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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  opera-
       tion 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 allo-
       cated 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 suit-
       ably 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 mal-
       loc() implementation  which  is  tunable  via  environment  variables.   When  MAL-
       LOC_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  MAL-
       LOC_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 imme-
       diately;  if set to 3, a diagnostic message is printed on stderr and the program is
       aborted.  Using a non-zero 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 sud-
       denly  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.

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

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



GNU                               2009-01-13                         MALLOC(3)

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