pthread_cond_wait doesn't unlock mutex

Question

I can't find any evidence online of pthread_cond_wait being strange on Mac OS X, but it seems to be failing the simplest test for me.

The function

int pthread_cond_wait(pthread_cond_t *, pthread_mutex_t * );

is supposed to unlock the mutex argument #2 and then wait for a signal to be sent on the condition argument #1. I wrote a simple program to test this, and also test for spurious wakeups:

#include <stdio.h> #include <pthread.h> pthread_t spin_thread; pthread_mutex_t spin_mutex; pthread_cond_t spin_cond; int actual = 0; void *condspin( void *v ) { int expected = 0; for ( ;; ) { if ( actual != expected ) printf( "unexpected %d\n", actual ); else printf( "expected %d\n", actual ); pthread_mutex_lock( &spin_mutex ); printf( "locked\n" ); expected = actual + 1; pthread_cond_wait( &spin_cond, &spin_mutex ); } return NULL; } int main( int argc, char ** argv ) { pthread_mutex_init( &spin_mutex, NULL ); pthread_cond_init( &spin_cond, NULL ); pthread_create( &spin_thread, NULL, &condspin, NULL ); for ( ;; ) { getchar(); pthread_cond_signal( &spin_cond ); printf( "signaled\n" ); ++ actual; } return 0; }

But it only acquires the lock once. The main thread doesn't even try to acquire the lock just to keep things simple.

Shadow:~ dkrauss$ cc condwait.c -o condwait Shadow:~ dkrauss$ ./condwait expected 0 locked signaled expected 1 signaled signaled

If I add a pthread_mutex_unlock after the pthread_cond_wait, it behaves as expected. (Or as well as you'd expect with only half a locking mechanism.) So, what gives?

bdonlan · Accepted Answer · 2009-08-24 00:12:09Z

8

pthread_cond_wait re-acquires the mutex when it is awoken. The standard pattern for using pthreads mutexes is:

pthread_mutex_lock(&mutex); // init work... while (!some_condition) pthread_cond_wait(&cond, &mutex); // finishing work... pthread_mutex_unlock(&mutex);

This behavior is described in the SUS documentation for pthread_cond_wait as:

Upon successful return, the mutex has been locked and is owned by the calling thread.

edited Aug 24, 2009 at 0:12

answered Aug 23, 2009 at 23:28

bdonlan

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3 Comments

Potatoswatter Over a year ago

Aha! Makes perfect sense! Now why don't any of the docs I read (and I read a few) say so???

bdonlan Over a year ago

it is documented in the SUS spec (citation added), and also in the linuxthreads manpage for pthread_cond_wait. I don't know about mac os x, but it should document this as well...

Potatoswatter Over a year ago

OS X has almost no documentation. It says this and only this: "The pthread_cond_wait() function atomically unlocks the mutex argument and waits on the cond argument." I did actually read the reference you provided but I was confused by its use of the passive voice. "The mutex has been locked" is a confusing way of saying "the function locks the mutex," especially in an interface where many requirements fall upon the user.

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