The “scheduling” thread attributes

Finally, if you do specify PTHREAD_EXPLICIT_SCHED for pthread_attr_setinheritsched(), then you'll need a way to specify both the scheduling policy and the priority of the thread you're about to create.

This is done with the two functions:

int
pthread_attr_setschedparam (pthread_attr_t *attr,
                            const struct sched_param *param);

int
pthread_attr_setschedpolicy (pthread_attr_t *attr,
                             int policy);

The policy is simple; it's one of SCHED_FIFO, SCHED_RR, or SCHED_OTHER.

The param is a structure that contains one member of relevance here: sched_priority. Set this value via direct assignment to the desired priority.

A common bug to watch out for is specifying PTHREAD_EXPLICIT_SCHED and then setting only the scheduling policy. The problem is that in an initialized attribute structure, the value of param.sched_priority is 0. This is the same priority as the IDLE process, meaning that your newly created thread will be competing for CPU with the IDLE process. Been there, done that, got the T-shirt. :-)

Enough people have been bitten by this that priority zero is reserved for the idle thread. You simply cannot run a thread at priority zero.