Using Mutex Lock/Unlock and Broadcast for pthreads in C

Question

This is my first post and I'm excited.

My problem is that I am creating a Rock, Paper, Scissors program in C where the parent process creates 2 threads. The 2 threads then throw a random rock, paper, or scissors and return the value to the parent where it gets counted and spits back the results of 3 rounds, then makes a final tally.

My problem is that I cannot get the threads to initiate correctly, I have them waiting in my thread_function1 but then they only complete one round and even then I don't get two threads back in the results. If someone could please shed some light I would really appreciate it! Thanks

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <pthread.h>
#include <sys/time.h>

#define NTHREADS 2
struct timeval tv;
void *thread_function1();
void *thread_function2();
char *guess_string(int g);
int wins[3];
int cmd_ready = 0;
int x1=0, x2=1, count=0;
int  guess, object, turns, i, j, k,l, winner, cmd, go, y;
pthread_mutex_t cv_m;
pthread_mutex_t count_mutex;
pthread_cond_t cv;
int myindex;
int flag; 
int throws[3];
int main(int argc, char *argv[])
{


  wins[0] = 0; wins[1] = 0;
  if ((argc != 2) || ((turns = atoi(argv[1])) <= 0))
    {
      fprintf(stderr,"Usage: %s turns\n", argv[0]);
      return 0;
    }

    pthread_t thread_id1, thread_id2;


    if (pthread_create(&thread_id1, NULL, thread_function1,&x1) != 0)
        perror("pthread_create"),
        exit(1); 

    if (pthread_create(&thread_id2, NULL, thread_function1,&x2) != 0)
        perror("pthread_create"),
        exit(1); 

    printf("Beginning %d Rounds...\nFight!\n", turns);
    printf("Child 1 TID: %d\n", (unsigned int) thread_id1);
    printf("Child 2 TID: %d\n", (unsigned int) thread_id2 );

    for(k=0; k<turns; k++)
    {

            pthread_mutex_lock (&cv_m);
            cmd = go; 
            cmd_ready = 2;
            pthread_cond_broadcast(&cv);    
            pthread_mutex_unlock(&cv_m);
            printf("------------------------\n");
            printf("Round: %d\n", k+1);

            printf("Child %d throws %s!\n",myindex+1, guess_string(myindex));

            pthread_mutex_lock (&count_mutex);

            winner = find_winner(throws[0], throws[1]);
            while(count == 2){
            if(winner >= 0)
            {
                printf("Child %d Wins!\n", winner+1);
                wins[winner]++;
                printf("6\n");
            }else
            {
                printf("Game is a Tie!\n");
            }
            go--;
            count = 0; 
            pthread_mutex_unlock(&count_mutex);
            }
    }

    pthread_join(thread_id1,NULL); 
    pthread_join(thread_id2,NULL);

    printf("------------------------\n");
    printf("------------------------\n");
    printf("Result:\n");
    printf("Child 1: %d\n", wins[0]);
    printf("Child 2: %d\n", wins[1]);
    printf("Ties: %d\n", turns - (wins[0] + wins[1]));
    printf("Child %d Wins!\n", (wins[0] > wins[1]) ? 1 : 2);

    pthread_mutex_destroy(&cv_m);
    pthread_cond_destroy(&cv);
    pthread_exit(NULL);
    return 0; 

}

void *thread_function1(void *p)
{
    struct timeval tv;
    myindex = *(int *)p;
    gettimeofday(&tv, NULL);
    srand(tv.tv_sec + tv.tv_usec + getpid());
    printf("1\n");
    pthread_mutex_lock (&cv_m);
    while(cmd_ready == 0)
    {
        printf("2\n");
        pthread_cond_wait(&cv, &cv_m);
    }
    printf("3\n");

    throws[myindex] = rand() % 3;
    cmd_ready--;
    printf("Ready: %d\n",cmd_ready);
    pthread_mutex_unlock (&cv_m);
    printf("4\n");

    pthread_mutex_lock (&count_mutex);
    count++;
    printf("Count %d\n", count);
    pthread_mutex_unlock(&count_mutex);

    while(count == 2){
        printf("5\n");
        return NULL;
    }

}

char *guess_string(int g){
    switch(g){
    case 0:
        return "Rock";
        break;
    case 1:
        return "Paper";
        break;
    case 2:
        return "Scissors";
        break;
    }
}

int find_winner(int g1, int g2){
    if(g1 == g2)
        return -1; 
    else if ((g1 == 2) && (g2 == 0))
        return 1; 
    else if ((g1 == 0) && (g2 == 2))
        return 0;
    else
        return (g1 > g2) ? 0 : 1;
}

Answer 1

You don't appear to be initializing your mutex or condition with pthread_mutex_init or pthread_cond_init .

The variable myindex is being modified by both threads without protection, the second thread to update this variable will be the one that appears to report back.

You are also counting on your threads to begin pending on the condition before main grabs the lock and issues the broadcast, you could have a case where main gets there first and your threads won't be ready.

That should be a start.