Producer/consumer

#include        <stdio.h>
#include        <stdlib.h>
#include        <string.h>
#include        <thread.h>
#include        <synch.h>
#define TRUE    1
#define FALSE   0
static  void    *producer(void*);
static  void    *consumer(void*);
static  char    Buff[BUFSIZ];
static  cond_t  Buff_cond;
static  mutex_t Buff_mutex;
static  int     DataInBuff = FALSE;
main()
{
        (void)mutex_init(&Buff_mutex, USYNC_THREAD,  NULL);
        (void)cond_init (&Buff_cond,  USYNC_THREAD,  NULL);
        (void)thr_create(NULL, 0, producer, NULL, 0, NULL);
        (void)thr_create(NULL, 0, consumer, NULL, 0, NULL);
        thr_exit(NULL);
        /*NOTREACHED*/
}
/*ARGSUSED*/
static void *producer(void *dummy)
{
	(void)mutex_lock(&Buff_mutex);
        for(;;){
                while(DataInBuff == TRUE)
                        cond_wait(&Buff_cond, &Buff_mutex);
		/* At this point,
		 * the buffer is empty (contents have been output).
		 * (Re)fill the buffer.
		 */
                if(fgets(Buff, sizeof(Buff), stdin) == NULL)
                        exit(EXIT_SUCCESS);
                DataInBuff = TRUE;
                cond_signal (&Buff_cond);
        }
        /*NOTREACHED*/
}
/*ARGSUSED*/
static void *consumer(void *dummy)
{
        (void)mutex_lock(&Buff_mutex);
        for(;;){
                while(DataInBuff == FALSE)
                        cond_wait(&Buff_cond, &Buff_mutex);
		/* At this point,
		 * the buffer has data to be output
		 */
                (void)fputs(Buff, stdout);
                DataInBuff = FALSE;
                cond_signal(&Buff_cond);
        }
        /*NOTREACHED*/
}

Producer/consumer

The program in ``Producer/consumer'' shows a simple producer/consumer example implemented using the condition variables facilities of the Threads Library.

• There are two threads, each running different functions. One runs producer, the other runs consumer.

• The item being produced and consumed is data in a common buffer.

  • The producer obtains that data with fgets (see gets(3S)) and places the data in the common buffer.

  • The consumer reads the data from the buffer. That data is output with fputs (see puts(3S)) so that its actions can be confirmed.

• The actions of producer and consumer threads are coordinated by the condition variable facility so that they run in strict alternation.

  • Nothing will be output (consumed) until something is placed in the buffer (produced).

  • Data in the buffer will not be overwritten until it is output.

• Note that this example differs from the pseudo-code shown earlier. The use of the condition variables are not bracketed by calls to mutex_lock(3synch) and mutex_unlock(3synch).

  • This curiosity arises because the actions of each of these threads is organized in a loop.

  • The semantics of cond_wait(3synch) guarantee that the named ``mutex'' will be released while a thread is waiting and reacquired before return from that function.

  • The condition (DataInBuff) is tested (for different values) by each thread only when the thread holds the ``mutex'' and the use of cond_wait by each threads allows the other to acquire the ``mutex''.

• The initial calls to mutex_lock(3synch) by each thread and the initial state of DataInBuff are organized so that:

  • Proper conditions are achieved for the initial pass by each thread.

  • The program will work correctly no matter which thread acquires the ``mutex'' on the first pass.

• The producer thread terminates the process with the exit(2) system call when it can obtain no more data from fgets (see gets(3S)).