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NAME

       pthread_setschedparam, pthread_getschedparam - set/get scheduling policy and parameters of
       a thread

SYNOPSIS

       #include <pthread.h>

       int pthread_setschedparam(pthread_t thread, int policy,
                                 const struct sched_param *param);
       int pthread_getschedparam(pthread_t thread, int *policy,
                                 struct sched_param *param);

       Compile and link with -pthread.

DESCRIPTION

       The pthread_setschedparam() function sets the scheduling  policy  and  parameters  of  the
       thread thread.

       policy  specifies  the new scheduling policy for thread.  The supported values for policy,
       and their semantics, are described in sched(7).

       The structure pointed to by param specifies the  new  scheduling  parameters  for  thread.
       Scheduling parameters are maintained in the following structure:

           struct sched_param {
               int sched_priority;     /* Scheduling priority */
           };

       As  can be seen, only one scheduling parameter is supported.  For details of the permitted
       ranges for scheduling priorities in each scheduling policy, see sched(7).

       The pthread_getschedparam() function returns the scheduling policy and parameters  of  the
       thread  thread, in the buffers pointed to by policy and param, respectively.  The returned
       priority   value   is   that   set   by   the   most    recent    pthread_setschedparam(),
       pthread_setschedprio(3),  or  pthread_create(3)  call  that affected thread.  The returned
       priority does not reflect any temporary priority adjustments as a result of calls  to  any
       priority    inheritance    or    priority    ceiling    functions   (see,   for   example,
       pthread_mutexattr_setprioceiling(3) and pthread_mutexattr_setprotocol(3)).

RETURN VALUE

       On success, these functions return 0; on error, they return a nonzero  error  number.   If
       pthread_setschedparam()  fails,  the  scheduling  policy  and parameters of thread are not
       changed.

ERRORS

       Both of these functions can fail with the following error:

       ESRCH  No thread with the ID thread could be found.

       pthread_setschedparam() may additionally fail with the following errors:

       EINVAL policy is not a recognized policy, or param does not make sense for the policy.

       EPERM  The caller does not have appropriate privileges to  set  the  specified  scheduling
              policy and parameters.

       POSIX.1  also  documents  an  ENOTSUP  ("attempt  was made to set the policy or scheduling
       parameters to an unsupported value") error for pthread_setschedparam().

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌─────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├─────────────────────────┼───────────────┼─────────┤
       │pthread_setschedparam(), │ Thread safety │ MT-Safe │
       │pthread_getschedparam()  │               │         │
       └─────────────────────────┴───────────────┴─────────┘

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008.

NOTES

       For  a  description of the permissions required to, and the effect of, changing a thread's
       scheduling policy and priority, and details of the permitted ranges for priorities in each
       scheduling policy, see sched(7).

EXAMPLE

       The    program    below    demonstrates    the    use   of   pthread_setschedparam()   and
       pthread_getschedparam(), as well as the  use  of  a  number  of  other  scheduling-related
       pthreads functions.

       In  the  following  run,  the  main thread sets its scheduling policy to SCHED_FIFO with a
       priority of 10, and initializes a  thread  attributes  object  with  a  scheduling  policy
       attribute  of  SCHED_RR  and a scheduling priority attribute of 20.  The program then sets
       (using pthread_attr_setinheritsched(3)) the inherit  scheduler  attribute  of  the  thread
       attributes  object  to  PTHREAD_EXPLICIT_SCHED,  meaning  that  threads created using this
       attributes object should take their  scheduling  attributes  from  the  thread  attributes
       object.   The  program  then creates a thread using the thread attributes object, and that
       thread displays its scheduling policy and priority.

           $ su      # Need privilege to set real-time scheduling policies
           Password:
           # ./a.out -mf10 -ar20 -i e
           Scheduler settings of main thread
               policy=SCHED_FIFO, priority=10

           Scheduler settings in 'attr'
               policy=SCHED_RR, priority=20
               inheritsched is EXPLICIT

           Scheduler attributes of new thread
               policy=SCHED_RR, priority=20

       In the above output, one can see that the scheduling policy and priority were  taken  from
       the values specified in the thread attributes object.

