getpriority(S)

getpriority, setpriority -- get/set program scheduling priority

Syntax

cc . . . -lc

#include <sys/time.h>
#include <sys/resource.h>

int getpriority(int which, id_t who);

int setpriority(int which, id_t who, int prio);

Description

The scheduling priority of the process, process group, or user, as indicated by which and who is obtained with getpriority and set with setpriority. The default priority is 0; lower priorities cause more favorable scheduling.

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.

getpriority returns the highest priority (lowest numerical value) enjoyed by any of the specified processes. setpriority sets the priorities of all of the specified processes to the value specified by prio. If prio is less than -20, a value of -20 is used; if it is greater than 20, a value of 20 is used. Only the privileged user may lower priorities.

Return values

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.

Diagnostics

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, or else who was not a valid process ID, process group ID or user ID.

In addition to the errors indicated above, setpriority may fail with the following error returned:

[EPERM]

A process was located, but one of the following is true:

Neither its effective nor real user ID matched the effective user ID of the caller, and neither the effective nor the real user ID of the process executing the setpriority was the privileged user.

The call to setpriority would have changed a process' priority to a value lower than its current value, and the effective user ID of the process executing the call was not that of the privileged user.

Notes

It is not possible for the process executing setpriority to lower any other process down to its current priority, without requiring privileged user privileges.

Files

/lib/libc.a: linking library

Standards conformance

getpriority and setpriority are conformant with:

X/Open Portability Guide Issue 4, Version 2 (Spec-1170).