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Full Discussion: Child threads communicating with main thread via pipes
Top Forums Programming Child threads communicating with main thread via pipes Post 302667479 by Corona688 on Friday 6th of July 2012 10:52:16 AM
Old 07-06-2012
Quote:
Originally Posted by Majortom71
I basically want to inform main that a client disconnected in order to decrement the thread counter.
Why not use a semaphore? They're threadsafe counters which do exactly what you want. See man sem_init and man sem_wait and man sem_post.

Code:
sem_t sem;

void main_thread(void)
{
        sem_init(&sem, 0, 5); // Create sem with value of 5
        while(1)
        {
                sem_wait(&sem); // If sem goes below 5, main will wait until something else adds
                if(!running) break;
                launch_thread(threadfunc);
        }
}

void threadfunc(void)
{
        while(1)
        {
                do_stuff;
                if(!running) break;
        }

        sem_post(&sem); // add 1 to sem
}

 

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LEARN ABOUT FREEBSD

sem_wait

SEM_WAIT(3)						   BSD Library Functions Manual 					       SEM_WAIT(3)

NAME

     sem_wait, sem_trywait -- decrement (lock) a semaphore

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <semaphore.h>

     int
     sem_wait(sem_t *sem);

     int
     sem_trywait(sem_t *sem);

DESCRIPTION

     The sem_wait() function decrements (locks) the semaphore pointed to by sem, but blocks if the value of sem is zero, until the value is non-
     zero and the value can be decremented.

     The sem_trywait() function decrements (locks) the semaphore pointed to by sem only if the value is non-zero.  Otherwise, the semaphore is not
     decremented and an error is returned.

RETURN VALUES

     Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS

     The sem_wait() and sem_trywait() functions will fail if:

     [EINVAL]		The sem argument points to an invalid semaphore.

     Additionally, sem_wait() will fail if:

     [EINTR]		A signal interrupted this function.

     Additionally, sem_trywait() will fail if:

     [EAGAIN]		The semaphore value was zero, and thus could not be decremented.

SEE ALSO

     sem_getvalue(3), sem_post(3), sem_timedwait(3)

STANDARDS

     The sem_wait() and sem_trywait() functions conform to ISO/IEC 9945-1:1996 (``POSIX.1'').

BSD
								  April 16, 2013							       BSD
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