       The  next  run is the same as the previous, except that the inherit scheduler attribute is
       set to PTHREAD_INHERIT_SCHED, meaning that threads created  using  the  thread  attributes
       object  should  ignore  the  scheduling  attributes specified in the attributes object and
       instead take their scheduling attributes from the creating thread.

           # ./a.out -mf10 -ar20 -i i
           Scheduler settings of main thread
               policy=SCHED_FIFO, priority=10

           Scheduler settings in 'attr'
               policy=SCHED_RR, priority=20
               inheritsched is INHERIT

           Scheduler attributes of new thread
               policy=SCHED_FIFO, priority=10

       In the above output, one can see that the scheduling policy and priority were  taken  from
       the creating thread, rather than the thread attributes object.

       Note  that  if  we had omitted the -i i option, the output would have been the same, since
       PTHREAD_INHERIT_SCHED is the default for the inherit scheduler attribute.

   Program source

       /* pthreads_sched_test.c */

       #include <pthread.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <errno.h>

       #define handle_error_en(en, msg) \
               do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)

       static void
       usage(char *prog_name, char *msg)
       {
           if (msg != NULL)
               fputs(msg, stderr);

           fprintf(stderr, "Usage: %s [options]\n", prog_name);
           fprintf(stderr, "Options are:\n");
       #define fpe(msg) fprintf(stderr, "\t%s", msg);          /* Shorter */
           fpe("-a<policy><prio> Set scheduling policy and priority in\n");
           fpe("                 thread attributes object\n");
           fpe("                 <policy> can be\n");
           fpe("                     f  SCHED_FIFO\n");
           fpe("                     r  SCHED_RR\n");
           fpe("                     o  SCHED_OTHER\n");
           fpe("-A               Use default thread attributes object\n");
           fpe("-i {e|i}         Set inherit scheduler attribute to\n");
           fpe("                 'explicit' or 'inherit'\n");
           fpe("-m<policy><prio> Set scheduling policy and priority on\n");
           fpe("                 main thread before pthread_create() call\n");
           exit(EXIT_FAILURE);
       }

       static int
       get_policy(char p, int *policy)
       {
           switch (p) {
           case 'f': *policy = SCHED_FIFO;     return 1;
           case 'r': *policy = SCHED_RR;       return 1;
           case 'o': *policy = SCHED_OTHER;    return 1;
           default:  return 0;
           }
       }

       static void
       display_sched_attr(int policy, struct sched_param *param)
       {
           printf("    policy=%s, priority=%d\n",
                   (policy == SCHED_FIFO)  ? "SCHED_FIFO" :
                   (policy == SCHED_RR)    ? "SCHED_RR" :
                   (policy == SCHED_OTHER) ? "SCHED_OTHER" :
                   "???",
                   param->sched_priority);
       }

       static void
       display_thread_sched_attr(char *msg)
       {
           int policy, s;
           struct sched_param param;

           s = pthread_getschedparam(pthread_self(), &policy, &param);
           if (s != 0)
               handle_error_en(s, "pthread_getschedparam");

           printf("%s\n", msg);
           display_sched_attr(policy, &param);
       }

       static void *
       thread_start(void *arg)
       {
           display_thread_sched_attr("Scheduler attributes of new thread");

           return NULL;
       }

       int
       main(int argc, char *argv[])
       {
           int s, opt, inheritsched, use_null_attrib, policy;
           pthread_t thread;
           pthread_attr_t attr;
           pthread_attr_t *attrp;
           char *attr_sched_str, *main_sched_str, *inheritsched_str;
           struct sched_param param;

           /* Process command-line options */

           use_null_attrib = 0;
           attr_sched_str = NULL;
           main_sched_str = NULL;
           inheritsched_str = NULL;

           while ((opt = getopt(argc, argv, "a:Ai:m:")) != -1) {
               switch (opt) {
               case 'a': attr_sched_str = optarg;      break;
               case 'A': use_null_attrib = 1;          break;
               case 'i': inheritsched_str = optarg;    break;
               case 'm': main_sched_str = optarg;      break;
               default:  usage(argv[0], "Unrecognized option\n");
               }
           }

           if (use_null_attrib &&
                   (inheritsched_str != NULL || attr_sched_str != NULL))
               usage(argv[0], "Can't specify -A with -i or -a\n");

