When to use pthread condition variables?

pthread question:

it appears that a condition variable only works if pthread_cond_wait is called before the other thread calls pthread_cond_notify. If notify somehow happens before wait then wait will be stuck.

My question is: when should condition variables be used?

The scheduler can preempt threads and a notify may happen before wait.

Waiting on semaphores does not have this problem -- these have a counter.

When is a conditional variable better than a semaphore?

Here is a test:

File condvar.c

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

// test of conditional variables;
// if cond-var is notified before wait starts, then wait never wakes up !!!
// better to use semaphores than this crap.

pthread_mutex_t cond_var_lock =  PTHREAD_MUTEX_INITIALIZER; 
pthread_cond_t cond_var = PTHREAD_COND_INITIALIZER;

int wait_first = 1;

void *tfunc(void *arg)
{
    (void) arg;

    if (!wait_first)
        sleep(1);

    fprintf(stderr,"on enter cond_var_lock %lx\n", pthread_self());
    pthread_mutex_lock( &cond_var_lock);
    fprintf(stderr,"before pthread_cond_wait %lx\n", pthread_self());
    pthread_cond_wait( &cond_var, &cond_var_lock);
    fprintf(stderr,"after pthread_cond_wait %lx\n", pthread_self());
    pthread_mutex_unlock( &cond_var_lock);
    fprintf(stderr,"after exit cond_var_lock %lx\n", pthread_self());

    return 0;
}

int main(int argc, char *argv[])
{
    pthread_t th;

    if (argc > 0) 
    wait_first = atoi( argv[1] );

    if (wait_first)
    {
        fprintf(stderr,"********* Wait first ***********\n");
    } else {
        fprintf(stderr,"********* Notify first *********\n");
    }


    pthread_create( &th, 0, tfunc, 0 );

    if (wait_first)
    {
        sleep(1);
    } 

    fprintf(stderr, "! on enter cond_var_lock %lx\n", pthread_self());
    pthread_mutex_lock( &cond_var_lock);
    fprintf(stderr, "! before pthread_cond_signal %lx\n", pthread_self());
    pthread_cond_signal( &cond_var );
    fprintf(stderr, "! after pthread_cond_signal %lx\n", pthread_self());
    pthread_mutex_unlock( &cond_var_lock);
    fprintf(stderr, "! after exit cond_var_lock %lx\n", pthread_self());

    sleep(5);
    return 0;    
}

File test.sh

#!/bin/sh

set -e
set -x

gcc condvar.c -o condvar -lpthread

./condvar 1

./condvar 0

Test output

Output:

+ gcc condvar.c -o condvar -lpthread
+ ./condvar 1
********* Wait first ***********
on enter cond_var_lock b7779b70
before pthread_cond_wait b7779b70
! on enter cond_var_lock b777a6c0
! before pthread_cond_signal b777a6c0
! after pthread_cond_signal b777a6c0
! after exit cond_var_lock b777a6c0
after pthread_cond_wait b7779b70
after exit cond_var_lock b7779b70
+ ./condvar 0
********* Notify first *********
! on enter cond_var_lock b785c6c0
! before pthread_cond_signal b785c6c0
! after pthread_cond_signal b785c6c0
! after exit cond_var_lock b785c6c0
on enter cond_var_lock b785bb70
before pthread_cond_wait b785bb70