illumos: 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