malloc(3) Library Functions Manual malloc(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 size);
void free(void *_Nullable ptr);
void *calloc(size_t n, size_t size);
void *realloc(void *_Nullable ptr, size_t size);
void *reallocarray(void *_Nullable ptr, size_t n, 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
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 n elements of
size bytes each and returns a pointer to the allocated memory. The
memory is set to zero. If n or size is 0, then calloc() returns a
unique pointer value that can later be successfully passed to free().
If the multiplication of n 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(n * 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 n
elements, each of which is size bytes. It is equivalent to the call
realloc(ptr, n * 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).
+--------------------------------------------+---------------+---------+
|Interface | Attribute | Value |
+--------------------------------------------+---------------+---------+
|malloc (), free (), calloc (), realloc () | Thread safety | MT-Safe |
+--------------------------------------------+---------------+---------+
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 n, size_t size);
int
main(void)
{
char *p;
p = MALLOCARRAY(32, char);
if (p == NULL)
err(EXIT_FAILURE, "malloc");
strlcpy(p, "foo", 32);
puts(p);
}
static inline void *
my_mallocarray(size_t n, size_t size)
{
return reallocarray(NULL, n, 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>.
Linux man-pages 6.14 2025-05-06 malloc(3)