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getpriority, setpriority
get/set process scheduling priority

#include <sys/resource.h>
getpriority(int which, id_t who);
setpriority(int which, id_t who, int 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. Priority values outside the range -20 to 20 are truncated to the appropriate limit. Only the superuser 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() will fail if:
No process was located using the which and who values specified.
which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER.
In addition, setpriority() will fail if:
A process was located, but neither its effective nor real user ID matched the effective user ID of the caller.
A non-superuser attempted to lower a process priority.

nice(1), fork(2), renice(8)

The getpriority() and setpriority() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

The predecessor of these functions, the former nice() system call, appeared in Version 3 AT&T UNIX and was removed in 4.3BSD-Reno. The getpriority() and setpriority() system calls appeared in 4.1cBSD.
September 10, 2015 OpenBSD-current