of BOUNDARY.  The BOUNDARY must be a power of two!  The function memalign works by allocating a somewhat larger block, and then returning an address within the block that is on the specified boundary.

 -- Function: void * valloc (size_t SIZE)
     Using valloc is like using memalign and passing the page size as the value of the second argument.  It is implemented like this:

          void *
          valloc (size_t size)
          {
            return memalign (getpagesize (), size);
          }

Malloc Tunable Parameters
-------------------------

You can adjust some parameters for dynamic memory allocation with the mallopt function.  This function is the general SVID/XPG interface, defined in malloc.h.

 -- Function: int mallopt (int PARAM, int VALUE)
     When calling mallopt, the PARAM argument specifies the parameter to be set, and VALUE the new value to be set.  Possible choices for PARAM, as defined in malloc.h, are:

     M_TRIM_THRESHOLD
          This is the minimum size (in bytes) of the top-most, releaseable chunk that will cause sbrk to be called with a negative argument in order to return memory to the system.

     M_TOP_PAD
          This parameter determines the amount of extra memory to obtain from the system when a call to sbrk is required.  It also specifies the number of bytes to retain when shrinking the heap by calling sbrk with a negative argument.  This provides the necessary hysteresis in heap size such that excessive amounts of system calls can be avoided.

     M_MMAP_THRESHOLD
          All chunks larger than this value are allocated outside the normal heap, using the mmap system call.  This way it is guaranteed that the memory for these chunks can be returned to the system on free.

     M_MMAP_MAX
          The maximum number of chunks to allocate with mmap.  Setting this to zero disables all use of mmap.

Heap Consistency Checking
-------------------------

You can ask malloc to check the consistency of dynamic storage by using the mcheck function.  This function is a GNU extension, declared in mcheck.h.

 -- Function: int mcheck (void (*ABORTFN) (enum mcheck_status STATUS))
     Calling mcheck tells malloc to perform occasional consistency checks.  These will catch things such as writing past the end of a block that was allocated with malloc.

     The ABORTFN argument is the function to call when an inconsistency is found.  If you supply a null pointer, then mcheck uses a default function which prints a message and calls abort (see Aborting a Program).  The function you supply is called with one argument, which says what sort of inconsistency was detected; its type is described below.

     It is too late to begin allocation checking once you have allocated anything with malloc.  So mcheck does nothing in that case.  The function returns -1 if you call it too late, and 0 otherwise (when it is successful).

     The easiest way to arrange to call mcheck early enough is to use the option -lmcheck when you link your program; then you don't need to modify your program source at all.  Alternatively you might use a debugger to insert a call to mcheck whenever the program is started, for example these gdb commands will automatically call mcheck whenever the program starts:

          (gdb) break main
          Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10
          (gdb) command 1
          Type commands for when breakpoint 1 is hit, one per line.
          End with a line saying just "end".
          >call mcheck(0)
          >continue
          >end
          (gdb) ...

     This will however only work if no initialization function of any object involved calls any of the malloc functions since mcheck must be called before the first such function.

 -- Function: enum mcheck_status mprobe (void *POINTER)
     The mprobe function lets you explicitly check for inconsistencies in a particular allocated block.  You must have already called mcheck at the beginning of the program, to do its occasional checks; calling mprobe requests an additional consistency check to be done at the time of the call.

     The argument POINTER must be a pointer returned by malloc or realloc.  mprobe returns a value that says what inconsistency, if any, was found.  The values are described below.

 -- Data Type: enum mcheck_status
     This enumerated type describes what kind of inconsistency was detected in an allocated block, if any.  Here are the possible values:

     MCHECK_DISABLED
          mcheck was not called before the first allocation. No consistency checking can be done.

     MCHECK_OK
          No inconsistency detected.

     MCHECK_HEAD
          The data immediately before the block was modified. This commonly happens when an array index or pointer is decremented too far.

     MCHECK_TAIL
          The data immediately after the block was modified. This commonly happens when an array index or pointer is incremented too far.

     MCHECK_FREE
          The block was already freed.

Another possibility to check for and guard against bugs in the use of malloc, realloc and free is to set the environment variable MALLOC_CHECK_.  When MALLOC_CHECK_ is set, a special (less efficient) implementation is used which is designed to be tolerant against simple errors, such as double calls of free with the same argument, or overruns of a single byte (off-by-one bugs).  Not all such errors can be protected against, however, and memory leaks can result.  If MALLOC_CHECK_ is set to 0, any detected heap corruption is silently ignored; if set to 1, a diagnostic is printed on stderr; if set to 2, abort is called immediately.  This can be useful because otherwise a crash may happen much later, and the true cause for the problem is then very hard to track down.

So, what's the difference between using MALLOC_CHECK_ and linking with -lmcheck?  MALLOC_CHECK_ is orthogonal with respect to -lmcheck.  -lmcheck has been added for backward compatibility.  Both MALLOC_CHECK_ and -lmcheck should uncover the same bugs - but using MALLOC_CHECK_ you don't need to recompile your application.

Storage Allocation Hooks
------------------------

The GNU C library lets you modify the behavior of malloc, realloc, and free by specifying appropriate hook functions.  You can use these hooks to help you debug programs that use dynamic storage allocation, for example.

The hook variables are declared in malloc.h.

