SETPRIORITY(3) Library Functions Manual SETPRIORITY(3)

getpriority setpriority — get and set the nice value


#include <sys/resource.h>

int getpriority(int which, id_t who);
int setpriority(int which, id_t who, int value);

The getpriority () function shall obtain the nice value of a process, process group, or user. The setpriority () function shall set the nice value of a process, process group, or user to value +{NZERO}.

Target processes are specified by the values of the which and who arguments. The which argument may be one of the following values: PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, indicating that the who argument is to be interpreted as a process ID, a process group ID, or an effective user ID, respectively. A 0 value for the who argument specifies the current process, process group, or user.

The nice value set with setpriority () shall be applied to the process. If the process is multi-threaded, the nice value shall affect all system scope threads in the process.

If more than one process is specified, getpriority () shall return value {NZERO} less than the lowest nice value pertaining to any of the specified processes, and setpriority () shall set the nice values of all of the specified processes to value +{NZERO}.

The default nice value is {NZERO}; lower nice values shall cause more favorable scheduling. While the range of valid nice values is [0,{NZERO}*2-1], implementations may enforce more restrictive limits. If value +{NZERO} is less than the system's lowest supported nice value, setpriority () shall set the nice value to the lowest supported value; if value +{NZERO} is greater than the system's highest supported nice value, setpriority () shall set the nice value to the highest supported value.

Only a process with appropriate privileges can lower its nice value.

Any processes or threads using SCHED_FIFO or SCHED_RR shall be unaffected by a call to setpriority (). This is not considered an error. A process which subsequently reverts to SCHED_OTHER need not have its priority affected by such a setpriority () call.

The effect of changing the nice value may vary depending on the process-scheduling algorithm in effect.

Since getpriority () can return the value -1 upon successful completion, it is necessary to set errno to 0 prior to a call to getpriority (). If getpriority () returns the value -1, then errno can be checked to see if an error occurred or if the value is a legitimate nice value.

Upon successful completion, getpriority () shall return an integer in the range -{NZERO} to {NZERO}-1. Otherwise, -1 shall be returned and errno set to indicate the error.

Upon successful completion, setpriority () shall return 0; otherwise, -1 shall be returned and errno set to indicate the error.

The getpriority () and setpriority () functions shall fail if:

No process could be located using the which and who argument values specified.
The value of the which argument was not recognized, or the value of the who argument is not a valid process ID, process group ID, or user ID.

In addition, setpriority () may fail if:

A process was located, but neither the real nor effective user ID of the executing process match the effective user ID of the process whose nice value is being changed.
A request was made to change the nice value to a lower numeric value and the current process does not have appropriate privileges.

The following example returns the current scheduling priority for the process ID returned by the call to getpid(3).

#include <sys/resource.h>
...
int which = PRIO_PROCESS;
id_t pid;
int ret;


pid = getpid();
ret = getpriority(which, pid);

The following example sets the priority for the current process ID to -20.

#include <sys/resource.h>
...
int which = PRIO_PROCESS;
id_t pid;
int priority = -20;
int ret;


pid = getpid();
ret = setpriority(which, pid, priority);

The getpriority () and setpriority () functions work with an offset nice value (nice value -{NZERO}). The nice value is in the range [0,2*{NZERO} -1], while the return value for getpriority () and the third parameter for setpriority () are in the range [-{NZERO},{NZERO} -1].

None.

None.

nice(3), sched_get_priority_max(3), sched_setscheduler(3)

XBD <sys/resource.h>

First released in Issue 4, Version 2.

Moved from X/OPEN UNIX extension to BASE.

The DESCRIPTION is reworded in terms of the nice value rather than to avoid confusion with functionality in the POSIX Realtime Extension.

June 14, 2024 posix.fail