reallocarray(3bsd)

NAME
reallocarray, recallocarray, freezero - memory allocation and deallocation

LIBRARY
libbsd

SYNOPSIS
#include <stdlib.h>
(See libbsd(7) for include usage.)
void *
reallocarray(void *ptr, size_t nmemb, size_t size);
void *
recallocarray(void *ptr, size_t oldnmemb, size_t nmemb, size_t size);
void
freezero(void *ptr, size_t size);

DESCRIPTION

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

Used for the allocation of memory holding sensitive data, the recallocarray() function guarantees that memory becoming unallocated is explicitly discarded, meaning cached free objects are cleared with explicit_bzero(3).

The recallocarray() function is similar to reallocarray() except it ensures newly allocated memory is cleared similar to calloc(). If ptr is NULL, oldnmemb is ignored and the call is equivalent to calloc(). If ptr is not NULL, oldnmemb must be a value such that oldnmemb * size is the size of the earlier allocation that returned ptr, otherwise the behavior is undefined. The freezero() function is similar to the free() function except it ensures memory is explicitly discarded. If ptr is NULL, no action occurs. If ptr is not NULL, the size argument must be equal to or smaller than the size of the earlier allocation that returned ptr. freezero() guarantees the memory range starting at ptr with length size is discarded while deallocating the whole object originally allocated.

RETURN VALUES
The reallocarray() and recallocarray() functions return a pointer to the allocated space if successful; otherwise, a null pointer is returned and errno is set to ENOMEM.

If multiplying nmemb and size results in integer overflow, reallocarray() and recallocarray() return NULL and set errno to ENOMEM.

If ptr is not NULL and multiplying oldnmemb and size results in integer overflow recallocarray() returns NULL and sets errno to EINVAL.

IDIOMS
Consider calloc() or the extensions reallocarray() and recallocarray() 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, NULL);

A drop-in replacement is reallocarray():

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

Alternatively, calloc() may be used at the cost of initialization overhead.

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) {
    	...

Calling realloc() with a NULL ptr is equivalent to calling malloc(). Instead of this idiom:

    if (p == NULL)
    	newp = malloc(newsize);
    else
    	newp = realloc(p, newsize);

Use the following:

    newp = realloc(p, newsize);

The recallocarray() function should be used for resizing objects containing sensitive data like keys. To avoid leaking information, it guarantees memory is cleared before placing it on the internal free list. Deallocation of such an object should be done by calling freezero().

SEE ALSO
malloc(3), calloc(3), alloca(3)

HISTORY
The reallocarray() function appeared in OpenBSD 5.6, and glibc 2.26. The recallocarray() function appeared in OpenBSD 6.1. The freezero() function appeared in OpenBSD 6.2.