I am supposed to reproduce the Strict Alternation algorithm using C and Posix Threads.

I coded the below solution but the printfs aren't being outputed at all. Sometimes just the my_id is outputed but that's all. What am I doing or understanding wrong?

For example, this was the output of one of the times I ran this code:

Oher output:

Here is the code I did:

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

int count = 0;
int turn = 1;

struct arg_struct {
    int my_id;
    int other;
};

void *runner(void *param) {
    struct arg_struct *args = (struct arg_struct *)param;
    int my_id = args -> my_id;
    int other = args -> other;
    printf("my_id %d\n", my_id);
    printf("other %d\n", other);
    printf("Running p_thread %d\n", my_id);

    while(1) {
        while (turn != my_id) { }

        // critical section
        count = count   my_id;
        printf("Critical section of p_thread %d, count was incremented by %d and is now %d\n", my_id, my_id, count);

        turn = other;

        // not critical section
        printf("Not critical section of p_thread %d\n", my_id);
    }

    pthread_exit(NULL);
}

int main(int argc, char *argv[]) {
    int thr_id1, thr_id2;
    pthread_t p_thread1;
    pthread_t p_thread2;

    struct arg_struct args_p_thread1;
    args_p_thread1.my_id = 1;
    args_p_thread1.other = 2;

    struct arg_struct args_p_thread2;
    args_p_thread2.my_id = 2;
    args_p_thread2.other = 1;

    thr_id1 = pthread_create(&p_thread1, NULL, runner, (void*)&args_p_thread1);
    thr_id2 = pthread_create(&p_thread2, NULL, runner, (void*)&args_p_thread2);

    return 0;
}

CodePudding user response：

The variable you use for your parameters is reused between the threads.

Your global variables are not immediately suitable for sharing between threads, so it’s possible they do not see a new value of turn and get stuck in your inner while loop.

Your main loop exits without waiting for the threads, so they may not get a chance to do much. Your program will exit at the end of the main thread, it will not automatically wait for your other threads.

CodePudding user response：

This code works for me on Repl.it.

I added volatile and pthread_join

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

int count = 0;
volatile int turn = 1;

struct arg_struct {
    int my_id;
    int other;
};

void *runner(void *param) {
    struct arg_struct *args = (struct arg_struct *)param;
    int my_id = args -> my_id;
    int other = args -> other;
    printf("my_id %d\n", my_id);
    printf("other %d\n", other);
    printf("Running p_thread %d\n", my_id);

    while(1) {
        while (turn != my_id) { }

        // critical section
        count = count   my_id;
        printf("Critical section of p_thread %d, count was incremented by %d and is now %d\n", my_id, my_id, count);

        turn = other;

        // not critical section
        printf("Not critical section of p_thread %d\n", my_id);
    }

    pthread_exit(NULL);
}

int main(int argc, char *argv[]) {
    int thr_id1, thr_id2;
    pthread_t p_thread1;
    pthread_t p_thread2;

    struct arg_struct args_p_thread1;
    args_p_thread1.my_id = 1;
    args_p_thread1.other = 2;

    struct arg_struct args_p_thread2;
    args_p_thread2.my_id = 2;
    args_p_thread2.other = 1;

    thr_id1 = pthread_create(&p_thread1, NULL, runner, (void*)&args_p_thread1);
    thr_id2 = pthread_create(&p_thread2, NULL, runner, (void*)&args_p_thread2);

  int join = 0;
    pthread_join(&p_thread1, &join);
  
  pthread_join(&p_thread2, &join);
  
    return 0;
}