Question

I’ve got some code that currently looks like this (simplified)

/* instance in global var *mystruct, count initialized to 0 */
typedef struct {
    volatile unsigned int count;
} mystruct_t;

pthread_mutex_t mymutex; // is initialized

/* one thread, goal: block while mystruct->count == 0 */
void x(void *n) {
    while(1) {
        pthread_mutex_lock(&mymutex);
        if(mystruct->count != 0) break;
        pthread_mutex_unlock(&mymutex);       
    }
    pthread_mutex_unlock(&mymutex);
    printf("count no longer equals zero");
    pthread_exit((void*) 0)
}

/* another thread */
void y(void *n) {
    sleep(10);
    pthread_mutex_lock(&mymutex);
    mystruct->count = 10;
    pthread_mutex_unlock(&mymutex);
}

This seems inefficient and wrong to me–but I don’t know a better way of doing it. Is there a better way, and if so, what is it?

Answer 1

A general solution is to use a POSIX semaphore. These are not part of the pthread library but work with pthreads just the same.

Since semaphores are provided in most other multi-threading APIs, it is a general technique that may be applied perhaps more portably; however perhaps more appropriate in this instance is a condition variable, which allows a thread to pend on the conditional value of a variable without polling, which seems to be exactly what you want.