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HOWTO build a simple RT application

The POSIX API forms the basis of real-time applications running under PREEMPT_RT. For the real-time thread a POSIX thread is used (pthread). Every real-time application needs proper handling in several basic areas like scheduling, priority, memory locking and stack prefaulting.

Basic prerequisites

Three basic prerequisites are introduced in the next subsections, followed by a short example illustrating those aspects.

Scheduling and priority

The scheduling policy as well as the priority must be set by the application explicitly. There are two possibilities for this:

  1. Using sched_setscheduler()
    This funcion needs to be called in the start routine of the pthread before calculating RT specific stuff.
  2. Using pthread attributes
    The functions pthread_attr_setschedpolicy() and pthread_attr_setschedparam()offer the interfaces to set policy and priority. Furthermore scheduler inheritance needs to be set properly to PTHREAD_EXPLICIT_SCHED by using pthread_attr_setinheritsched(). This forces the new thread to use the policy and priority specified by the pthread attributes and not to use the inherit scheduling of the thread which created the real-time thread.

Memory locking

See here

Stack for RT thread

See here


 * POSIX Real Time Example
 * using a single pthread as RT thread
#include <limits.h>
#include <pthread.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
void *thread_func(void *data)
        /* Do RT specific stuff here */
        return NULL;
int main(int argc, char* argv[])
        struct sched_param param;
        pthread_attr_t attr;
        pthread_t thread;
        int ret;
        /* Lock memory */
        if(mlockall(MCL_CURRENT|MCL_FUTURE) == -1) {
                printf("mlockall failed: %m\n");
        /* Initialize pthread attributes (default values) */
        ret = pthread_attr_init(&attr);
        if (ret) {
                printf("init pthread attributes failed\n");
                goto out;
        /* Set a specific stack size  */
        ret = pthread_attr_setstacksize(&attr, PTHREAD_STACK_MIN);
        if (ret) {
            printf("pthread setstacksize failed\n");
            goto out;
        /* Set scheduler policy and priority of pthread */
        ret = pthread_attr_setschedpolicy(&attr, SCHED_FIFO);
        if (ret) {
                printf("pthread setschedpolicy failed\n");
                goto out;
        param.sched_priority = 80;
        ret = pthread_attr_setschedparam(&attr, &param);
        if (ret) {
                printf("pthread setschedparam failed\n");
                goto out;
        /* Use scheduling parameters of attr */
        ret = pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED);
        if (ret) {
                printf("pthread setinheritsched failed\n");
                goto out;
        /* Create a pthread with specified attributes */
        ret = pthread_create(&thread, &attr, thread_func, NULL);
        if (ret) {
                printf("create pthread failed\n");
                goto out;
        /* Join the thread and wait until it is done */
        ret = pthread_join(thread, NULL);
        if (ret)
                printf("join pthread failed: %m\n");
        return ret;
realtime/documentation/howto/applications/application_base.txt · Last modified: 2017/12/05 08:22 by anna-maria