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

NAME         top

       malloc, free, calloc, realloc - 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);

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().

       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).
       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         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 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.

ERRORS         top

       calloc(), malloc(), and realloc() 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         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

       POSIX.1-2001, POSIX.1-2008, C89, C99.

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).  Allocations performed using mmap(2) are
       unaffected by the RLIMIT_DATA resource limit (see getrlimit(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

       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         top

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GNU                              2015-08-08                        MALLOC(3)