 -- Variable: __malloc_hook
     The value of this variable is a pointer to the function that malloc uses whenever it is called.  You should define this function to look like malloc; that is, like:

          void *FUNCTION (size_t SIZE, void *CALLER)

     The value of CALLER is the return address found on the stack when the malloc function was called.  This value allows you to trace the memory consumption of the program.

 -- Variable: __realloc_hook
     The value of this variable is a pointer to function that realloc uses whenever it is called.  You should define this function to look like realloc; that is, like:

          void *FUNCTION (void *PTR, size_t SIZE, void *CALLER)

     The value of CALLER is the return address found on the stack when the realloc function was called.  This value allows to trace the memory consumption of the program.

 -- Variable: __free_hook
     The value of this variable is a pointer to function that free uses whenever it is called.  You should define this function to look like free; that is, like:

          void FUNCTION (void *PTR, void *CALLER)

     The value of CALLER is the return address found on the stack when the free function was called.  This value allows to trace the memory consumption of the program.

 -- Variable: __memalign_hook
     The value of this variable is a pointer to function that memalign uses whenever it is called.  You should define this function to look like memalign; that is, like:

          void *FUNCTION (size_t SIZE, size_t ALIGNMENT)

You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first!  Otherwise, your program will get stuck in an infinite recursion.  Before calling the function recursively, one should make sure to restore all the hooks to their previous value.  When coming back from the recursive call, all the hooks should be resaved since a hook might modify itself.

Here is an example showing how to use __malloc_hook and __free_hook properly.  It installs a function that prints out information every time malloc or free is called.  We just assume here that realloc and memalign are not used in our program.

     /* Global variables used to hold underlaying hook values.  */
     static void *(*old_malloc_hook) (size_t);
     static void (*old_free_hook) (void*);

     /* Prototypes for our hooks.  */
     static void *my_malloc_hook (size_t);
     static void my_free_hook(void*);

     static void *
     my_malloc_hook (size_t size)
     {
       void *result;
       /* Restore all old hooks */
       __malloc_hook = old_malloc_hook;
       __free_hook = old_free_hook;
       /* Call recursively */
       result = malloc (size);
       /* Save underlaying hooks */
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       /* printf might call malloc, so protect it too. */
       printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
       /* Restore our own hooks */
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
       return result;
     }

     static void *
     my_free_hook (void *ptr)
     {
       /* Restore all old hooks */
       __malloc_hook = old_malloc_hook;
       __free_hook = old_free_hook;
       /* Call recursively */
       free (ptr);
       /* Save underlaying hooks */
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       /* printf might call free, so protect it too. */
       printf ("freed pointer %p\n", ptr);
       /* Restore our own hooks */
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
     }

     main ()
     {
       ...
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
       ...
     }

The mcheck function (see Heap Consistency Checking) works by installing such hooks.

Statistics for Storage Allocation with malloc
---------------------------------------------

You can get information about dynamic storage allocation by calling the mallinfo function.  This function and its associated data type are declared in malloc.h; they are an extension of the standard SVID/XPG version.

 -- Data Type: struct mallinfo
     This structure type is used to return information about the dynamic storage allocator.  It contains the following members:

     int arena
          This is the total size of memory allocated with sbrk by malloc, in bytes.

     int ordblks
          This is the number of chunks not in use.  (The storage allocator internally gets chunks of memory from the operating system, and then carves them up to satisfy individual malloc requests; see Efficiency and Malloc.)

     int smblks
          This field is unused.

     int hblks
          This is the total number of chunks allocated with mmap.

     int hblkhd
          This is the total size of memory allocated with mmap, in bytes.

     int usmblks
          This field is unused.

     int fsmblks
          This field is unused.

     int uordblks
          This is the total size of memory occupied by chunks handed out by malloc.

     int fordblks
          This is the total size of memory occupied by free (not in use) chunks.

     int keepcost
          This is the size of the top-most releaseable chunk that normally borders the end of the heap (i.e. the ``brk'' of the process).

 -- Function: struct mallinfo mallinfo (void)
     This function returns information about the current dynamic memory usage in a structure of type struct mallinfo.

Summary of malloc-Related Functions
-----------------------------------

Here is a summary of the functions that work with malloc:

void *malloc (size_t SIZE)
     Allocate a block of SIZE bytes.  See Basic Allocation.

void free (void *ADDR)
     Free a block previously allocated by malloc.  See Freeing after Malloc.

void *realloc (void *ADDR, size_t SIZE)
     Make a block previously allocated by malloc larger or smaller, possibly by copying it to a new location.  See Changing Block Size.

void *calloc (size_t COUNT, size_t ELTSIZE)
     Allocate a block of COUNT * ELTSIZE bytes using malloc, and set its contents to zero.  See Allocating Cleared Space.

void *valloc (size_t SIZE)
     Allocate a block of SIZE bytes, starting on a page boundary. See Aligned Memory Blocks.

void *memalign (size_t SIZE, size_t BOUNDARY)
     Allocate a block of SIZE bytes, starting on an address that is a multiple of BOUNDARY.  See Aligned Memory Blocks.

int mallopt (int PARAM, int VALUE)
     Adjust a tunable parameter.  See Malloc Tunable Parameters.

int mcheck (void (*ABORTFN) (void))
     Tell malloc to perform occasional consistency checks on dynamically allocated memory, and to call ABORTFN when an inconsistency is found.  See Heap Consistency Checking.

void *(*__malloc_hook) (size_t SIZE, void *CALLER)
     A pointer to a function that malloc uses whenever it is called.

void *(*__realloc_hook) (void *PTR, size_t SIZE, void *CALLER)
     A pointer to a function that realloc uses whenever it is called.

void (*__free_hook) (void *PTR, void *CALLER)
     A pointer to a function that free uses whenever it is called.

void (*__memalign_hook) (size_t SIZE, size_t ALIGNMENT)
     A pointer to a function that memalign uses whenever it is called.

struct mallinfo mallinfo (void)
     Return information about the current dynamic memory usage. See Statistics of Malloc.

