Ubuntu Manpage: sched_setscheduler, sched_getscheduler - set and get scheduling policy/parameters

NAME

       sched_setscheduler, sched_getscheduler - set and get scheduling policy/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-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 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 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.

STANDARDS

       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.