#include <sys/time.h> #include <sys/resource.h>
The scheduling priority of the process, process group, or user, as indicated by which and who is obtained with the getpriority call and set with the setpriority call. Which 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. Prio is a value in the range -20 to 20. The default priority is 0; lower priorities cause more favorable scheduling.
The getpriority call returns the highest priority (lowest numerical value) enjoyed by any of the specified processes. The setpriority 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 afterwards to determine if a -1 is an error or a legitimate value. The setpriority call returns 0 if there is no error, or -1 if there is.
The actual priority range varies between kernel versions. Linux before 1.3.36 had -infinity..15. Linux since 1.3.43 has -20..19, and the system call getpriority returns 40..1 for these values (since negative numbers are error codes). The library call converts N into 20-N.
Including <sys/time.h> is not required these days, but increases portability. (Indeed, <sys/resource.h> defines the rusage structure with fields of type struct timeval defined in <sys/time.h>.)
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD).