realloc(3) — Linux manual page


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

NAME         top

       malloc,  free,  calloc, realloc, reallocarray - allocate and free dy‐
       namic memory

SYNOPSIS         top

       #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)):

           Since glibc 2.29:
           Glibc 2.28 and earlier:

DESCRIPTION         top

       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

       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 malloc(), 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 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).  Un‐
       less 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 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 over‐
       flow occurs, reallocarray() returns NULL, sets errno to ENOMEM, and
       leaves the original block of memory unchanged.

RETURN VALUE         top

       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, 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 untouched; it is not freed or

       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         top

       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).

VERSIONS         top

       reallocarray() first appeared in glibc in version 2.26.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface            Attribute     Value   │
       │malloc(), free(),    │ Thread safety │ MT-Safe │
       │calloc(), realloc()  │               │         │

CONFORMING TO         top

       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         top

       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-

       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).  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 contention
       for these mutexes.  To scalably handle memory allocation in
       multithreaded applications, 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 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         top

       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 

COLOPHON         top

       This page is part of release 5.09 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

GNU                              2020-06-09                        MALLOC(3)

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