Fwd: Why was the reallocarray function not added to glibc?

[Zack Weinberg <zackw at panix dot com>]

On Mon, Apr 10, 2017 at 6:15 AM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>
> the idiomatic unsigned mul overflow check is
>
>   if (b && -1/b < a)
>     return 0;
>   c = a*b;

I've never seen this construct before; it is not obvious to me why it
checks for unsigned mul overflow.  It would need, at the very least,
an explanation in comments.  Also, this...

#include <stdlib.h>

void *reallocarray(void *ptr, size_t size, size_t nmemb)
{
    if (!size || !nmemb) {
        free(ptr);
        return 0;
    }
    if (-1/nmemb < size) {
        return 0;
    }
    return realloc(ptr, size * nmemb);
}

... generates code with a divide instruction in it, using either gcc
6.3.0 or clang 3.8.1, so I do not think we can use it yet.

zw