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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.
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.TH MALLOC 3  2017-09-15 "GNU" "Linux Programmer's Manual"
.SH NAME
malloc, free, calloc, realloc \- allocate and free dynamic memory
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.PP
.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" );
.BI "void *reallocarray(void " "*ptr" ", size_t " nmemb ", size_t "  "size" );
.fi
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.BR reallocarray ():
.ad l
_GNU_SOURCE
.RE
    Since glibc 2.29:
        _DEFAULT_SOURCE
    Glibc 2.28 and earlier:
        _GNU_SOURCE
.ad
.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 ().
If the multiplication of
.I nmemb
and
.I size
would result in integer overflow, then
.BR calloc ()
returns an error.
By contrast,
an integer overflow would not be detected in the following call to
.BR malloc (),
with the result that an incorrectly sized block of memory would be allocated:
.PP
.in +4n
.EX
malloc(nmemb * size);
.EE
.in
.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.
.PP
The
.BR reallocarray ()
function changes the size of the memory block pointed to by
.I ptr
to be large enough for an array of
.I nmemb
elements, each of which is
.I size
bytes.
It is equivalent to the call
.PP
.in +4n
    realloc(ptr, nmemb * size);
.in
.PP
However, unlike that
.BR realloc ()
call,
.BR reallocarray ()
fails safely in the case where the multiplication would overflow.
If such an overflow occurs,
.BR reallocarray ()
returns NULL, sets
.I errno
to
.BR ENOMEM ,
and leaves the original block of memory unchanged.
.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