Allocation Debugging
====================

A complicated task when programming with languages which do not use garbage collected dynamic memory allocation is to find memory leaks. Long running programs must assure that dynamically allocated objects are freed at the end of their lifetime.  If this does not happen the system runs out of memory, sooner or later.

The malloc implementation in the GNU C library provides some simple means to detect such leaks and obtain some information to find the location.  To do this the application must be started in a special mode which is enabled by an environment variable.  There are no speed penalties for the program if the debugging mode is not enabled.

How to install the tracing functionality
----------------------------------------

 -- Function: void mtrace (void)
     When the mtrace function is called it looks for an environment variable named MALLOC_TRACE.  This variable is supposed to contain a valid file name.  The user must have write access.  If the file already exists it is truncated.  If the environment variable is not set or it does not name a valid file which can be opened for writing nothing is done.  The behaviour of malloc etc. is not changed. For obvious reasons this also happens if the application is installed with the SUID or SGID bit set.

     If the named file is successfully opened mtrace installs special handlers for the functions malloc, realloc, and free (see Hooks for Malloc).  From now on all uses of these functions are traced and protocolled into the file.  There is now of course a speed penalty for all calls to the traced functions so tracing should not be enabled during their normal use.

     This function is a GNU extension and generally not available on other systems.  The prototype can be found in mcheck.h.

 -- Function: void muntrace (void)
     The muntrace function can be called after mtrace was used to enable tracing the malloc calls.  If no (succesful) call of mtrace was made muntrace does nothing.

     Otherwise it deinstalls the handlers for malloc, realloc, and free and then closes the protocol file.  No calls are protocolled anymore and the program runs again at full speed.

     This function is a GNU extension and generally not available on other systems.  The prototype can be found in mcheck.h.

Example program excerpts
------------------------

Even though the tracing functionality does not influence the runtime behaviour of the program it is not a good idea to call mtrace in all programs.  Just imagine that you debug a program using mtrace and all other programs used in the debugging session also trace their malloc calls.  The output file would be the same for all programs and thus is unusable.  Therefore one should call mtrace only if compiled for debugging.  A program could therefore start like this:

     #include <mcheck.h>

     int
     main (int argc, char *argv[])
     {
     #ifdef DEBUGGING
       mtrace ();
     #endif
       ...
     }

This is all what is needed if you want to trace the calls during the whole runtime of the program.  Alternatively you can stop the tracing at any time with a call to muntrace.  It is even possible to restart the tracing again with a new call to mtrace.  But this can cause unreliable results since there may be calls of the functions which are not called.  Please note that not only the application uses the traced functions, also libraries (including the C library itself) use these functions.

This last point is also why it is no good idea to call muntrace before the program terminated.  The libraries are informed about the termination of the program only after the program returns from main or calls exit and so cannot free the memory they use before this time.

So the best thing one can do is to call mtrace as the very first function in the program and never call muntrace.  So the program traces almost all uses of the malloc functions (except those calls which are executed by constructors of the program or used libraries).

Some more or less clever ideas
------------------------------

You know the situation.  The program is prepared for debugging and in all debugging sessions it runs well.  But once it is started without debugging the error shows up.  A typical example is a memory leak that becomes visible only when we turn off the debugging.  If you foresee such situations you can still win.  Simply use something equivalent to the following little program:

     #include <mcheck.h>
     #include <signal.h>

     static void
     enable (int sig)
     {
       mtrace ();
       signal (SIGUSR1, enable);
     }

     static void
     disable (int sig)
     {
       muntrace ();
       signal (SIGUSR2, disable);
     }

     int
     main (int argc, char *argv[])
     {
       ...

       signal (SIGUSR1, enable);
       signal (SIGUSR2, disable);

       ...
     }

I.e., the user can start the memory debugger any time s/he wants if the program was started with MALLOC_TRACE set in the environment. The output will of course not show the allocations which happened before the first signal but if there is a memory leak this will show up nevertheless.

Interpreting the traces
-----------------------

If you take a look at the output it will look similar to this:

     = Start
      [0x8048209] - 0x8064cc8
      [0x8048209] - 0x8064ce0
      [0x8048209] - 0x8064cf8
      [0x80481eb] + 0x8064c48 0x14
      [0x80481eb] + 0x8064c60 0x14
      [0x80481eb] + 0x8064c78 0x14
      [0x80481eb] + 0x8064c90 0x14
     = End

What this all means is not really important since the trace file is not meant to be read by a human.  Therefore no attention is given to readability.  Instead there is a program which comes with the GNU C library which interprets the traces and outputs a summary in an user-friendly way.  The program is called mtrace (it is in fact a Perl script) and it takes one or two arguments.  In any case the name of the file with the trace output must be specified.  If an optional argument precedes the name of the trace file this must be the name of the program which generated the trace.

     drepper$ mtrace tst-mtrace log
     No memory leaks.

