The scheduling priority of the process, process group, or user, as indicated by which and who is obtained with the getpriority system call and set with the setpriority system call. The which argument is one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, and who is interpreted relative to which (a process identifier for PRIO_PROCESS, process group identifier for PRIO_PGRP, and a user ID for PRIO_USER). A zero value of who denotes the current process, process group, or user. The prio argument is a value in the range -20 to 20. The default priority is 0; lower priorities cause more favorable scheduling.
The getpriority system call returns the highest priority (lowest numerical value) enjoyed by any of the specified processes. The setpriority system call sets the priorities of all of the specified processes to the specified value. Only the super-user may lower priorities.
Since getpriority can legitimately return the value -1, it is necessary to clear the external variable errno prior to the call, then check it afterward to determine if a -1 is an error or a legitimate value.
.Rv -std setpriority
The getpriority and setpriority system calls will fail if: