C: thread_mutex Sync

Question

I am having a problem with multiple thread synchronization, I will post only an example of what I am doing to "try" the synchronization because the original code is larger. Basically I have a heavy process with 3 threads attached, to lock/unlock the threads I use functions that modifies the mutex state in each thread. Something like this:

## thread.c ##
    #include "thread.h"

extern int socks;

pthread_mutex_t sem = PTHREAD_MUTEX_INITIALIZER;

void lock()
{
    pthread_mutex_lock(&sem);
}

void unlock()
{
    pthread_mutex_unlock(&sem);
}

void* thread(void* arg)
{
    printf("Socks: %d\n",socks);
    lock();
    lock();

    printf("Unlocked\n");
    return;
}


## main.c ##
#include "defines.h"
#include "thread.h"

int socks;

int main()
{
    pthread_t tid;
    socks= 9;

    pthread_create(&tid, NULL, thread, NULL);

    //sleep(2);
    unlock();
    pthread_join(tid,NULL);
}

What I get after execute the code is:

./test

Socks: 9

Clearly I have some concepts wrong, but I can't find the problem. The thread shouldn't be unlocked with unlock()? Why the program works when I use sleep(2) before the unlock call? Which way should I use for avoid problems? Each code are in separated files.

Thanks in advance! PS: sorry for my crappy English!

Answer 1

Since the thread runs asynchronously, it might happen that the main thread joins before the thread has hit the lock. In this case I would suspect that the unlock() just happens too early, and the behaviour you get isn't well defined. If you let the main thread sleep for a while before you unlock, the other thread seems to have time to hit the lock and can be unlocked, resulting in the behaviour you seem to expect.