In this case the program tst-mtrace was run and it produced a trace file log.  The message printed by mtrace shows there are no problems with the code, all allocated memory was freed afterwards.

If we call mtrace on the example trace given above we would get a different outout:

     drepper$ mtrace errlog
     - 0x08064cc8 Free 2 was never alloc'd 0x8048209
     - 0x08064ce0 Free 3 was never alloc'd 0x8048209
     - 0x08064cf8 Free 4 was never alloc'd 0x8048209

     Memory not freed:
     -----------------
        Address     Size     Caller
     0x08064c48     0x14  at 0x80481eb
     0x08064c60     0x14  at 0x80481eb
     0x08064c78     0x14  at 0x80481eb
     0x08064c90     0x14  at 0x80481eb

We have called mtrace with only one argument and so the script has no chance to find out what is meant with the addresses given in the trace.  We can do better:

     drepper$ mtrace tst errlog
     - 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39
     - 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39
     - 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39

     Memory not freed:
     -----------------
        Address     Size     Caller
     0x08064c48     0x14  at /home/drepper/tst.c:33
     0x08064c60     0x14  at /home/drepper/tst.c:33
     0x08064c78     0x14  at /home/drepper/tst.c:33
     0x08064c90     0x14  at /home/drepper/tst.c:33

Suddenly the output makes much more sense and the user can see immediately where the function calls causing the trouble can be found.

Interpreting this output is not complicated.  There are at most two different situations being detected.  First, free was called for pointers which were never returned by one of the allocation functions. This is usually a very bad problem and what this looks like is shown in the first three lines of the output.  Situations like this are quite rare and if they appear they show up very drastically: the program normally crashes.

The other situation which is much harder to detect are memory leaks.  As you can see in the output the mtrace function collects all this information and so can say that the program calls an allocation function from line 33 in the source file /home/drepper/tst-mtrace.c four times without freeing this memory before the program terminates. Whether this is a real problem remains to be investigated.

Obstacks
========

An obstack is a pool of memory containing a stack of objects.  You can create any number of separate obstacks, and then allocate objects in specified obstacks.  Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other.

Aside from this one constraint of order of freeing, obstacks are totally general: an obstack can contain any number of objects of any size.  They are implemented with macros, so allocation is usually very fast as long as the objects are usually small.  And the only space overhead per object is the padding needed to start each object on a suitable boundary.

Creating Obstacks
-----------------

The utilities for manipulating obstacks are declared in the header file obstack.h.

 -- Data Type: struct obstack
     An obstack is represented by a data structure of type struct obstack.  This structure has a small fixed size; it records the status of the obstack and how to find the space in which objects are allocated. It does not contain any of the objects themselves.  You should not try to access the contents of the structure directly; use only the functions described in this chapter.

You can declare variables of type struct obstack and use them as obstacks, or you can allocate obstacks dynamically like any other kind of object.  Dynamic allocation of obstacks allows your program to have a variable number of different stacks.  (You can even allocate an obstack structure in another obstack, but this is rarely useful.)

All the functions that work with obstacks require you to specify which obstack to use.  You do this with a pointer of type struct obstack *.  In the following, we often say ``an obstack'' when strictly speaking the object at hand is such a pointer.

The objects in the obstack are packed into large blocks called chunks.  The struct obstack structure points to a chain of the chunks currently in use.

The obstack library obtains a new chunk whenever you allocate an object that won't fit in the previous chunk.  Since the obstack library manages chunks automatically, you don't need to pay much attention to them, but you do need to supply a function which the obstack library should use to get a chunk.  Usually you supply a function which uses malloc directly or indirectly.  You must also supply a function to free a chunk. These matters are described in the following section.

Preparing for Using Obstacks
----------------------------

Each source file in which you plan to use the obstack functions must include the header file obstack.h, like this:

     #include <obstack.h>

Also, if the source file uses the macro obstack_init, it must declare or define two functions or macros that will be called by the obstack library.  One, obstack_chunk_alloc, is used to allocate the chunks of memory into which objects are packed.  The other, obstack_chunk_free, is used to return chunks when the objects in them are freed.  These macros should appear before any use of obstacks in the source file.

Usually these are defined to use malloc via the intermediary xmalloc (see Unconstrained Allocation).  This is done with the following pair of macro definitions:

     #define obstack_chunk_alloc xmalloc
     #define obstack_chunk_free free

Though the storage you get using obstacks really comes from malloc, using obstacks is faster because malloc is called less often, for larger blocks of memory.  See Obstack Chunks, for full details.

At run time, before the program can use a struct obstack object as an obstack, it must initialize the obstack by calling obstack_init.

 -- Function: int obstack_init (struct obstack *OBSTACK-PTR)
     Initialize obstack OBSTACK-PTR for allocation of objects.  This function calls the obstack's obstack_chunk_alloc function.  If allocation of memory fails, the function pointed to by obstack_alloc_failed_handler is called.  The obstack_init function always returns 1 (Compatibility notice: Former versions of obstack returned 0 if allocation failed).

