thread_c.c

#include <pthread.h>
#include <unistd.h>

void *work(void *arg) {
    printf("Sleeping for %d seconds.\n", *(int *) arg);
    sleep(*(uint *) arg);
    printf("%d\n", *(int*) arg);
    return arg;
} 

int main() {
    pthread_t t1, t2;

    int magic_nr_1 = 46;
    int magic_nr_2 = 3;

    pthread_create(&t1, NULL, work, (void *) &magic_nr_1);
    pthread_create(&t2, NULL, work, (void *) &magic_nr_2);

    pthread_join(t1,NULL);
    pthread_join(t2,NULL);
}

This has the correct name for the thread worker function, as well as the correct casting for the arguments to the work function.