realloc(3)

NAME#

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

LIBRARY#

Standard C library (libc, -lc)

SYNOPSIS#

#include <stdlib.h>

void *malloc(size_t \nsize\n);\n
\nvoid free(void *_Nullable \nptr\n);\n
\nvoid *calloc(size_t \nnmemb\n, size_t \nsize\n);\n
\nvoid *realloc(void *_Nullable \nptr\n, size_t \nsize\n);\n
\nvoid *reallocarray(void *_Nullable \nptr\n, size_t \nnmemb\n, size_t \nsize\n);

reallocarray():

\n
    Since glibc 2.29:
\n
        _DEFAULT_SOURCE
\n
    glibc 2.28 and earlier:
\n
        _GNU_SOURCE

DESCRIPTION#

malloc()#

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 a unique pointer value that can later be successfully passed to free(). (See "Nonportable behavior" for portability issues.)

free()#

The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc() or related functions. Otherwise, or if ptr has already been freed, undefined behavior occurs. If ptr is NULL, no operation is performed.

calloc()#

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 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);

realloc()#

The realloc() function changes the size of the memory block pointed to by ptr to size bytes. The contents of the memory 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) (but see "Nonportable behavior" for portability issues).

Unless ptr is NULL, it must have been returned by an earlier call to malloc or related functions. If the area pointed to was moved, a free(ptr) is done.

reallocarray()#

The reallocarray() function changes the size of (and possibly moves) 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 an error.

RETURN VALUE#

The malloc(), calloc(), realloc(), and reallocarray() functions return a pointer to the allocated memory, which is suitably aligned for any type that fits into the requested size or less. On error, these functions return NULL and set errno. Attempting to allocate more than PTRDIFF_MAX bytes is considered an error, as an object that large could cause later pointer subtraction to overflow.

The free() function returns no value, and preserves errno.

The realloc() and reallocarray() functions return NULL if ptr is not NULL and the requested size is zero; this is not considered an error. (See "Nonportable behavior" for portability issues.) Otherwise, 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 these functions fail, the original block is left untouched; it is not freed or moved.

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). Another reason could be that the number of mappings created by the caller process exceeded the limit specified by /proc/sys/vm/max_map_count.

ATTRIBUTES#

For an explanation of the terms used in this section, see attributes(7).

STANDARDS#

malloc()

free()

calloc()

realloc()

C11, POSIX.1-2008.

reallocarray()

None.

HISTORY#

malloc()

free()

calloc()

realloc()

POSIX.1-2001, C89.

reallocarray()

glibc 2.26. OpenBSD 5.6, FreeBSD 11.0.

malloc() and related functions rejected sizes greater than PTRDIFF_MAX starting in glibc 2.30.

free() preserved errno starting in glibc 2.33.

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.

If your program uses a private memory allocator, it should do so by replacing malloc(), free(), calloc(), and realloc(). The replacement functions must implement the documented glibc behaviors, including errno handling, size-zero allocations, and overflow checking; otherwise, other library routines may crash or operate incorrectly. For example, if the replacement free() does not preserve errno, then seemingly unrelated library routines may fail without having a valid reason in errno. Private memory allocators may also need to replace other glibc functions; see "Replacing malloc" in the glibc manual for details.

Crashes in memory allocators 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 these functions when the requested size is zero is glibc specific; other implementations may return NULL without setting errno, and portable POSIX programs should tolerate such behavior. See realloc(3p).

POSIX requires memory allocators to set errno upon failure. However, the C standard does not require this, and applications portable to non-POSIX platforms should not assume this.

Portable programs should not use private memory allocators, as POSIX and the C standard do not allow replacement of malloc(), free(), calloc(), and realloc().

EXAMPLES#

#include <err.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MALLOCARRAY(n, type)  ((type *) my_mallocarray(n, sizeof(type)))
#define MALLOC(type)          MALLOCARRAY(1, type)
static inline void *my_mallocarray(size_t nmemb, size_t size);
int
main(void)
{
\n
    char  *p;
\n
    p = MALLOCARRAY(32, char);
\n
    if (p == NULL)
\n
        err(EXIT_FAILURE, "malloc");
\n
    strlcpy(p, "foo", 32);
\n
    puts(p);
}
static inline void *
my_mallocarray(size_t nmemb, size_t size)
{
\n
    return reallocarray(NULL, nmemb, size);
}

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↗ (opens in new tab).