Here are two examples of how to allocate the space for an obstack and initialize it.  First, an obstack that is a static variable:

     static struct obstack myobstack;
     ...
     obstack_init (&myobstack);

Second, an obstack that is itself dynamically allocated:

     struct obstack *myobstack_ptr
       = (struct obstack *) xmalloc (sizeof (struct obstack));

     obstack_init (myobstack_ptr);

 -- Variable: obstack_alloc_failed_handler
     The value of this variable is a pointer to a function that obstack uses when obstack_chunk_alloc fails to allocate memory.  The default action is to print a message and abort. You should supply a function that either calls exit (see Program Termination) or longjmp (see Non-Local Exits) and doesn't return.

          void my_obstack_alloc_failed (void)
          ...
          obstack_alloc_failed_handler = &my_obstack_alloc_failed;

Allocation in an Obstack
------------------------

The most direct way to allocate an object in an obstack is with obstack_alloc, which is invoked almost like malloc.

 -- Function: void * obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)
     This allocates an uninitialized block of SIZE bytes in an obstack and returns its address.  Here OBSTACK-PTR specifies which obstack to allocate the block in; it is the address of the struct obstack object which represents the obstack.  Each obstack function or macro requires you to specify an OBSTACK-PTR as the first argument.

     This function calls the obstack's obstack_chunk_alloc function if it needs to allocate a new chunk of memory; it calls obstack_alloc_failed_handler if allocation of memory by obstack_chunk_alloc failed.

For example, here is a function that allocates a copy of a string STR in a specific obstack, which is in the variable string_obstack:

     struct obstack string_obstack;

     char *
     copystring (char *string)
     {
       size_t len = strlen (string) + 1;
       char *s = (char *) obstack_alloc (&string_obstack, len);
       memcpy (s, string, len);
       return s;
     }

To allocate a block with specified contents, use the function obstack_copy, declared like this:

 -- Function: void * obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     This allocates a block and initializes it by copying SIZE bytes of data starting at ADDRESS.  It calls obstack_alloc_failed_handler if allocation of memory by obstack_chunk_alloc failed.

 -- Function: void * obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     Like obstack_copy, but appends an extra byte containing a null character.  This extra byte is not counted in the argument SIZE.

The obstack_copy0 function is convenient for copying a sequence of characters into an obstack as a null-terminated string.  Here is an example of its use:

     char *
     obstack_savestring (char *addr, int size)
     {
       return obstack_copy0 (&myobstack, addr, size);
     }

Contrast this with the previous example of savestring using malloc (see Basic Allocation).

Freeing Objects in an Obstack
-----------------------------

To free an object allocated in an obstack, use the function obstack_free.  Since the obstack is a stack of objects, freeing one object automatically frees all other objects allocated more recently in the same obstack.

 -- Function: void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT)
     If OBJECT is a null pointer, everything allocated in the obstack is freed.  Otherwise, OBJECT must be the address of an object allocated in the obstack.  Then OBJECT is freed, along with everything allocated in OBSTACK since OBJECT.

Note that if OBJECT is a null pointer, the result is an uninitialized obstack.  To free all storage in an obstack but leave it valid for further allocation, call obstack_free with the address of the first object allocated on the obstack:

     obstack_free (obstack_ptr, first_object_allocated_ptr);

Recall that the objects in an obstack are grouped into chunks.  When all the objects in a chunk become free, the obstack library automatically frees the chunk (see Preparing for Obstacks).  Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.

Obstack Functions and Macros
----------------------------

The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler.  The obstack facility works with all C compilers, including both ISO C and traditional C, but there are precautions you must take if you plan to use compilers other than GNU C.

If you are using an old-fashioned non-ISO C compiler, all the obstack ``functions'' are actually defined only as macros.  You can call these macros like functions, but you cannot use them in any other way (for example, you cannot take their address).

Calling the macros requires a special precaution: namely, the first operand (the obstack pointer) may not contain any side effects, because it may be computed more than once.  For example, if you write this:

     obstack_alloc (get_obstack (), 4);

you will find that get_obstack may be called several times. If you use *obstack_list_ptr++ as the obstack pointer argument, you will get very strange results since the incrementation may occur several times.

In ISO C, each function has both a macro definition and a function definition.  The function definition is used if you take the address of the function without calling it.  An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here:

     char *x;
     void *(*funcp) ();
     /* Use the macro.  */
     x = (char *) obstack_alloc (obptr, size);
     /* Call the function.  */
     x = (char *) (obstack_alloc) (obptr, size);
     /* Take the address of the function.  */
     funcp = obstack_alloc;

This is the same situation that exists in ISO C for the standard library functions.  See Macro Definitions.

Warning: When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ISO C.

If you use the GNU C compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once.

Growing Objects
---------------

Because storage in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object.  With this technique, you do not need to know how much data you will put in the object until you come to the end of it.  We call this the technique of growing objects.  The special functions for adding data to the growing object are described in this section.

You don't need to do anything special when you start to grow an object. Using one of the functions to add data to the object automatically starts it.  However, it is necessary to say explicitly when the object is finished.  This is done with the function obstack_finish.

The actual address of the object thus built up is not known until the object is finished.  Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk.

While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object.  If you try to do so, the space already added to the growing object will become part of the other object.

