MichaelMoser MichaelMoser - 1 year ago 80
Linux Question

When to use pthread conditional variables?

pthread question;

it appears that conditional 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 conditional variables be used ? scheduler can preempt threads and notify well happen before wait;

waiting on semaphores does not have this problem - these have a counter, still when is a conditional variable better?

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)

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)

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());

return 0;

**** file test.sh


set -e
set -x

gcc condvar.c -o condvar -lpthread

./condvar 1

./condvar 0

**** test 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

Answer Source

Condition variables should be used as a place to wait and be notified. They are not the condition itself and they are not events. The condition is contained in the surrounding programming logic. The typical usage pattern of condition variables is

// safely examine the condition, prevent other threads from
// altering it
pthread_mutex_lock (&lock);
while ( SOME-CONDITION is false)
    pthread_cond_wait (&cond, &lock);

// Do whatever you need to do when condition becomes true
pthread_mutex_unlock (&lock);

On the other hand, a thread, signaling the condition variable, typically looks like

// ensure we have exclusive access to whathever comprises the condition
pthread_mutex_lock (&lock);


// Wakeup any threads that may be waiting on the condition
pthread_cond_signal (&cond);

// allow others to proceed
pthread_mutex_unlock (&lock)