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.\"	$OpenBSD: malloc.3,v 1.78 2014/05/01 18:41:59 jmc Exp $
.\"
.Dd $Mdocdate: May 1 2014 $
.Dt MALLOC 3
.Os
.Sh NAME
.Nm reallocarray
.Nd memory allocation and deallocation
.Sh LIBRARY
.ds str-Lb-libbsd Utility functions from BSD systems (libbsd, \-lbsd)
.Lb libbsd
.Sh SYNOPSIS
.In bsd/stdlib.h
.Ft void *
.Fn reallocarray "void *ptr" "size_t nmemb" "size_t size"
.Sh DESCRIPTION
.Pp
When using
.Fn malloc
be careful to avoid the following idiom:
.Bd -literal -offset indent
if ((p = malloc(num * size)) == NULL)
	err(1, "malloc");
.Ed
.Pp
The multiplication may lead to an integer overflow, which can
be avoided using the extension
.Fn reallocarray ,
as follows:
.Bd -literal -offset indent
if ((p = reallocarray(NULL, num, size)) == NULL)
	err(1, "malloc");
.Ed
.Pp
Alternatively
.Fn calloc
is a more portable solution which comes with the cost of clearing memory.
.Pp
If
.Fn malloc
must be used, be sure to test for overflow:
.Bd -literal -offset indent
if (size && num > SIZE_MAX / size) {
	errno = ENOMEM;
	err(1, "overflow");
}
.Ed
.Pp
The use of
.Fn reallocarray
or
.Fn calloc
is strongly encouraged when allocating multiple sized objects
in order to avoid possible integer overflows.
.Sh RETURN VALUES
The
.Fn reallocarray
function returns a pointer to the allocated space if successful; otherwise,
a null pointer is returned and
.Va errno
is set to
.Er ENOMEM .
.Sh SEE ALSO
.Xr malloc 3 ,
.Xr calloc 3 ,
.Xr alloca 3
.Sh HISTORY
.Fn reallocarray
appeared in
.Ox 5.6 .