 -- Function: void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)
     The most basic function for adding to a growing object is obstack_blank, which adds space without initializing it.

 -- Function: void obstack_grow (struct obstack *OBSTACK-PTR, void *DATA, int SIZE)
     To add a block of initialized space, use obstack_grow, which is the growing-object analogue of obstack_copy.  It adds SIZE bytes of data to the growing object, copying the contents from DATA.

 -- Function: void obstack_grow0 (struct obstack *OBSTACK-PTR, void *DATA, int SIZE)
     This is the growing-object analogue of obstack_copy0.  It adds SIZE bytes copied from DATA, followed by an additional null character.

 -- Function: void obstack_1grow (struct obstack *OBSTACK-PTR, char C)
     To add one character at a time, use the function obstack_1grow. It adds a single byte containing C to the growing object.

 -- Function: void obstack_ptr_grow (struct obstack *OBSTACK-PTR, void *DATA)
     Adding the value of a pointer one can use the function obstack_ptr_grow.  It adds sizeof (void *) bytes containing the value of DATA.

 -- Function: void obstack_int_grow (struct obstack *OBSTACK-PTR, int DATA)
     A single value of type int can be added by using the obstack_int_grow function.  It adds sizeof (int) bytes to the growing object and initializes them with the value of DATA.

 -- Function: void * obstack_finish (struct obstack *OBSTACK-PTR)
     When you are finished growing the object, use the function obstack_finish to close it off and return its final address.

     Once you have finished the object, the obstack is available for ordinary allocation or for growing another object.

     This function can return a null pointer under the same conditions as obstack_alloc (see Allocation in an Obstack).

When you build an object by growing it, you will probably need to know afterward how long it became.  You need not keep track of this as you grow the object, because you can find out the length from the obstack just before finishing the object with the function obstack_object_size, declared as follows:

 -- Function: int obstack_object_size (struct obstack *OBSTACK-PTR)
     This function returns the current size of the growing object, in bytes. Remember to call this function before finishing the object. After it is finished, obstack_object_size will return zero.

If you have started growing an object and wish to cancel it, you should finish it and then free it, like this:

     obstack_free (obstack_ptr, obstack_finish (obstack_ptr));

This has no effect if no object was growing.

You can use obstack_blank with a negative size argument to make the current object smaller.  Just don't try to shrink it beyond zero length---there's no telling what will happen if you do that.

Extra Fast Growing Objects
--------------------------

The usual functions for growing objects incur overhead for checking whether there is room for the new growth in the current chunk.  If you are frequently constructing objects in small steps of growth, this overhead can be significant.

You can reduce the overhead by using special ``fast growth'' functions that grow the object without checking.  In order to have a robust program, you must do the checking yourself.  If you do this checking in the simplest way each time you are about to add data to the object, you have not saved anything, because that is what the ordinary growth functions do.  But if you can arrange to check less often, or check more efficiently, then you make the program faster.

The function obstack_room returns the amount of room available in the current chunk.  It is declared as follows:

 -- Function: int obstack_room (struct obstack *OBSTACK-PTR)
     This returns the number of bytes that can be added safely to the current growing object (or to an object about to be started) in obstack OBSTACK using the fast growth functions.

While you know there is room, you can use these fast growth functions for adding data to a growing object:

 -- Function: void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char C)
     The function obstack_1grow_fast adds one byte containing the character C to the growing object in obstack OBSTACK-PTR.

 -- Function: void obstack_ptr_grow_fast (struct obstack *OBSTACK-PTR, void *DATA)
     The function obstack_ptr_grow_fast adds sizeof (void *) bytes containing the value of DATA to the growing object in obstack OBSTACK-PTR.

 -- Function: void obstack_int_grow_fast (struct obstack *OBSTACK-PTR, int DATA)
     The function obstack_int_grow_fast adds sizeof (int) bytes containing the value of DATA to the growing object in obstack OBSTACK-PTR.

 -- Function: void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)
     The function obstack_blank_fast adds SIZE bytes to the growing object in obstack OBSTACK-PTR without initializing them.

When you check for space using obstack_room and there is not enough room for what you want to add, the fast growth functions are not safe.  In this case, simply use the corresponding ordinary growth function instead.  Very soon this will copy the object to a new chunk; then there will be lots of room available again.

So, each time you use an ordinary growth function, check afterward for sufficient space using obstack_room.  Once the object is copied to a new chunk, there will be plenty of space again, so the program will start using the fast growth functions again.

Here is an example:

     void
     add_string (struct obstack *obstack, const char *ptr, int len)
     {
       while (len > 0)
         {
           int room = obstack_room (obstack);
           if (room == 0)
             {
               /* Not enough room. Add one character slowly,
                  which may copy to a new chunk and make room.  */
               obstack_1grow (obstack, *ptr++);
               len--;
             }
           else
             {
               if (room > len)
                 room = len;
               /* Add fast as much as we have room for. */
               len -= room;
               while (room-- > 0)
                 obstack_1grow_fast (obstack, *ptr++);
             }
         }
     }

Status of an Obstack
--------------------

Here are functions that provide information on the current status of allocation in an obstack.  You can use them to learn about an object while still growing it.

