sched_setscheduler(2) System Calls Manual sched_setscheduler(2)
NAME
sched_setscheduler, sched_getscheduler - set and get scheduling pol-
icy/parameters
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sched.h>
int sched_setscheduler(pid_t pid, int policy,
const struct sched_param *param);
int sched_getscheduler(pid_t pid);
DESCRIPTION
The sched_setscheduler() system call sets both the scheduling policy and
parameters for the thread whose ID is specified in pid. If pid equals
zero, the scheduling policy and parameters of the calling thread will be
set.
The scheduling parameters are specified in the param argument, which is
a pointer to a structure of the following form:
struct sched_param {
...
int sched_priority;
...
};
In the current implementation, the structure contains only one field,
sched_priority. The interpretation of param depends on the selected
policy.
Currently, Linux supports the following "normal" (i.e., non-real-time)
scheduling policies as values that may be specified in policy:
SCHED_OTHER the standard round-robin time-sharing policy;
SCHED_BATCH for "batch" style execution of processes; and
SCHED_IDLE for running very low priority background jobs.
For each of the above policies, param->sched_priority must be 0.
Various "real-time" policies are also supported, for special time-criti-
cal 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 sched(7). The real-time
policies that may be specified in policy are:
SCHED_FIFO a first-in, first-out policy; and
SCHED_RR a round-robin policy.
For each of the above policies, param->sched_priority specifies a sched-
uling priority for the thread. This is a number in the range returned
by calling sched_get_priority_min(2) and sched_get_priority_max(2) with
the specified policy. On Linux, these system calls return, respec-
tively, 1 and 99.
Since Linux 2.6.32, the SCHED_RESET_ON_FORK flag can be ORed in policy
when calling sched_setscheduler(). As a result of including this flag,
children created by fork(2) do not inherit privileged scheduling poli-
cies. See sched(7) for details.
sched_getscheduler() returns the current scheduling policy of the thread
identified by pid. If pid equals zero, the policy of the calling thread
will be retrieved.
RETURN VALUE
On success, sched_setscheduler() returns zero. On success,
sched_getscheduler() returns the policy for the thread (a nonnegative
integer). On error, both calls return -1, and errno is set to indicate
the error.
ERRORS
EINVAL Invalid arguments: pid is negative or param is NULL.
EINVAL (sched_setscheduler()) policy is not one of the recognized poli-
cies.
EINVAL (sched_setscheduler()) param does not make sense for the speci-
fied policy.
EPERM The calling thread does not have appropriate privileges.
ESRCH The thread whose ID is pid could not be found.
VERSIONS
POSIX.1 does not detail the permissions that an unprivileged thread re-
quires in order to call 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.
The scheduling policy and parameters are in fact per-thread attributes
on Linux. The value returned from a call to gettid(2) can be passed in
the argument pid. Specifying pid as 0 will operate on the attributes of
the calling thread, and passing the value returned from a call to get-
pid(2) will operate on the attributes of the main thread of the thread
group. (If you are using the POSIX threads API, then use
pthread_setschedparam(3), pthread_getschedparam(3), and
pthread_setschedprio(3), instead of the sched_*(2) system calls.)
STANDARDS
POSIX.1-2008 (but see BUGS below).
SCHED_BATCH and SCHED_IDLE are Linux-specific.
HISTORY
POSIX.1-2001.
NOTES
Further details of the semantics of all of the above "normal" and "real-
time" scheduling policies can be found in the sched(7) manual page.
That page also describes an additional policy, SCHED_DEADLINE, which is
settable only via sched_setattr(2).
POSIX systems on which sched_setscheduler() and sched_getscheduler() are
available define _POSIX_PRIORITY_SCHEDULING in <unistd.h>.
BUGS
POSIX.1 says that on success, sched_setscheduler() should return the
previous scheduling policy. Linux sched_setscheduler() does not conform
to this requirement, since it always returns 0 on success.
SEE ALSO
chrt(1), nice(2), sched_get_priority_max(2), sched_get_priority_min(2),
sched_getaffinity(2), sched_getattr(2), sched_getparam(2),
sched_rr_get_interval(2), sched_setaffinity(2), sched_setattr(2),
sched_setparam(2), sched_yield(2), setpriority(2), capabilities(7),
cpuset(7), sched(7)
Linux man-pages 6.9.1 2024-05-02 sched_setscheduler(2)
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