           /* Optionally set scheduling attributes of main thread,
              and display the attributes */

           if (main_sched_str != NULL) {
               if (!get_policy(main_sched_str[0], &policy))
                   usage(argv[0], "Bad policy for main thread (-m)\n");
               param.sched_priority = strtol(&main_sched_str[1], NULL, 0);

               s = pthread_setschedparam(pthread_self(), policy, &param);
               if (s != 0)
                   handle_error_en(s, "pthread_setschedparam");
           }

           display_thread_sched_attr("Scheduler settings of main thread");
           printf("\n");

           /* Initialize thread attributes object according to options */

           attrp = NULL;

           if (!use_null_attrib) {
               s = pthread_attr_init(&attr);
               if (s != 0)
                   handle_error_en(s, "pthread_attr_init");
               attrp = &attr;
           }

           if (inheritsched_str != NULL) {
               if (inheritsched_str[0] == 'e')
                   inheritsched = PTHREAD_EXPLICIT_SCHED;
               else if (inheritsched_str[0] == 'i')
                   inheritsched = PTHREAD_INHERIT_SCHED;
               else
                   usage(argv[0], "Value for -i must be 'e' or 'i'\n");

               s = pthread_attr_setinheritsched(&attr, inheritsched);
               if (s != 0)
                   handle_error_en(s, "pthread_attr_setinheritsched");
           }

           if (attr_sched_str != NULL) {
               if (!get_policy(attr_sched_str[0], &policy))
                   usage(argv[0],
                           "Bad policy for 'attr' (-a)\n");
               param.sched_priority = strtol(&attr_sched_str[1], NULL, 0);

               s = pthread_attr_setschedpolicy(&attr, policy);
               if (s != 0)
                   handle_error_en(s, "pthread_attr_setschedpolicy");
               s = pthread_attr_setschedparam(&attr, &param);
               if (s != 0)
                   handle_error_en(s, "pthread_attr_setschedparam");
           }

           /* If we initialized a thread attributes object, display
              the scheduling attributes that were set in the object */

           if (attrp != NULL) {
               s = pthread_attr_getschedparam(&attr, &param);
               if (s != 0)
                   handle_error_en(s, "pthread_attr_getschedparam");
               s = pthread_attr_getschedpolicy(&attr, &policy);
               if (s != 0)
                   handle_error_en(s, "pthread_attr_getschedpolicy");

               printf("Scheduler settings in 'attr'\n");
               display_sched_attr(policy, &param);

               s = pthread_attr_getinheritsched(&attr, &inheritsched);
               printf("    inheritsched is %s\n",
                       (inheritsched == PTHREAD_INHERIT_SCHED)  ? "INHERIT" :
                       (inheritsched == PTHREAD_EXPLICIT_SCHED) ? "EXPLICIT" :
                       "???");
               printf("\n");
           }

           /* Create a thread that will display its scheduling attributes */

           s = pthread_create(&thread, attrp, &thread_start, NULL);
           if (s != 0)
               handle_error_en(s, "pthread_create");

           /* Destroy unneeded thread attributes object */

           if (!use_null_attrib) {
             s = pthread_attr_destroy(&attr);
             if (s != 0)
                 handle_error_en(s, "pthread_attr_destroy");
           }

           s = pthread_join(thread, NULL);
           if (s != 0)
               handle_error_en(s, "pthread_join");

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       getrlimit(2), sched_get_priority_min(2), pthread_attr_init(3),
       pthread_attr_setinheritsched(3), pthread_attr_setschedparam(3),
       pthread_attr_setschedpolicy(3), pthread_create(3), pthread_self(3),
       pthread_setschedprio(3), pthreads(7), sched(7)

COLOPHON

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