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30 April 2012

If I must use multiple threads, I communicate with channels1. Channels allow different threads to produce and consume objects in a structured fashion.

This is also a good demonstration of the difference between a mutex and a semaphore. The mutex in the following is used to prevent multiple threads removing items at the same time, or adding them at the same time. It would not be necessary if there is only one consumer and one producer.

The two semaphores allow a thread to wait until there is something for it to do. One semaphore is for consumers to wait on, the other is for producers. After consuming or producing they then signal the other semaphore to allow the other type to act.

Here is an example of it in use. If you have two functions of type void *() (void *) called producer and consumer the producer can add a char by calling channel_add(channel, char) and the producer uses channel_get(channel, ptr_to_char).

#include <channel.h>

pthread_t prod, cons;
channel_t channel;

/* Channel with a single char */
channel_init(&amp;channel, 1, sizeof (char));

pthread_create(&amp;prod, NULL, producer, (void *) &amp;channel);
pthread_create(&amp;cons, NULL, consumer, (void *) &amp;channel);

pthread_join(prod, NULL);
pthread_join(cons, NULL);

  1. If I can’t use threads then I run multiple non-communicating processes.