Subj : pthread_cond_broadcast [a little long]
To   : comp.programming.threads
From : fabrizio
Date : Tue Jan 25 2005 01:28 pm

Hello,

I am trying to synchronize a certain number of threads
using condvars and encounter some problems.

I create a thread that wait for x signals and then broadcast
a convar that should start all other threads.
I create x threads that signal a condvar, to indicate that they are
ready, then wait for the start order.

Unfortunately, as the last ready thread signal the ready condvar
the thread waiting for the ready event, immediately signal the start 
order and the last thread has not yet reach the wait instruction.

As I imagine it is not very clear, here is a simple program and its
output that explicit my problem :
-------------
#include <pthread.h>
#include <stdio.h>

pthread_cond_t readyvar;
pthread_mutex_t readymut;

pthread_cond_t startvar;
pthread_mutex_t startmut;


void* routine0(void* data) {
	int n=3;
	while(n > 0) {
		pthread_mutex_lock(&readymut);
		printf("%d\twaiting for %d ready\n",(int)pthread_self(),n);
		pthread_cond_wait(&readyvar,&readymut);
		n--;
		printf("%d\tready received\n",(int)pthread_self());
		pthread_mutex_unlock(&readymut);
	}

	pthread_mutex_lock(&startmut);
	pthread_cond_broadcast(&startvar);
	printf("%d\tsignal start\n",(int)pthread_self());
	pthread_mutex_unlock(&startmut);
	return(NULL);
}

void* routine1(void* data) {
	pthread_mutex_lock(&readymut);
	pthread_cond_signal(&readyvar);
	printf("%d\tsignal ready\n",(int)pthread_self());
	pthread_mutex_unlock(&readymut);

	pthread_mutex_lock(&startmut);
	printf("%d\twaiting for start order\n",(int)pthread_self());
	pthread_cond_wait(&startvar,&startmut);
	printf("%d\tstart order received\n",(int)pthread_self());
	pthread_mutex_unlock(&startmut);
	return(NULL);
}

int main() {
   pthread_t t0,t1,t2,t3;

   pthread_create(&t0,NULL,routine0,NULL);

   pthread_create(&t1,NULL,routine1,NULL);
   pthread_create(&t2,NULL,routine1,NULL);
   pthread_create(&t3,NULL,routine1,NULL);

   pthread_join(t0,NULL);
}
-----------
I modify the output a little so it can be read more easily

16386   waiting for 3 ready
	32771   signal ready
16386   ready received
16386   waiting for 2 ready
	32771   waiting for start order
		49156   signal ready
16386   ready received
16386   waiting for 1 ready
		49156   waiting for start order
			65541   signal ready
16386   ready received
16386   signal start		AT THIS POINT THE START IS SIGNALED
				BUT THE LAST THREAD IS NOT WAITING FOR
	32771   start order received
		49156   start order received
			65541   waiting for start order
------------------

My question is :
how should I design my threads for doing such synchronization ?

Any hint on this would be greatly appreciated.
Fabrice

.