.\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\"
.TH sched_setscheduler 2 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
sched_setscheduler, sched_getscheduler \-
set and get scheduling policy/parameters
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sched.h>
.P
.BI "int sched_setscheduler(pid_t " pid ", int " policy ,
.BI "                       const struct sched_param *" param );
.BI "int sched_getscheduler(pid_t " pid );
.fi
.SH DESCRIPTION
The
.BR sched_setscheduler ()
system call
sets both the scheduling policy and parameters for the
thread whose ID is specified in \fIpid\fP.
If \fIpid\fP equals zero, the
scheduling policy and parameters of the calling thread will be set.
.P
The scheduling parameters are specified in the
.I param
argument, which is a pointer to a structure of the following form:
.P
.in +4n
.EX
struct sched_param {
    ...
    int sched_priority;
    ...
};
.EE
.in
.P
In the current implementation, the structure contains only one field,
.IR sched_priority .
The interpretation of
.I param
depends on the selected policy.
.P
Currently, Linux supports the following "normal"
(i.e., non-real-time) scheduling policies as values that may be specified in
.IR policy :
.TP 14
.B SCHED_OTHER
the standard round-robin time-sharing policy;
.\" In the 2.6 kernel sources, SCHED_OTHER is actually called
.\" SCHED_NORMAL.
.TP
.B SCHED_BATCH
for "batch" style execution of processes; and
.TP
.B SCHED_IDLE
for running
.I very
low priority background jobs.
.P
For each of the above policies,
.I param\->sched_priority
must be 0.
.P
Various "real-time" policies are also supported,
for special time-critical applications that need precise control over
the way in which runnable threads are selected for execution.
For the rules governing when a process may use these policies, see
.BR sched (7).
The real-time policies that may be specified in
.I policy
are:
.TP 14
.B SCHED_FIFO
a first-in, first-out policy; and
.TP
.B SCHED_RR
a round-robin policy.
.P
For each of the above policies,
.I param\->sched_priority
specifies a scheduling priority for the thread.
This is a number in the range returned by calling
.BR sched_get_priority_min (2)
and
.BR sched_get_priority_max (2)
with the specified
.IR policy .
On Linux, these system calls return, respectively, 1 and 99.
.P
Since Linux 2.6.32, the
.B SCHED_RESET_ON_FORK
flag can be ORed in
.I policy
when calling
.BR sched_setscheduler ().
As a result of including this flag, children created by
.BR fork (2)
do not inherit privileged scheduling policies.
See
.BR sched (7)
for details.
.P
.BR sched_getscheduler ()
returns the current scheduling policy of the thread
identified by \fIpid\fP.
If \fIpid\fP equals zero, the policy of the
calling thread will be retrieved.
.SH RETURN VALUE
On success,
.BR sched_setscheduler ()
returns zero.
On success,
.BR sched_getscheduler ()
returns the policy for the thread (a nonnegative integer).
On error, both calls return \-1, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
Invalid arguments:
.I pid
is negative or
.I param
is NULL.
.TP
.B EINVAL
.RB ( sched_setscheduler ())
.I policy
is not one of the recognized policies.
.TP
.B EINVAL
.RB ( sched_setscheduler ())
.I param
does not make sense for the specified
.IR policy .
.TP
.B EPERM
The calling thread does not have appropriate privileges.
.TP
.B ESRCH
The thread whose ID is \fIpid\fP could not be found.
.SH VERSIONS
POSIX.1 does not detail the permissions that an unprivileged
thread requires in order to call
.BR sched_setscheduler (),
and details vary across systems.
For example, the Solaris 7 manual page says that
the real or effective user ID of the caller must
match the real user ID or the save set-user-ID of the target.
.P
The scheduling policy and parameters are in fact per-thread
attributes on Linux.
The value returned from a call to
.BR gettid (2)
can be passed in the argument
.IR pid .
Specifying
.I pid
as 0 will operate on the attributes of the calling thread,
and passing the value returned from a call to
.BR getpid (2)
will operate on the attributes of the main thread of the thread group.
(If you are using the POSIX threads API, then use
.BR pthread_setschedparam (3),
.BR pthread_getschedparam (3),
and
.BR pthread_setschedprio (3),
instead of the
.BR sched_* (2)
system calls.)
.SH STANDARDS
POSIX.1-2008 (but see BUGS below).
.P
.B SCHED_BATCH
and
.B SCHED_IDLE
are Linux-specific.
.SH HISTORY
POSIX.1-2001.
.SH NOTES
Further details of the semantics of all of the above "normal"
and "real-time" scheduling policies can be found in the
.BR sched (7)
manual page.
That page also describes an additional policy,
.BR SCHED_DEADLINE ,
which is settable only via
.BR sched_setattr (2).
.P
POSIX systems on which
.BR sched_setscheduler ()
and
.BR sched_getscheduler ()
are available define
.B _POSIX_PRIORITY_SCHEDULING
in \fI<unistd.h>\fP.
.SH BUGS
POSIX.1 says that on success,
.BR sched_setscheduler ()
should return the previous scheduling policy.
Linux
.BR sched_setscheduler ()
does not conform to this requirement,
since it always returns 0 on success.
.SH SEE ALSO
.ad l
.nh
.BR chrt (1),
.BR nice (2),
.BR sched_get_priority_max (2),
.BR sched_get_priority_min (2),
.BR sched_getaffinity (2),
.BR sched_getattr (2),
.BR sched_getparam (2),
.BR sched_rr_get_interval (2),
.BR sched_setaffinity (2),
.BR sched_setattr (2),
.BR sched_setparam (2),
.BR sched_yield (2),
.BR setpriority (2),
.BR capabilities (7),
.BR cpuset (7),
.BR sched (7)
.ad