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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.

       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().  Other-
       wise, 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().

RETURN VALUE
       For calloc() and malloc(), the value returned is a pointer to the allo-
       cated  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
       ANSI-C

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(), free() or realloc() 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  vari-
       ables.  When MALLOC_CHECK_ is set, a special (less efficient) implemen-
       tation 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 immedi-
       ately.  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 suf-
       ficiently 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)
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