PTHREAD_SETSCHEDPARAM(3)				     Linux Programmer's Manual					  PTHREAD_SETSCHEDPARAM(3)

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 follow-
       ing 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 schedul-
       ing 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_mutex-
       attr_setprotocol(3)).

RETURN VALUE

       On  success, these functions return 0; on error, they return a nonzero error number.  If pthread_setschedparam() fails, the scheduling pol-
       icy 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).

       +-------------------------+---------------+---------+
       |Interface		 | Attribute	 | Value   |
       +-------------------------+---------------+---------+
       |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]
", prog_name);
	   fprintf(stderr, "Options are:
");
       #define fpe(msg) fprintf(stderr, "	%s", msg);	       /* Shorter */
	   fpe("-a<policy><prio> Set scheduling policy and priority in
");
	   fpe("		 thread attributes object
");
	   fpe("		 <policy> can be
");
	   fpe("		     f	SCHED_FIFO
");
	   fpe("		     r	SCHED_RR
");
	   fpe("		     o	SCHED_OTHER
");
	   fpe("-A		 Use default thread attributes object
");
	   fpe("-i {e|i}	 Set inherit scheduler attribute to
");
	   fpe("		 'explicit' or 'inherit'
");
	   fpe("-m<policy><prio> Set scheduling policy and priority on
");
	   fpe("		 main thread before pthread_create() call
");
	   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
",
		   (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
", 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
");
	       }
	   }

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

	   /* 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)
");
	       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("
");

	   /* 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'
");

	       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)
");
	       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'
");
	       display_sched_attr(policy, &param);

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

	   /* 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

       This page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-15						  PTHREAD_SETSCHEDPARAM(3)