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BSD 2.11 - man page for getpriority (bsd section 2)

GETPRIORITY(2)							System Calls Manual						    GETPRIORITY(2)

getpriority, setpriority - get/set program scheduling priority
#include <sys/resource.h> prio = getpriority(which, who) int prio, which, who; setpriority(which, who, prio) int which, who, prio;
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 afterward 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.
Getpriority and setpriority may return one of the following errors: [ESRCH] No process was located using the which and who values specified. [EINVAL] Which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER. In addition to the errors indicated above, setpriority may fail with one of the following errors returned: [EPERM] A process was located, but neither its effective nor real user ID matched the effective user ID of the caller. [EACCES] A non super-user attempted to lower a process priority.
nice(1), fork(2), renice(8) 4th Berkeley Distribution May 22, 1986 GETPRIORITY(2)