realloc

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():
           Since glibc 2.29:
               _DEFAULT_SOURCE
           Glibc 2.28 and earlier:
               _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().  If the multiplication of nmemb and size would result
       in integer overflow, then calloc() returns an error.  By  contrast,  an
       integer  overflow  would  not be detected in the following call to mal-
       loc(), with the result that an incorrectly sized block of memory  would
       be allocated:

           malloc(nmemb * size);

       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(),  cal-
       loc(),  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 equivalent 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 oc-
       curs, 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  cal-
       loc() 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, or NULL if the request
       failed.   The returned pointer may be the same as ptr if the allocation
       was not moved (e.g., there was room to expand the allocation in-place),
       or different from ptr if the allocation was moved to a new address.  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 un-
       touched; 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  at-
       tributes(7).

       +---------------------+---------------+---------+
       |Interface            | Attribute     | Value   |
       +---------------------+---------------+---------+
       |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  Open-
       BSD 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/overcommit_memory and /proc/sys/vm/oom_adj in proc(5), and
       the   Linux  kernel  source  file  Documentation/vm/overcommit-account-
       ing.rst.

       Normally, malloc() allocates memory from the heap, and adjusts the size
       of the heap as required, using sbrk(2).  When allocating blocks of mem-
       ory 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  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 in-
       ternally to protect the memory-management data structures  employed  by
       these  functions.   In a multithreaded application in which threads si-
       multaneously allocate and free memory, there could  be  contention  for
       these  mutexes.   To scalably handle memory allocation in multithreaded
       applications, glibc creates additional memory allocation arenas if  mu-
       tex  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 corruption, 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)

       For details of the GNU C library implementation, see
       <https://sourceware.org/glibc/wiki/MallocInternals>.

COLOPHON
       This page is part of release 5.05 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                               2020-02-09                         MALLOC(3)