realloc

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



NAME
       malloc, free, calloc, realloc, reallocarray - 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 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, 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
       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
       reallocarray() first appeared in glibc in version 2.26.

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 5.09 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
       https://www.kernel.org/doc/man-pages/.




GNU                                2020-06-09                          MALLOC(3)