Tribblix: manual page: getpriority.3c

GETPRIORITY(3C) Standard C Library Functions GETPRIORITY(3C)

NAME

getpriority, setpriority - get and set the nice value

SYNOPSIS

#include <sys/resource.h>

int getpriority(int which, id_t who);

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

DESCRIPTION

The getpriority() function obtains the nice value of a process,
thread, or set of processes. The setpriority() function sets the
nice value of a process, thread, or set of processes to value+NZERO,
where NZERO is defined to be 20.

Target entities are specified by the values of the which and who
arguments. The which argument can be one of the following values:
PRIO_PROCESS, PRIO_PGRP, PRIO_USER, PRIO_GROUP, PRIO_SESSION,
PRIO_LWP, PRIO_TASK, PRIO_PROJECT, PRIO_ZONE, or PRIO_CONTRACT,
indicating that the who argument is to be interpreted as a process
ID, a process group ID, an effective user ID, an effective group ID,
a session ID, a thread (lwp) ID, a task ID, a project ID, a zone ID,
or a process contract ID, respectively. A 0 value for the who
argument specifies the current process, process group, or user. A 0
value for the who argument is treated as valid group ID, session ID,
thread (lwp) ID, task ID, project ID, zone ID, or process contract
ID. A P_MYID value for the who argument can be used to specify the
current group, session, thread, task, project, zone, or process
contract, respectively.

If a specified process is multi-threaded, the nice value set with
setpriority() affects all threads in the process.

If more than one process is specified, getpriority() returns NZERO
less than the lowest nice value pertaining to any of the specified
entities, and setpriority() sets the nice values of all of the
specified processes to value+NZERO.

The default nice value is NZERO. Lower nice values cause more
favorable scheduling. The range of valid nice values is 0 to
NZERO*2-1. If value+NZERO is less than the system's lowest supported
nice value, setpriority() sets the nice value to the lowest supported
value. If value+NZERO is greater than the system's highest supported
nice value, setpriority() sets the nice value to the highest
supported value.

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

Any process or thread using SCHED_FIFO or SCHED_RR is unaffected by a
call to setpriority(). This is not considered an error. A process or
thread that subsequently reverts to SCHED_OTHER will not have its
priority affected by such a setpriority() call.

The effect of changing the nice value varies depending on the
scheduling policy in effect.

Since getpriority() can return the value -1 on 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.

RETURN VALUES

Upon successful completion, getpriority() returns an integer in the
range from -NZERO to NZERO-1. Otherwise, -1 is returned and errno is
set to indicate the error.

Upon successful completion, setpriority() returns 0. Otherwise, -1 is
returned and errno is set to indicate the error.

ERRORS

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

ESRCH
No process or thread could be located using the which and
who argument values specified.

EINVAL
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, user ID, group ID, session ID, thread
(lwp) ID, task ID, project ID, or zone ID.

In addition, setpriority() may fail if:

EPERM
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.

EACCES
A request was made to change the nice value to a lower
numeric value and the current process does not have
appropriate privileges.

EXAMPLES

Example 1: Example using getpriority()

The following example returns the current scheduling priority for the
process ID returned by the call to getpid(2).

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

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

Example 2: Example using setpriority()

The following example sets the nice value for the current process to
0.

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

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

USAGE

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

ATTRIBUTES