malloc.3

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.

       malloc() allocates size bytes and returns a pointer to the allocated
       memory.  The memory is not cleared.

       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
       behaviour 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, the call is equivalent to malloc(size); if size is equal to zero,
       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(), the value returned is a pointer to the
       allocated memory, which is suitably aligned for any kind of variable,
       or NULL if the request fails.

       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.

SEE ALSO
       brk(2), posix_memalign(3)

NOTES
       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 GNU libc (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 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.

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.



GNU                               1993-04-04                         MALLOC(3)