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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
MALLOC(3) Linux Programmer's Manual MALLOC(3)
malloc, free, calloc, realloc - allocate and free dynamic memory
#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
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().
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.
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.
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).
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├─────────────────────┼───────────────┼─────────┤
│malloc(), free(), │ Thread safety │ MT-Safe │
│calloc(), realloc() │ │ │
└─────────────────────┴───────────────┴─────────┘
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.
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.
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). Allocations performed using mmap(2) are
unaffected by the RLIMIT_DATA resource limit (see getrlimit(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.
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)
This page is part of release 4.12 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 2017-07-13 MALLOC(3)
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