Selecting a random thread in c - c

Question

I need to randomly select two out of three threads, and I've come up with a few ideas but I'm not sure if they would work.
Is it better to have an array of 3 threads and then have another thread choose from there, or to have three independent threads? And in both cases, how would you even write the function that would randomly select them? From what I understand, rand() can't be used because it's only for integers, and other than that I have no clue what to use.
This is kind of a game project, and the winner of the two selected threads plays with the remaining thread. Can the same function be used for both instances then, or does there have to be a new one? I'm guessing the most useful thing would be the same thread using the same function, only with its choices narrowed down?
I know this is probably simple but I'm only just starting out with threads, so excuse my poor skills. Any help is greatly appreciated!
EDIT: Thank you everyone! However, since I'm already confused I really don't want to go into semaphores now. I've written some code based on one of the comments but it's not giving me the wanted output. Here's what I have:
EDIT #2: I somehow managed to write it! The only thing left is for the function to call itself again but the pointers kind of confuse me so I don't know how to call it in the else part, and I don't know how else to stop the randomized values from repeating.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *play (void *myvar);
void *select (void *var);
pthread_t tid[3], th;
int i, ra1, ra2;
int main(int argc, char *argv[]) {
pthread_create(&th, NULL, &select, NULL);
for (i = 0; i < 3; i++) {
pthread_create(&tid[i], NULL, &play, NULL);
}
pthread_join(th, NULL);
for (i = 0; i < 3; i++) {
pthread_join(tid[i], NULL);
}
return 0;
}
void *play (void *myvar) {
printf("Thread%i hits the ball!\n", i);
return NULL;
}
void *select (void *var) {
srand ( time(NULL) );
ra1 = rand() % 3 + 1;
ra2 = rand() % 3 + 1;
if (ra1 != ra2) {
printf("Threads have been chosen! Thread%i and Thread%i will start the game!\n",
ra1, ra2);
}
else //what to do?;
return NULL;
}

Answer 1

I'm assuming that you want 2 of 3 threads to perform some task, and that this selection should be random. I further assume that you want these 3 threads to do the selection "in common", i.e. not requiring a forth thread to do the selection (or to have one of the three doing a special selection procedure).
A possible approach would be to use a counting semaphore to "protect" the task and only allow 2 threads to use it, i.e. initialize the semaphore to 2. You can then have each thread sleep a random (small) delay, and try to acquire the semaphore.
Without much comment, see this example:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
struct data {
sem_t * semaphore;
int random;
char const * name;
};
void task(char const * name) {
printf("WOOOOO %s\n", name);
}
void * operation(void * d) {
struct data * data = d;
usleep(data->random % 1000);
if (sem_trywait(data->semaphore) == 0) {
task(data->name);
} else {
printf("FAILED, such a shame for %s\n", data->name);;
}
return NULL;
}
int main(void) { // NOTE: Demo, thus no error checking
srand(time(NULL));
sem_t * semaphore = malloc(sizeof(*semaphore));
struct data data[3] = {
{semaphore, rand(), "John"},
{semaphore, rand(), "Marry"},
{semaphore, rand(), "Rupert"}
};
pthread_t others[2];
sem_init(semaphore, 0, 2);
pthread_create(&(others[0]), NULL, &operation, &(data[0]));
pthread_create(&(others[1]), NULL, &operation, &(data[1]));
(void)operation(&(data[2]));
pthread_join(others[0], NULL);
pthread_join(others[1], NULL);
sem_destroy(semaphore);
free(semaphore);
return 0;
}
(Live on ideone)
Note that using rand makes this choice here actually pseudo-random. Getting rid of the sleeping part can lead to a real random selection, especially when the threads have been doing different things previously. (Although I can imagine a situation where one would be able to influence the threads from the outside to achieve a particular selection)

Answer 2

Array of thread ID should server your purpose. Use the rand() function to randomly select the index of the array. Whatever thread ID is stored at that index of your array should be your selected thread.