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NAME

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

LIBRARY

       POSIX threads library (libpthread, -lpthread)

SYNOPSIS

       #include <pthread.h>

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

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(), pthread_getschedparam()                              │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       POSIX.1-2008.

HISTORY

       glibc 2.0 POSIX.1-2001.

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).

EXAMPLES

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

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

       [[noreturn]]
       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, const 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);

               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)