malloc(3) — Linux manual page


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

NAME         top

       malloc, free, calloc, realloc, reallocarray - allocate and free
       dynamic 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

           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 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
       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) (this behavior is nonportable;
       see NOTES).  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 equivalent to the

               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         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

       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 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         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(), calloc(), realloc() │ Thread safety │ MT-Safe │

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

       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.

   Nonportable behavior
       The behavior of realloc() when size is equal to zero, and ptr is
       not NULL, is glibc specific; other implementations may return
       NULL, and set errno.  Portable POSIX programs should avoid it.
       See realloc(3p).

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 5.13 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                            2021-03-22                      MALLOC(3)

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