.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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.\" Modified Sat Jul 24 19:00:59 1993 by Rik Faith (faith@cs.unc.edu)
.\" Clarification concerning realloc, iwj10@cus.cam.ac.uk (Ian Jackson), 950701
.\" Documented MALLOC_CHECK_, Wolfram Gloger (wmglo@dent.med.uni-muenchen.de)
.\" 2007-09-15 mtk: added notes on malloc()'s use of sbrk() and mmap().
.\"
.\" FIXME . Review http://austingroupbugs.net/view.php?id=374
.\" to see what changes are required on this page.
.\"
.TH MALLOC 3  2014-05-21 "GNU" "Linux Programmer's Manual"
.SH NAME
malloc, free, calloc, realloc \- allocate and free dynamic memory
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.BI "void *malloc(size_t " "size" );
.BI "void free(void " "*ptr" );
.BI "void *calloc(size_t " "nmemb" ", size_t " "size" );
.BI "void *realloc(void " "*ptr" ", size_t "  "size" );
.fi
.SH DESCRIPTION
.PP
The
.BR malloc ()
function allocates
.I size
bytes and returns a pointer to the allocated memory.
.IR "The memory is not initialized" .
If
.I size
is 0, then
.BR malloc ()
returns either NULL,
.\" glibc does this:
or a unique pointer value that can later be successfully passed to
.BR free ().
.PP
The
.BR free ()
function frees the memory space pointed to by
.IR ptr ,
which must have been returned by a previous call to
.BR malloc (),
.BR calloc (),
or
.BR realloc ().
Otherwise, or if
.I free(ptr)
has already been called before, undefined behavior occurs.
If
.I ptr
is NULL, no operation is performed.
.PP
The
.BR calloc ()
function allocates memory for an array of
.I nmemb
elements of
.I size
bytes each and returns a pointer to the allocated memory.
The memory is set to zero.
If
.I nmemb
or
.I size
is 0, then
.BR calloc ()
returns either NULL,
.\" glibc does this:
or a unique pointer value that can later be successfully passed to
.BR free ().
.PP
The
.BR realloc ()
function changes the size of the memory block pointed to by
.I ptr
to
.I 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
.I not
be initialized.
If
.I ptr
is NULL, then the call is equivalent to
.IR malloc(size) ,
for all values of
.IR size ;
if
.I size
is equal to zero,
and
.I ptr
is not NULL, then the call is equivalent to
.IR free(ptr) .
Unless
.I ptr
is NULL, it must have been returned by an earlier call to
.BR malloc (),
.BR calloc ()
or
.BR realloc ().
If the area pointed to was moved, a
.I free(ptr)
is done.
.SH RETURN VALUE
The
.BR malloc ()
and
.BR 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
.BR malloc ()
with a
.I size
of zero,
or by a successful call to
.BR calloc ()
with
.I nmemb
or
.I size
equal to zero.
.PP
The
.BR free ()
function returns no value.
.PP
The
.BR realloc ()
function returns a pointer to the newly allocated memory, which is suitably
aligned for any built-in type and may be different from
.IR ptr ,
or NULL if the request fails.
If
.I size
was equal to 0, either NULL or a pointer suitable to be passed to
.BR free ()
is returned.
If
.BR realloc ()
fails, the original block is left untouched; it is not freed or moved.
.SH CONFORMING TO
C89, C99.
.SH NOTES
By default, Linux follows an optimistic memory allocation strategy.
This means that when
.BR 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
.IR /proc/sys/vm/overcommit_memory
and
.IR /proc/sys/vm/oom_adj
in
.BR proc (5),
and the Linux kernel source file
.IR Documentation/vm/overcommit-accounting .

Normally,
.BR malloc ()
allocates memory from the heap, and adjusts the size of the heap
as required, using
.BR sbrk (2).
When allocating blocks of memory larger than
.B MMAP_THRESHOLD
bytes, the glibc
.BR malloc ()
implementation allocates the memory as a private anonymous mapping using
.BR mmap (2).
.B MMAP_THRESHOLD
is 128 kB by default, but is adjustable using
.BR mallopt (3).
Allocations performed using
.BR mmap (2)
are unaffected by the
.B RLIMIT_DATA
resource limit (see
.BR 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
.IR "memory allocation arenas"
if mutex contention is detected.
Each arena is a large region of memory that is internally allocated
by the system
(using
.BR brk (2)
or
.BR mmap (2)),
and managed with its own mutexes.

The UNIX 98 standard requires
.BR malloc (),
.BR calloc (),
and
.BR realloc ()
to set
.I errno
to
.B 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
.IR errno ,
then certain library routines may fail without having
a reason in
.IR errno .
.LP
Crashes in
.BR malloc (),
.BR calloc (),
.BR realloc (),
or
.BR free ()
are almost always related to heap corruption, such as overflowing
an allocated chunk or freeing the same pointer twice.
.PP
The
.BR malloc ()
implementation is tunable via environment variables; see
.BR mallopt (3)
for details.
.SH SEE ALSO
.\" http://g.oswego.edu/dl/html/malloc.html
.\" A Memory Allocator - by Doug Lea
.\"
.\" http://www.bozemanpass.com/info/linux/malloc/Linux_Heap_Contention.html
.\" Linux Heap, Contention in free() - David Boreham
.\"
.\" http://www.citi.umich.edu/projects/linux-scalability/reports/malloc.html
.\" malloc() Performance in a Multithreaded Linux Environment -
.\"     Check Lever, David Boreham
.\"
.ad l
.nh
.BR brk (2),
.BR mmap (2),
.BR alloca (3),
.BR malloc_get_state (3),
.BR malloc_info (3),
.BR malloc_trim (3),
.BR malloc_usable_size (3),
.BR mallopt (3),
.BR mcheck (3),
.BR mtrace (3),
.BR posix_memalign (3)
.SH COLOPHON
This page is part of release 3.74 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%http://www.kernel.org/doc/man\-pages/.