*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->NetBSD man pages -> realloc (3)              



NAME    [Toc]    [Back]

     malloc, calloc, realloc, free - general purpose memory allocation functions

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <stdlib.h>

     void *
     malloc(size_t size);

     void *
     calloc(size_t number, size_t size);

     void *
     realloc(void *ptr, size_t size);

     free(void *ptr);

     char * malloc_options;

DESCRIPTION    [Toc]    [Back]

     The malloc() function allocates size bytes of memory.  The allocated
     space is suitably aligned (after possible pointer coercion) for storage
     of any type of object.  If the space is at least pagesize bytes in length
     (see getpagesize(3)), the returned memory will be page boundary aligned
     as well.  If malloc() fails, a NULL pointer is returned, and the errno
     variable is set to ENOMEM.

     The calloc() function allocates space for number objects, each size bytes
     in length.  The result is identical to calling malloc() with an argument
     of ``number * size'', with the exception that the allocated memory is
     initialized to all bits zero.

     The realloc() function changes the size of the previously allocated memory
 referenced by ptr to size bytes.  The contents of the memory are
     unchanged up to the lesser of the new and old sizes.  If the new size is
     larger, the value of the newly allocated portion of the memory is undefined.
  If the requested memory cannot be allocated, NULL is returned and
     the memory referenced by ptr is valid and unchanged.  If ptr is NULL, the
     realloc() function behaves identically to malloc() for the specified

     The free() function causes the allocated memory referenced by ptr to be
     made available for future allocations.  If ptr is NULL, no action occurs.

TUNING    [Toc]    [Back]

     Once, when the first call is made to one of these memory allocation routines,
 various flags will be set or reset, which affect the workings of
     this allocation implementation.

     The ``name'' of the file referenced by the symbolic link named
     /etc/malloc.conf, the value of the environment variable MALLOC_OPTIONS,
     and the string pointed to by the global variable malloc_options will be
     interpreted, in that order, character by character as flags.

     Most flags are single letters, where uppercase indicates that the behavior
 is set, or on, and lowercase means that the behavior is not set, or

     A       All warnings (except for the warning about unknown flags being
             set), and failure to allocate memory become fatal.  The process
             will call abort(3) in these cases.

     J       Each byte of new memory allocated by malloc() or realloc() as
             well as all memory returned by free() or realloc() will be initialized
 to 0xd0.  This options also sets the ``R'' option.  This
             is intended for debugging and will impact performance negatively.

     H       Pass a hint to the kernel about pages unused by the allocation
             functions.  This will help performance if the system is paging
             excessively.  This option is off by default.

     R       Causes the realloc() function to always reallocate memory even if
             the initial allocation was sufficiently large.  This can substantially
 aid in compacting memory.

     U       Generate ``utrace'' entries for ktrace(1), for all operations.
             Consult the source for details on this option.

     V       Attempting to allocate zero bytes will return a NULL pointer
             instead of a valid pointer.  (The default behavior is to make a
             minimal allocation and return a pointer to it.)  This option is
             provided for System V compatibility.

     X       Rather than return failure for any allocation function, display a
             diagnostic message on stderr and cause the program to drop core
             (using abort(3)).  This option should be set at compile time by
             including the following in the source code:

                   extern char *malloc_options;
                   malloc_options = "X";

     Z       This option implicitly sets the ``J'' and ``R'' options, and then
             zeros out the bytes that were requested.  This is intended for
             debugging and will impact performance negatively.

     <       Reduce the size of the cache by a factor of two.  The default
             cache size is 16 pages.  This option can be specified multiple

     >       Double the size of the cache by a factor of two.  The default
             cache size is 16 pages.  This option can be specified multiple

     The ``J'' and ``Z'' options are intended for testing and debugging.  An
     application which changes its behavior when these options are used is

RETURN VALUES    [Toc]    [Back]

     The malloc() and calloc() functions return a pointer to the allocated
     memory if successful; otherwise a NULL pointer is returned.

     The realloc() function returns a pointer, possibly identical to ptr, to
     the allocated memory if successful; otherwise a NULL pointer is returned,
     in which case the memory referenced by ptr is still available and intact.

     The free() function returns no value.

ENVIRONMENT    [Toc]    [Back]

     The following environment variables affect the execution of the allocation

          If the environment variable MALLOC_OPTIONS is set, the characters it
          contains will be interpreted as flags to the allocation functions.

EXAMPLES    [Toc]    [Back]

     To set a systemwide reduction of cache size, and to dump core whenever a
     problem occurs:

           ln -s 'A<' /etc/malloc.conf

     To specify in the source that a program does no return value checking on
     calls to these functions:

           extern char *malloc_options;
           malloc_options = "X";


     The major difference between this implementation and other allocation
     implementations is that the free pages are not accessed unless allocated,
     and are aggressively returned to the kernel for reuse.

