sched_attr.c

/*
 * sched_attr.c
 *
 * Demonstrate use of POSIX 1003.1c-1995 thread priority
 * scheduling attributes, by creating an attributes object with
 * realtime scheduling policy and priority.
 *
 * Special notes: Although Solaris 2.5 defines
 * _POSIX_THREAD_PRIORITY_SCHEDULING, it does not support the
 * SCHED_RR policy for threads.
 */
#include <unistd.h>
#include <pthread.h>
#include <sched.h>
#include "errors.h"

/*
 * Thread start routine. If priority scheduling is supported,
 * report the thread's scheduling attributes.
 */
void *thread_routine (void *arg)
{
    int my_policy;
    struct sched_param my_param;
    int status;

    /*
     * If the priority scheduling option is not defined, then we
     * can do nothing with the output of pthread_getschedparam,
     * so just report that the thread ran, and exit.
     */
#if defined (_POSIX_THREAD_PRIORITY_SCHEDULING) && !defined (sun)
    status = pthread_getschedparam (
        pthread_self (), &my_policy, &my_param);
    if (status != 0)
        err_abort (status, "Get sched");
    printf ("thread_routine running at %s/%d\n",
        (my_policy == SCHED_FIFO ? "FIFO"
            : (my_policy == SCHED_RR ? "RR"
            : (my_policy == SCHED_OTHER ? "OTHER"
            : "unknown"))),
        my_param.sched_priority);
#else
    printf ("thread_routine running\n");
#endif
    return NULL;
}

int main (int argc, char *argv[])
{
    pthread_t thread_id;
    pthread_attr_t thread_attr;
    int thread_policy;
    struct sched_param thread_param;
    int status, rr_min_priority, rr_max_priority;

    status = pthread_attr_init (&thread_attr);
    if (status != 0)
        err_abort (status, "Init attr");

    /*
     * If the priority scheduling option is defined, set various scheduling
     * parameters. Note that it is particularly important that you remember
     * to set the inheritsched attribute to PTHREAD_EXPLICIT_SCHED, or the
     * policy and priority that you've set will be ignored! The default
     * behavior is to inherit scheduling information from the creating
     * thread.
     */
#if defined (_POSIX_THREAD_PRIORITY_SCHEDULING) && !defined (sun)
    status = pthread_attr_getschedpolicy (
        &thread_attr, &thread_policy);
    if (status != 0)
        err_abort (status, "Get policy");
    status = pthread_attr_getschedparam (
        &thread_attr, &thread_param);
    if (status != 0)
        err_abort (status, "Get sched param");
    printf (
        "Default policy is %s, priority is %d\n",
        (thread_policy == SCHED_FIFO ? "FIFO"
         : (thread_policy == SCHED_RR ? "RR"
            : (thread_policy == SCHED_OTHER ? "OTHER"
               : "unknown"))),
        thread_param.sched_priority);

    status = pthread_attr_setschedpolicy (
        &thread_attr, SCHED_RR);
    if (status != 0)
        printf ("Unable to set SCHED_RR policy.\n");
    else {
        /*
         * Just for the sake of the exercise, we'll use the
         * middle of the priority range allowed for
         * SCHED_RR. This should ensure that the thread will be
         * run, without blocking everything else. Because any
         * assumptions about how a thread's priority interacts
         * with other threads (even in other processes) are
         * nonportable, especially on an implementation that
         * defaults to System contention scope, you may have to
         * adjust this code before it will work on some systems.
         */
        rr_min_priority = sched_get_priority_min (SCHED_RR);
        if (rr_min_priority == -1)
            errno_abort ("Get SCHED_RR min priority");
        rr_max_priority = sched_get_priority_max (SCHED_RR);
        if (rr_max_priority == -1)
            errno_abort ("Get SCHED_RR max priority");
        thread_param.sched_priority =
            (rr_min_priority + rr_max_priority)/2;
        printf (
            "SCHED_RR priority range is %d to %d: using %d\n",
            rr_min_priority,
            rr_max_priority,
            thread_param.sched_priority);
        status = pthread_attr_setschedparam (
            &thread_attr, &thread_param);
        if (status != 0)
            err_abort (status, "Set params");
        printf (
            "Creating thread at RR/%d\n",
            thread_param.sched_priority);
        status = pthread_attr_setinheritsched (
            &thread_attr, PTHREAD_EXPLICIT_SCHED);
        if (status != 0)
            err_abort (status, "Set inherit");
    }
#else
    printf ("Priority scheduling not supported\n");
#endif
    status = pthread_create (
        &thread_id, &thread_attr, thread_routine, NULL);
    if (status != 0)
        err_abort (status, "Create thread");
    status = pthread_join (thread_id, NULL);
    if (status != 0)
        err_abort (status, "Join thread");
    printf ("Main exiting\n");
    return 0;
}