Mastering Embedded Linux Programming - Second Edition

Changing conditions

Cooperating threads need a method of alerting one another that something has changed and needs attention. That thing is called a condition and the alert is sent through a condition variable, or condvar.

A condition is just something that you can test to give a true or false result. A simple example is a buffer that contains either zero or some items. One thread takes items from the buffer and sleeps when it is empty. Another thread places items into the buffer and signals the other thread that it has done so because the condition that the other thread is waiting on has changed. If it is sleeping, it needs to wake up and do something. The only complexity is that the condition is, by definition, a shared resource and so ...