           Most  allocation  implementations  will store a data structure containing
 a linked list in the free chunks of memory, used to tie all
           the  free  memory  together.  That can be suboptimal, as every time
           the free-list is traversed, the otherwise unused, and likely  paged
           out,  pages  are faulted into primary memory.  On systems which are
           paging, this can result in a factor of five increase in the  number
           of page-faults done by a process.

     A side effect of this architecture is that many minor transgressions on
     the interface which would traditionally not be detected are in fact
     detected.  As a result, programs that have been running happily for years
     may suddenly start to complain loudly, when linked with this allocation

     The first and most important thing to do is to set the ``A'' option.
     This option forces a coredump (if possible) at the first sign of trouble,
     rather than the normal policy of trying to continue if at all possible.

     It is probably also a good idea to recompile the program with suitable
     options and symbols for debugger support.

     If the program starts to give unusual results, coredump or generally
     behave differently without emitting any of the messages listed in the
     next section, it is likely because it depends on the storage being filled
     with nul bytes.  Try running it with ``Z'' option set; if that improves
     the situation, this diagnosis has been confirmed.  If the program still
     misbehaves, the likely problem is accessing memory outside the allocated
     area, more likely after than before the allocated area.

     Alternatively, if the symptoms are not easy to reproduce, setting the
     ``J'' option may help provoke the problem.

     In truly difficult cases, the ``U'' option, if supported by the kernel,
     can provide a detailed trace of all calls made to these functions.

     Unfortunately this implementation does not provide much detail about the
     problems it detects, the performance impact for storing such information
     would be prohibitive.  There are a number of allocation implementations
     available on the 'Net which focus on detecting and pinpointing problems
     by trading performance for extra sanity checks and detailed diagnostics.


     If malloc(), calloc(), realloc() or free() detect an error or warning
     condition, a message will be printed to file descriptor STDERR_FILENO.
     Errors will result in the process dumping core.  If the ``A'' option is
     set, all warnings are treated as errors.

     The following is a brief description of possible error messages and their

     (ES): mumble mumble mumble
             The allocation functions were compiled with ``EXTRA_SANITY''
             defined, and an error was found during the additional error
             checking.  Consult the source code for further information.

     allocation failed
             If the ``A'' option is specified it is a fatal error for an allocation
 function to fail.

     mmap(2) failed, check limits
             This most likely means that the system is dangerously overloaded
             or that the process' limits are incorrectly specified.

     freelist is destroyed
             The internal free-list has been corrupted.

     The following is a brief description of possible warning messages and
     their meanings:

     chunk/page is already free
             The process attempted to free() memory which had already been

     junk pointer ...
             A pointer specified to one of the allocation functions points
             outside the bounds of the memory of which they are aware.

     malloc() has never been called
             No memory has been allocated, yet something is being freed or

     modified (chunk-/page-) pointer
             The pointer passed to free() or realloc() has been modified.

     pointer to wrong page
             The pointer that malloc() or calloc() is trying to free does not
             reference a possible page.

     recursive call
             A process has attempted to call an allocation function recursively.
  This is not permitted.  In particular, signal handlers
             should not attempt to allocate memory.

     out of memory
             The ``X'' option was specified and an allocation of memory

     unknown char in MALLOC_OPTIONS
             An unknown option was specified.  Even with the ``A'' option set,
             this warning is still only a warning.

SEE ALSO    [Toc]    [Back]

     brk(2), alloca(3), getpagesize(3), memory(3)

STANDARDS    [Toc]    [Back]

     The malloc(), calloc(), realloc() and free() functions conform to ANSI
     X3.159-1989 (``ANSI C'').

HISTORY    [Toc]    [Back]

     The present allocation implementation started out as a filesystem for a
     drum attached to a 20bit binary challenged computer which was built with
     discrete germanium transistors.  It has since graduated to handle primary
     storage rather than secondary.  It first appeared in its new shape and
     ability in FreeBSD 2.2, and then in NetBSD 1.5.

BUGS    [Toc]    [Back]

     The messages printed in case of problems provide no detail about the
     actual values.

     It can be argued that returning a null pointer when asked to allocate
     zero bytes is a silly response to a silly question.

     This implementation was authored by Poul-Henning Kamp.  Please report any
     problems to him at <phk@FreeBSD.org>.

BSD                             August 2, 1999                             BSD
[ Back ]
 Similar pages
Name OS Title
memory NetBSD general memory allocation operations
memory FreeBSD general memory allocation operations
malloc OpenBSD memory allocation and deallocation
realloc OpenBSD memory allocation and deallocation
cfree OpenBSD memory allocation and deallocation
calloc OpenBSD memory allocation and deallocation
free OpenBSD memory allocation and deallocation
malloc.conf OpenBSD memory allocation and deallocation
xconfirm IRIX general purpose dialog box
memget FreeBSD memory allocation/deallocation system
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service