NAME

sched_setscheduler, sched_getscheduler - set and get scheduling policy/parameters

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-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 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 scheduling 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, respectively, 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 policies. 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 appropriately.

ERRORS

EINVAL: Invalid arguments: pid is negative or param is NULL.

EINVAL: (sched_setscheduler()) policy is not one of the recognized policies.

EINVAL: (sched_setscheduler()) param does not make sense for the specified policy.

EPERM: The calling thread does not have appropriate privileges.

ESRCH: The thread whose ID is pid could not be found.

CONFORMING TO

POSIX.1-2001, POSIX.1-2008 (but see BUGS below). The SCHED_BATCH and SCHED_IDLE policies are Linux-specific.

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>.

POSIX.1 does not detail the permissions that an unprivileged thread requires 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 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 pthread_setschedparam(3), pthread_getschedparam(3), and pthread_setschedprio(3), instead of the sched_*(2) system calls.)

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.