Question

So basically I am working on a class project where we make our own implementation of a lock. I am fairly certain my lock code is correct, however my testing code is not working properly. we are using a version of pthreads called sthreads that my professor gave to the class.
here is my testing code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include "sync.h"
#include "sthread.h"

sthread_mutex_t *mutex;
int locktest(void* arg){
    int threadNum = (int)arg;
    //int i;
    for(;;){ 
        int x  = sthread_mutex_lock(mutex);
        printf("return value: %d\n", x);
        //sleep(1);
        printf("thread %d has the lock\n", threadNum);
        sleep(1);
        sthread_mutex_unlock(mutex);
    }
    return 0;
}


int main(){
sthread_t thr1, thr2, thr3, thr4, thr5;

if (sthread_init() == -1){
    return -1;
}

if(sthread_mutex_init(mutex)){
    return -1;
}

sthread_mutex_lock(mutex);

if (sthread_create(&thr1, locktest, (void *)1) == -1){
    return -1;
}
if (sthread_create(&thr2, locktest, (void *)2) == -1){
    return -1;
}
if (sthread_create(&thr3, locktest, (void *)3) == -1){
    return -1;
}
if (sthread_create(&thr4, locktest, (void *)4) == -1){
    return -1;
}
if (sthread_create(&thr5, locktest, (void *)5) == -1){
    return -1;
}

sleep(100);
sthread_mutex_unlock(mutex);
sthread_mutex_destroy(mutex);
return 0;

}

I’ve descovered that for some reason, even though mutex is global, each thread has a different instance of it. I know this because the return value for each call to sthread_mutex_lock(mutex) in the function locktest (which each thread runs) is not 0. this indicates the lock has not been initialized and is thus a null pointer, even though you see I initialized it in main in the second if statement.

does anyone have any clue as to why this is happening?

Answer 1

You never initialize mutex to point at anything, so it has the value NULL. sthread_mutex_init(mutex) isn’t going to work, since it was basically passed the value NULL.

You probably want sthread_mutex_init(&mutex) (declare sthread_mutex_init as taking a sthread_mutex_t **).