#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
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:
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.
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.
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.
malloc() and related functions rejected sizes greater than PTRDIFF_MAX starting in glibc 2.30.
free() preserved errno starting in glibc 2.33.
Interface | Attribute | Value |
malloc(), free(), calloc(), realloc() | Thread safety | MT-Safe |
reallocarray() is a nonstandard extension that first appeared in OpenBSD 5.6 and FreeBSD 11.0.
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.
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().
For details of the GNU C library implementation, see