freebsd-dev/lib/libc/stdlib/reallocarray.3

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.Dd May 1, 2015
.Dt REALLOCARRAY 3
.Os
.Sh NAME
.Nm reallocarray
.Nd memory reallocation function
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft void *
.Fn reallocarray "void *ptr" "size_t nmemb" "size_t size"
.Sh DESCRIPTION
The
.Fn reallocarray
function is similar to the
.Fn realloc
function
except it operates on
.Fa nmemb
members of size
.Fa size
and checks for integer overflow in the calculation
.Fa nmemb
*
.Fa size .
.Sh RETURN VALUES
The
.Fn reallocarray
function returns a pointer to the allocated space; otherwise, a
.Dv NULL
pointer is returned and
.Va errno
is set to
.Er ENOMEM .
.Sh EXAMPLES
Consider
.Fn reallocarray
when there is multiplication in the
.Fa size
argument of
.Fn malloc
or
.Fn realloc .
For example, avoid this common idiom as it may lead to integer overflow:
.Bd -literal -offset indent
if ((p = malloc(num * size)) == NULL)
	err(1, "malloc");
.Ed
.Pp
A drop-in replacement is the
.Ox
extension
.Fn reallocarray :
.Bd -literal -offset indent
if ((p = reallocarray(NULL, num, size)) == NULL)
	err(1, "reallocarray");
.Ed
.Pp
When using
.Fn realloc ,
be careful to avoid the following idiom:
.Bd -literal -offset indent
size += 50;
if ((p = realloc(p, size)) == NULL)
	return (NULL);
.Ed
.Pp
Do not adjust the variable describing how much memory has been allocated
until the allocation has been successful.
This can cause aberrant program behavior if the incorrect size value is used.
In most cases, the above sample will also result in a leak of memory.
As stated earlier, a return value of
.Dv NULL
indicates that the old object still remains allocated.
Better code looks like this:
.Bd -literal -offset indent
newsize = size + 50;
if ((newp = realloc(p, newsize)) == NULL) {
	free(p);
	p = NULL;
	size = 0;
	return (NULL);
}
p = newp;
size = newsize;
.Ed
.Pp
As with
.Fn malloc ,
it is important to ensure the new size value will not overflow;
i.e. avoid allocations like the following:
.Bd -literal -offset indent
if ((newp = realloc(p, num * size)) == NULL) {
	...
.Ed
.Pp
Instead, use
.Fn reallocarray :
.Bd -literal -offset indent
if ((newp = reallocarray(p, num, size)) == NULL) {
	...
.Ed
.Sh SEE ALSO
.Xr realloc 3
.Sh HISTORY
The
.Fn reallocarray
function first appeared in
.Ox 5.6
and
.Fx 11.0 .