How to set REX prefix when using RDRAND?

[Florian Weimer]

On 10/12/2015 07:41 PM, Jeffrey Walton wrote:

>     while (size && safety)
>     {
>         char rc;
>         __asm__ volatile(
> #if BOOL_X86
>           "rdrandl %0 ; setc %1"
> #else
>           "rdrandq %0 ; setc %1"
> #endif
>           : "=rm" (val), "=qm" (rc)
>           :
>           : "cc"
>         );

I don't think RDRAND supports memory operand, so the constraint is
incorrect, and a width suffix is not required.

>         if (rc)
>         {
>             size_t count = (size < sizeof(val) ? size : sizeof(val));
>             memcpy(output, &val, count);
>             size =- count;

You forgot to increment the output variable.

Florian