path: root/man3/malloc.3

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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().
.\"
.TH MALLOC 3  2010-10-18 "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 *realloc(void " "*ptr" ", size_t "  "size" );
.BI "void *calloc(size_t " "nmemb" ", 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
that is suitably aligned for any kind of variable.
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 kind of variable 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 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).
.\" FIXME . there is no mallopt(3) man page yet.
Allocations performed using
.BR mmap (2)
are unaffected by the
.B RLIMIT_DATA
resource limit (see
.BR getrlimit (2)).

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
Recent versions of Linux libc (later than 5.4.23) and glibc (2.x)
include a
.BR malloc ()
implementation which is tunable via environment variables.
When
.B MALLOC_CHECK_
is set, a special (less efficient) implementation is used which
is designed to be tolerant against simple errors, such as double
calls of
.BR free ()
with the same argument, or overruns of a single byte (off-by-one
bugs).
Not all such errors can be protected against, however, and
memory leaks can result.
If
.B MALLOC_CHECK_
is set to 0, any detected heap corruption is silently ignored;
if set to 1, a diagnostic message is printed on \fIstderr\fP;
if set to 2,
.BR abort (3)
is called immediately;
if set to 3, a diagnostic message is printed on \fIstderr\fP
and the program is aborted.
Using a nonzero
.B MALLOC_CHECK_
value can be useful because otherwise
a crash may happen much later, and the true cause for the problem
is then very hard to track down.
.SH "SEE ALSO"
.BR brk (2),
.\" .BR mallopt (3),
.BR mmap (2),
.BR alloca (3),
.BR posix_memalign (3)