rdrand.s

.global do_rdrand

/* Signature: uint32_t do_rdrand(void); */

do_rdrand:
.Lrdrand_retry:
    rdrand %rax
    jc .Lrdrand_retry           /* Fail? */
    ret

test_rdrand.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <pthread.h>

uint32_t do_rdrand(void);

void * threadfunc(void *unused) {
    printf("Value: %08x\n", do_rdrand());
    return NULL;
}

int main(int argc, char **argv) {
    int nThread = atoi(argv[1]);
    pthread_t *threads = (pthread_t *) malloc(nThread * sizeof(pthread_t));
    for (int i = 0; i < nThread; i++) {
        pthread_create(&threads[i], NULL, threadfunc, NULL);
    }
    for (int i = 0; i < nThread; i++) {
        pthread_join(threads[i], NULL);
    }
    return 0;
}