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MALLOC(3)                  Linux Programmer's Manual                 MALLOC(3)

NAME
       calloc, malloc, free, realloc - Allocate and free dynamic memory

SYNOPSIS
       #include <stdlib.h>

       void *calloc(size_t nmemb, size_t size);
       void *malloc(size_t size);
       void free(void *ptr);
       void *realloc(void *ptr, size_t size);

DESCRIPTION
       calloc() allocates memory for an array of nmemb elements of size bytes
       each and returns a pointer to the allocated memory.  The memory is set
       to zero.  If nmemb or size is 0, then calloc() returns either NULL, or
       a unique pointer value that can later be successfully passed to free().

       malloc() allocates size bytes and returns a pointer to the allocated
       memory.  The memory is not cleared.  If size is 0, then malloc()
       returns either NULL, or a unique pointer value that can later be
       successfully passed to free().

       free() frees the memory space pointed to by ptr, which must have been
       returned by a previous call to malloc(), calloc() or realloc().
       Otherwise, or if free(ptr) has already been called before, undefined
       behavior occurs.  If ptr is NULL, no operation is performed.

       realloc() changes the size of the memory block pointed to by ptr to
       size bytes.  The contents will be unchanged to the minimum of the old
       and new sizes; newly allocated memory will be uninitialized.  If ptr is
       NULL, then the call is equivalent to malloc(size), for all values of
       size; if size is equal to zero, and ptr is not NULL, then the call is
       equivalent to free(ptr).  Unless ptr is NULL, it must have been
       returned by an earlier call to malloc(), calloc() or realloc().  If the
       area pointed to was moved, a free(ptr) is done.

RETURN VALUE
       For calloc() and malloc(), return a pointer to the allocated memory,
       which is suitably aligned for any kind of variable.  On error, these
       functions return NULL.  NULL may also be returned by a successful call
       to malloc() with a size of zero, or by a successful call to calloc()
       with nmemb or size equal to zero.

       free() returns no value.

       realloc() returns a pointer to the newly allocated memory, which is
       suitably aligned for any kind of variable and may be different from
       ptr, or NULL if the request fails.  If size was equal to 0, either NULL
       or a pointer suitable to be passed to free() is returned.  If realloc()
       fails the original block is left untouched; it is not freed or moved.

CONFORMING TO
       C89, C99.

NOTES
       Normally, malloc() allocates memory from the heap, and adjusts the size
       of the heap as required, using sbrk(2).  When allocating blocks of
       memory larger than MMAP_THRESHOLD bytes, the glibc malloc()
       implementation allocates the memory as a private anonymous mapping
       using mmap(2).  MMAP_THRESHOLD is 128 kB by default, but is adjustable
       using mallopt(3).  Allocations performed using mmap(2) are unaffected
       by the RLIMIT_DATA resource limit (see getrlimit(2)).

       The Unix98 standard requires malloc(), calloc(), and realloc() to set
       errno to ENOMEM upon failure.  Glibc assumes that this is done (and the
       glibc versions of these routines do this); if you use a private malloc
       implementation that does not set errno, then certain library routines
       may fail without having a reason in errno.

       Crashes in malloc(), calloc(), realloc(), or free() are almost always
       related to heap corruption, such as overflowing an allocated chunk or
       freeing the same pointer twice.

       Recent versions of Linux libc (later than 5.4.23) and glibc (2.x)
       include a malloc() implementation which is tunable via environment
       variables.  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 message is printed on stderr; if set
       to 2, abort(3) is called immediately; if set to 3, a diagnostic message
       is printed on stderr and the program is aborted.  Using a nonzero
       MALLOC_CHECK_ value 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.

BUGS
       By default, Linux follows an optimistic memory allocation strategy.
       This means that when malloc() returns non-NULL there is no guarantee
       that the memory really is available.  This is a really bad bug.  In
       case it turns out that the system is out of memory, one or more
       processes will be killed by the infamous OOM killer.  In case Linux is
       employed under circumstances where it would be less desirable to
       suddenly lose some randomly picked processes, and moreover the kernel
       version is sufficiently recent, one can switch off this overcommitting
       behavior using a command like:

           # echo 2 > /proc/sys/vm/overcommit_memory

       See also the kernel Documentation directory, files
       vm/overcommit-accounting and sysctl/vm.txt.

SEE ALSO
       brk(2), mmap(2), alloca(3), posix_memalign(3)

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

GNU                               2009-01-13                         MALLOC(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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