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 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 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 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.
ERRORS
The getpriority() and setpriority() functions shall fail if:
ESRCH No process 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,
or user 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.
The following sections are informative.
EXAMPLES
Using getpriority()
The following example returns the current scheduling priority for the
process ID returned by the call to getpid().
#include <sys/resource.h>
...
int which = PRIO_PROCESS;
id_t pid;
int ret;
pid = getpid();
ret = getpriority(which, pid);
Using setpriority()
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);
APPLICATION USAGE
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].
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
nice() , sched_get_priority_max() , sched_setscheduler() , the Base
Definitions volume of IEEE Std 1003.1-2001, <sys/resource.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .