free

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
           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
       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).  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 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).

       ┌─────────────────────┬───────────────┬─────────┐
       │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
       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/overcommit_memory and /proc/sys/vm/oom_adj in proc(5), and
       the Linux kernel source file Documentation/vm/overcommit-
       accounting.rst.

       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
       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 5.01 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                               2019-03-06                         MALLOC(3)