 -- Function: void * obstack_base (struct obstack *OBSTACK-PTR)
     This function returns the tentative address of the beginning of the currently growing object in OBSTACK-PTR.  If you finish the object immediately, it will have that address.  If you make it larger first, it may outgrow the current chunk---then its address will change!

     If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk).

 -- Function: void * obstack_next_free (struct obstack *OBSTACK-PTR)
     This function returns the address of the first free byte in the current chunk of obstack OBSTACK-PTR.  This is the end of the currently growing object.  If no object is growing, obstack_next_free returns the same value as obstack_base.

 -- Function: int obstack_object_size (struct obstack *OBSTACK-PTR)
     This function returns the size in bytes of the currently growing object. This is equivalent to

          obstack_next_free (OBSTACK-PTR) - obstack_base (OBSTACK-PTR)

Alignment of Data in Obstacks
-----------------------------

Each obstack has an alignment boundary; each object allocated in the obstack automatically starts on an address that is a multiple of the specified boundary.  By default, this boundary is 4 bytes.

To access an obstack's alignment boundary, use the macro obstack_alignment_mask, whose function prototype looks like this:

 -- Macro: int obstack_alignment_mask (struct obstack *OBSTACK-PTR)
     The value is a bit mask; a bit that is 1 indicates that the corresponding bit in the address of an object should be 0.  The mask value should be one less than a power of 2; the effect is that all object addresses are multiples of that power of 2.  The default value of the mask is 3, so that addresses are multiples of 4.  A mask value of 0 means an object can start on any multiple of 1 (that is, no alignment is required).

     The expansion of the macro obstack_alignment_mask is an lvalue, so you can alter the mask by assignment.  For example, this statement:

          obstack_alignment_mask (obstack_ptr) = 0;

     has the effect of turning off alignment processing in the specified obstack.

Note that a change in alignment mask does not take effect until after the next time an object is allocated or finished in the obstack.  If you are not growing an object, you can make the new alignment mask take effect immediately by calling obstack_finish. This will finish a zero-length object and then do proper alignment for the next object.

Obstack Chunks
--------------

Obstacks work by allocating space for themselves in large chunks, and then parceling out space in the chunks to satisfy your requests.  Chunks are normally 4096 bytes long unless you specify a different chunk size. The chunk size includes 8 bytes of overhead that are not actually used for storing objects.  Regardless of the specified size, longer chunks will be allocated when necessary for long objects.

The obstack library allocates chunks by calling the function obstack_chunk_alloc, which you must define.  When a chunk is no longer needed because you have freed all the objects in it, the obstack library frees the chunk by calling obstack_chunk_free, which you must also define.

These two must be defined (as macros) or declared (as functions) in each source file that uses obstack_init (see Creating Obstacks). Most often they are defined as macros like this:

     #define obstack_chunk_alloc malloc
     #define obstack_chunk_free free

Note that these are simple macros (no arguments).  Macro definitions with arguments will not work!  It is necessary that obstack_chunk_alloc or obstack_chunk_free, alone, expand into a function name if it is not itself a function name.

If you allocate chunks with malloc, the chunk size should be a power of 2.  The default chunk size, 4096, was chosen because it is long enough to satisfy many typical requests on the obstack yet short enough not to waste too much memory in the portion of the last chunk not yet used.

 -- Macro: int obstack_chunk_size (struct obstack *OBSTACK-PTR)
     This returns the chunk size of the given obstack.

Since this macro expands to an lvalue, you can specify a new chunk size by assigning it a new value.  Doing so does not affect the chunks already allocated, but will change the size of chunks allocated for that particular obstack in the future.  It is unlikely to be useful to make the chunk size smaller, but making it larger might improve efficiency if you are allocating many objects whose size is comparable to the chunk size.  Here is how to do so cleanly:

     if (obstack_chunk_size (obstack_ptr) < NEW-CHUNK-SIZE)
       obstack_chunk_size (obstack_ptr) = NEW-CHUNK-SIZE;

Summary of Obstack Functions
----------------------------

Here is a summary of all the functions associated with obstacks.  Each takes the address of an obstack (struct obstack *) as its first argument.

void obstack_init (struct obstack *OBSTACK-PTR)
     Initialize use of an obstack.  See Creating Obstacks.

void *obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)
     Allocate an object of SIZE uninitialized bytes. See Allocation in an Obstack.

void *obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     Allocate an object of SIZE bytes, with contents copied from ADDRESS.  See Allocation in an Obstack.

void *obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     Allocate an object of SIZE+1 bytes, with SIZE of them copied from ADDRESS, followed by a null character at the end. See Allocation in an Obstack.

void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT)
     Free OBJECT (and everything allocated in the specified obstack more recently than OBJECT).  See Freeing Obstack Objects.

void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)
     Add SIZE uninitialized bytes to a growing object. See Growing Objects.

void obstack_grow (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     Add SIZE bytes, copied from ADDRESS, to a growing object. See Growing Objects.

void obstack_grow0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)
     Add SIZE bytes, copied from ADDRESS, to a growing object, and then add another byte containing a null character.  See Growing Objects.

void obstack_1grow (struct obstack *OBSTACK-PTR, char DATA-CHAR)
     Add one byte containing DATA-CHAR to a growing object. See Growing Objects.

void *obstack_finish (struct obstack *OBSTACK-PTR)
     Finalize the object that is growing and return its permanent address. See Growing Objects.

