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 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);

reallocarray():

    Since glibc 2.29:


        _DEFAULT_SOURCE


    glibc 2.28 and earlier:


        _GNU_SOURCE

DESCRIPTION¶

malloc(nmemb * size);

realloc(ptr, nmemb * size);

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).

VERSIONS¶

reallocarray() was added in glibc 2.26.

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

free() preserved errno starting in glibc 2.33.

ATTRIBUTES¶

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

STANDARDS¶

malloc(), free(), calloc(), realloc(): POSIX.1-2001, POSIX.1-2008, 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.

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.

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.