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

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

SYNOPSIS
       #include <stdlib.h>

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

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       reallocarray():
           _GNU_SOURCE

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

       The  free()  function  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.

       The calloc() function 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().

       The  realloc()  function  changes  the  size of the memory block pointed to by ptr to size
       bytes.  The contents will be unchanged in the range from the start of the region up to the
       minimum  of the old and new sizes.  If the new size is larger than the old size, the added
       memory will not be initialized.  If ptr is NULL, then  the  call  is  equivalent  to  mal-
       loc(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.

       The reallocarray() function changes the size of the memory block pointed to by ptr  to  be
       large  enough  for an array of nmemb elements, each of which is size bytes.  It is equiva-
       lent to the call

               realloc(ptr, nmemb * size);

       However, unlike that realloc() call, reallocarray() fails safely in  the  case  where  the
       multiplication  would  overflow.  If such an overflow occurs, reallocarray() returns NULL,
       sets errno to ENOMEM, and leaves the original block of memory unchanged.

RETURN VALUE
       The malloc() and calloc() functions return a pointer to the  allocated  memory,  which  is
       suitably  aligned for any built-in type.  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.

       The free() function returns no value.

       The  realloc() function returns a pointer to the newly allocated memory, which is suitably
       aligned for any built-in type 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.

       On  success,  the reallocarray() function returns a pointer to the newly allocated memory.
       On failure, it returns NULL and the original block of memory is left untouched.

ERRORS
       calloc(), malloc(), realloc(), and reallocarray() can fail with the following error:

       ENOMEM Out of memory.  Possibly, the application hit the RLIMIT_AS  or  RLIMIT_DATA  limit
              described in getrlimit(2).

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       -Œ----------------------¬----------------¬----------
       -‚Interface            -‚ Attribute     -‚ Value   -‚
       "----------------------¼----------------¼----------¤
       -‚malloc(), free(),    -‚ Thread safety -‚ MT-Safe -‚
       -‚calloc(), realloc()  -‚               -‚         -‚
       -----------------------´----------------´---------"
CONFORMING TO
       malloc(), free(), calloc(), realloc(): POSIX.1-2001, POSIX.1-2008, C89, C99.

       reallocarray()  is  a nonstandard extension that first appeared in OpenBSD 5.6 and FreeBSD
       11.0.

NOTES
       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.  In
       case it turns out that the system is out of memory, one or more processes will  be  killed
       by  the  OOM  killer.   For more information, see the description of /proc/sys/vm/overcom-
       mit_memory and /proc/sys/vm/oom_adj in proc(5), and the Linux kernel source file  Documen-
       tation/vm/overcommit-accounting.

       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 map-
       ping using mmap(2).  MMAP_THRESHOLD is 128 kB by default, but  is  adjustable  using  mal-
       lopt(3).   Prior  to  Linux 4.7 allocations performed using mmap(2) were unaffected by the
       RLIMIT_DATA resource limit; since Linux 4.7, this limit is also enforced  for  allocations
       performed using mmap(2).

       To  avoid corruption in multithreaded applications, mutexes are used internally to protect
       the memory-management data structures employed by these  functions.   In  a  multithreaded
       application  in which threads simultaneously allocate and free memory, there could be con-
       tention for these mutexes.  To scalably handle memory allocation in multithreaded applica-
       tions,  glibc creates additional memory allocation arenas if mutex contention is detected.
       Each arena is a large region of memory that is internally allocated by the  system  (using
       brk(2) or mmap(2)), and managed with its own mutexes.

       SUSv2  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 cor-
       ruption, such as overflowing an allocated chunk or freeing the same pointer twice.

       The  malloc()  implementation  is  tunable  via  environment variables; see mallopt(3) for
       details.

SEE ALSO
       valgrind(1), brk(2), mmap(2), alloca(3), malloc_get_state(3), malloc_info(3),
       malloc_trim(3), malloc_usable_size(3), mallopt(3), mcheck(3), mtrace(3), posix_memalign(3)

COLOPHON
       This page is part of release 4.15 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.

GNU                                         2017-09-15                                  MALLOC(3)