Manual Pages  — REALLOCARRAY

reallocarray – memory reallocation function

LIBRARY SYNOPSIS DESCRIPTION RETURN VALUES EXAMPLES SEE ALSO HISTORY

Standard C Library (libc, -lc)

#include <stdlib.h>

void *
reallocarray(void *ptr, size_t nmemb, size_t size);

The reallocarray() function is similar to the realloc() function except it operates on nmemb members of size size and checks for integer overflow in the calculation nmemb * size.

The reallocarray() function returns a pointer to the allocated space; otherwise, a NULL pointer is returned and errno is set to ENOMEM.

Consider reallocarray() when there is multiplication in the size argument of malloc() or realloc(). For example, avoid this common idiom as it may lead to integer overflow:

if ((p = malloc(num * size)) == NULL)
        err(1, "malloc");

A drop-in replacement is the OpenBSD extension reallocarray():

if ((p = reallocarray(NULL, num, size)) == NULL)
        err(1, "reallocarray");

When using realloc(), be careful to avoid the following idiom:

size += 50;
if ((p = realloc(p, size)) == NULL)
        return (NULL);

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 NULL indicates that the old object still remains allocated. Better code looks like this:

newsize = size + 50;
if ((newp = realloc(p, newsize)) == NULL) {
        free(p);
        p = NULL;
        size = 0;
        return (NULL);
}
p = newp;
size = newsize;

As with malloc(), it is important to ensure the new size value will not overflow; i.e. avoid allocations like the following:

if ((newp = realloc(p, num * size)) == NULL) {
        ...

Instead, use reallocarray():

if ((newp = reallocarray(p, num, size)) == NULL) {
        ...

The reallocarray() function first appeared in OpenBSD 5.6 and FreeBSD 11.0 .