Try making x86 RDRAND fail.

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>

// Try making RDRAND fail.
// gcc -Wall -Wextra -pedantic rdrand.c -pthread

void *fseed(void *arg) {
        int id = *(int *)arg;

        fprintf(stderr, "Thread SEED%02d started.\n", id);

        asm volatile (
                "x:\n\t"
                "pause\n\t"
                "rdseed %%rax\n\t"
                "jmp x\n\t"
                : : : "rax"
        );

        return NULL;
}

void *frand(void *arg) {
        int id = *(int *)arg;
        unsigned x = 0;
        unsigned long tries = 0;

        fprintf(stderr, "Thread RAND%02d started.\n", id);

        asm volatile (
                "xor %1, %1\n\t"
                "retry:\n\t"
                "pause\n\t"
                "inc %1\n\t"
                "rdrand %0\n\t"
                "jc retry\n\t"
                "done:\n\t"
                : "=a"(x), "=r"(tries) : :
        );

        fprintf(stderr, "Thread RAND%02d done after %lu tries.\n", id, tries);
        exit(0);

        return NULL;
}

#define NSEED 1
#define NRAND 10

int main(void) {
        pthread_t st[NSEED], rt[NRAND];
        int sids[NSEED], rids[NRAND];

        for (int i = 0; i < NSEED; i++) {
                sids[i] = i;

                if ((errno = pthread_create(st + i, NULL, fseed, &sids[i])) != 0) {
                        fprintf(stderr, "Thread SEED%02d startup failed: %s\n", i, strerror(errno));
                        return 1;
                }
        }

        for (int i = 0; i < NRAND; i++) {
                rids[i] = i;

                if ((errno = pthread_create(rt + i, NULL, frand, &rids[i])) != 0) {
                        fprintf(stderr, "Thread RAND%02d startup failed: %s\n", i, strerror(errno));
                        return 1;
                }
        }

        sleep(1000);
        fputs("Timed out.\n", stderr);

        return 0;
}