int obstack_object_size (struct obstack *OBSTACK-PTR)
     Get the current size of the currently growing object.  See Growing Objects.

void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)
     Add SIZE uninitialized bytes to a growing object without checking that there is enough room.  See Extra Fast Growing.

void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char DATA-CHAR)
     Add one byte containing DATA-CHAR to a growing object without checking that there is enough room.  See Extra Fast Growing.

int obstack_room (struct obstack *OBSTACK-PTR)
     Get the amount of room now available for growing the current object. See Extra Fast Growing.

int obstack_alignment_mask (struct obstack *OBSTACK-PTR)
     The mask used for aligning the beginning of an object.  This is an lvalue.  See Obstacks Data Alignment.

int obstack_chunk_size (struct obstack *OBSTACK-PTR)
     The size for allocating chunks.  This is an lvalue.  See Obstack Chunks.

void *obstack_base (struct obstack *OBSTACK-PTR)
     Tentative starting address of the currently growing object. See Status of an Obstack.

void *obstack_next_free (struct obstack *OBSTACK-PTR)
     Address just after the end of the currently growing object. See Status of an Obstack.

Automatic Storage with Variable Size
====================================

The function alloca supports a kind of half-dynamic allocation in which blocks are allocated dynamically but freed automatically.

Allocating a block with alloca is an explicit action; you can allocate as many blocks as you wish, and compute the size at run time.  But all the blocks are freed when you exit the function that alloca was called from, just as if they were automatic variables declared in that function.  There is no way to free the space explicitly.

The prototype for alloca is in stdlib.h.  This function is a BSD extension.

 -- Function: void * alloca (size_t SIZE);
     The return value of alloca is the address of a block of SIZE bytes of storage, allocated in the stack frame of the calling function.

Do not use alloca inside the arguments of a function call---you will get unpredictable results, because the stack space for the alloca would appear on the stack in the middle of the space for the function arguments.  An example of what to avoid is foo (x, alloca (4), y).

alloca Example
--------------

As an example of the use of alloca, here is a function that opens a file name made from concatenating two argument strings, and returns a file descriptor or minus one signifying failure:

     int
     open2 (char *str1, char *str2, int flags, int mode)
     {
       char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
       stpcpy (stpcpy (name, str1), str2);
       return open (name, flags, mode);
     }

Here is how you would get the same results with malloc and free:

     int
     open2 (char *str1, char *str2, int flags, int mode)
     {
       char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
       int desc;
       if (name == 0)
         fatal ("virtual memory exceeded");
       stpcpy (stpcpy (name, str1), str2);
       desc = open (name, flags, mode);
       free (name);
       return desc;
     }

As you can see, it is simpler with alloca.  But alloca has other, more important advantages, and some disadvantages.

Advantages of alloca
--------------------

Here are the reasons why alloca may be preferable to malloc:

  * Using alloca wastes very little space and is very fast.  (It is open-coded by the GNU C compiler.)

  * Since alloca does not have separate pools for different sizes of block, space used for any size block can be reused for any other size. alloca does not cause storage fragmentation.

  * Nonlocal exits done with longjmp (see Non-Local Exits) automatically free the space allocated with alloca when they exit through the function that called alloca.  This is the most important reason to use alloca.

     To illustrate this, suppose you have a function open_or_report_error which returns a descriptor, like open, if it succeeds, but does not return to its caller if it fails.  If the file cannot be opened, it prints an error message and jumps out to the command level of your program using longjmp. Let's change open2 (see Alloca Example) to use this subroutine:

          int
          open2 (char *str1, char *str2, int flags, int mode)
          {
            char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
            stpcpy (stpcpy (name, str1), str2);
            return open_or_report_error (name, flags, mode);
          }

     Because of the way alloca works, the storage it allocates is freed even when an error occurs, with no special effort required.

     By contrast, the previous definition of open2 (which uses malloc and free) would develop a storage leak if it were changed in this way.  Even if you are willing to make more changes to fix it, there is no easy way to do so.

Disadvantages of alloca
-----------------------

These are the disadvantages of alloca in comparison with malloc:

  * If you try to allocate more storage than the machine can provide, you don't get a clean error message.  Instead you get a fatal signal like the one you would get from an infinite recursion; probably a segmentation violation (see Program Error Signals).

  * Some non-GNU systems fail to support alloca, so it is less portable.  However, a slower emulation of alloca written in C is available for use on systems with this deficiency.

GNU C Variable-Size Arrays
--------------------------

In GNU C, you can replace most uses of alloca with an array of variable size.  Here is how open2 would look then:

     int open2 (char *str1, char *str2, int flags, int mode)
     {
       char name[strlen (str1) + strlen (str2) + 1];
       stpcpy (stpcpy (name, str1), str2);
       return open (name, flags, mode);
     }

But alloca is not always equivalent to a variable-sized array, for several reasons:

  * A variable size array's space is freed at the end of the scope of the name of the array.  The space allocated with alloca remains until the end of the function.

  * It is possible to use alloca within a loop, allocating an additional block on each iteration.  This is impossible with variable-sized arrays.

Note: If you mix use of alloca and variable-sized arrays within one function, exiting a scope in which a variable-sized array was declared frees all blocks allocated with alloca during the execution of that scope.