MALLOC
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
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NAME
malloc, free, calloc, realloc - allocate and free dynamic memory
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);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
reallocarray():
-
_GNU_SOURCE
DESCRIPTION
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.
RETURN VALUE
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.
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).
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
|
CONFORMING TO
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.
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.
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.
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.
SEE ALSO
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)
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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