sched_setscheduler(2)

1sched_setscheduler(2)         System Calls Manual        sched_setscheduler(2)
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NAME

6       sched_setscheduler,  sched_getscheduler  -  set and get scheduling pol‐
7       icy/parameters
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LIBRARY

10       Standard C library (libc, -lc)
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SYNOPSIS

13       #include <sched.h>
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15       int sched_setscheduler(pid_t pid, int policy,
16                              const struct sched_param *param);
17       int sched_getscheduler(pid_t pid);
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DESCRIPTION

20       The sched_setscheduler() system call sets both  the  scheduling  policy
21       and  parameters  for  the  thread whose ID is specified in pid.  If pid
22       equals zero, the scheduling policy and parameters of the calling thread
23       will be set.
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25       The scheduling parameters are specified in the param argument, which is
26       a pointer to a structure of the following form:
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28           struct sched_param {
29               ...
30               int sched_priority;
31               ...
32           };
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34       In the current implementation, the structure contains only  one  field,
35       sched_priority.   The  interpretation  of param depends on the selected
36       policy.
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38       Currently, Linux supports the following "normal" (i.e.,  non-real-time)
39       scheduling policies as values that may be specified in policy:
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41       SCHED_OTHER   the standard round-robin time-sharing policy;
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43       SCHED_BATCH   for "batch" style execution of processes; and
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45       SCHED_IDLE    for running very low priority background jobs.
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47       For each of the above policies, param->sched_priority must be 0.
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49       Various "real-time" policies are also supported, for special time-crit‐
50       ical applications that need precise  control  over  the  way  in  which
51       runnable  threads  are selected for execution.  For the rules governing
52       when a process may use these policies,  see  sched(7).   The  real-time
53       policies that may be specified in policy are:
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55       SCHED_FIFO    a first-in, first-out policy; and
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57       SCHED_RR      a round-robin policy.
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59       For  each  of  the  above  policies,  param->sched_priority specifies a
60       scheduling priority for the thread.  This is a number in the range  re‐
61       turned   by   calling  sched_get_priority_min(2)  and  sched_get_prior‐
62       ity_max(2) with the specified policy.  On Linux, these system calls re‐
63       turn, respectively, 1 and 99.
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65       Since  Linux 2.6.32, the SCHED_RESET_ON_FORK flag can be ORed in policy
66       when calling sched_setscheduler().  As a result of including this flag,
67       children  created by fork(2) do not inherit privileged scheduling poli‐
68       cies.  See sched(7) for details.
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70       sched_getscheduler() returns  the  current  scheduling  policy  of  the
71       thread  identified by pid.  If pid equals zero, the policy of the call‐
72       ing thread will be retrieved.
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RETURN VALUE

75       On   success,   sched_setscheduler()   returns   zero.    On   success,
76       sched_getscheduler()  returns  the policy for the thread (a nonnegative
77       integer).  On error, both calls return -1, and errno is set to indicate
78       the error.
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ERRORS

81       EINVAL Invalid arguments: pid is negative or param is NULL.
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83       EINVAL (sched_setscheduler()) policy is not one of the recognized poli‐
84              cies.
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86       EINVAL (sched_setscheduler()) param does not make sense for the  speci‐
87              fied policy.
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89       EPERM  The calling thread does not have appropriate privileges.
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91       ESRCH  The thread whose ID is pid could not be found.
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VERSIONS

94       POSIX.1 does not detail the permissions that an unprivileged thread re‐
95       quires in order to call sched_setscheduler(), and details  vary  across
96       systems.   For example, the Solaris 7 manual page says that the real or
97       effective user ID of the caller must match the real user ID or the save
98       set-user-ID of the target.
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100       The  scheduling policy and parameters are in fact per-thread attributes
101       on Linux.  The value returned from a call to gettid(2) can be passed in
102       the  argument  pid.  Specifying pid as 0 will operate on the attributes
103       of the calling thread, and passing the value returned from  a  call  to
104       getpid(2)  will  operate  on  the  attributes of the main thread of the
105       thread group.  (If you are  using  the  POSIX  threads  API,  then  use
106       pthread_setschedparam(3),         pthread_getschedparam(3),         and
107       pthread_setschedprio(3), instead of the sched_*[22m(2) system calls.)
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STANDARDS

110       POSIX.1-2008 (but see BUGS below).
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112       SCHED_BATCH and SCHED_IDLE are Linux-specific.
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HISTORY

115       POSIX.1-2001.
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NOTES

118       Further details of the semantics of  all  of  the  above  "normal"  and
119       "real-time"  scheduling  policies  can  be found in the sched(7) manual
120       page.  That page also describes an additional  policy,  SCHED_DEADLINE,
121       which is settable only via sched_setattr(2).
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123       POSIX  systems  on  which sched_setscheduler() and sched_getscheduler()
124       are available define _POSIX_PRIORITY_SCHEDULING in <unistd.h>.
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BUGS

127       POSIX.1 says that on success, sched_setscheduler()  should  return  the
128       previous  scheduling  policy.  Linux sched_setscheduler() does not con‐
129       form to this requirement, since it always returns 0 on success.
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