pthread_setschedprio( ) function shall set the scheduling priority for the thread whose thread ID is given by thread to the value given by prio. See Scheduling Policies (on page 44) for a description on how this function call affects the ordering of the thread in the thread list for its new priority. If the pthread_setschedprio( ) function fails, the scheduling priority of the target thread shall not be changed. If successful, the pthread_setschedprio( ) function shall return zero; otherwise, an error number shall be returned to indicate the error. If a thread whose policy or priority has been modified by pthread_setschedprio( ) is a running thread or is runnable, the effect on its position in the thread list depends on the direction of the modification, as follows: a. If the priority is raised, the thread becomes the tail of the thread list. b. If the priority is unchanged, the thread does not change position in the thread list. c. If the priority is lowered, the thread becomes the head of the thread list.