Real time task (periodic task) - c

Question

Good evening everyone,
I'm still learning real-time programming, and I'm trying to synchronize two threads using semaphores, the first thread calculates the sum and returns a value (sum). the sum will be passed as a parameter to the 2nd thread that will use it to calculate an average (this is just an example for manipulating semaphores). my problem now and that the two tasks are not periodic because once the thread returns a result it leaves the loop while and the main() finishes the work !!! now how to make the tasks period ?? thank you for helping me and here is my source code.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
sem_t evt;
//first task
void *tache1(void *arg)
{
int j;
int s=0;
struct timespec time;
clock_gettime(CLOCK_REALTIME, &time);
while(1){
printf("first THREADDDDDDDDD \n");
for(j=0; j<100; j++)
s= s + j;
return (void*) s;
sem_post(&evt);
sleep(3);
}
}
//second task
void *tache2(void *arg){
int moyenne = 1;
int sum = *((int*) arg);
struct timespec time;
clock_gettime(CLOCK_REALTIME, &time);
while(3){
sem_wait(&evt);
printf("second THREADDDDDDDDD \n");
moyenne= sum/10;
return(void*)moyenne;
sleep(2);
}
}
int main()
{
pthread_t th1, th2;
int sum;
int moyenne;
int status;
sem_init(&evt, 0,0);
printf("start main\n") ;
pthread_create(&th1, NULL, tache1, NULL);
pthread_join(th1, (void*) &sum);
pthread_create(&th2, NULL, tache2, &sum);
pthread_join(th2, (void*) &moyenne);
printf("the sum in the main is %d.\n", sum);
printf("AVG %d.\n", moyenne);
printf("End main\n") ;
return 0;
}

Answer 1

When you do this:
pthread_create(&th1, NULL, tache1, NULL);
pthread_join(th1, (void*) &sum);
pthread_create(&th2, NULL, tache2, &sum);
pthread_join(th2, (void*) &moyenne);
You start a thread, wait for it to finish, then start another thread, then wait for that to finish. That's not multithreading. You want both threads to be active at the same time.
Also, both of your thread functions include a return statement with more statements after them. Once a return statement is hit, the entire function returns immediately. No statements after them are executed. Note also that you don't need a while (1) (or while (3)) loop in either of these functions.
The idea behind semaphores (and mutexes) is that multiple threads are running at once, and both thread need to operate on global data so one thread needs to wait for the other thread to do something before it can access the global data.
So make sum and moyenne global variables, start both threads at once, and get rid of the extra loops in the thread functions.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
sem_t evt;
int sum;
int moyenne;
void *tache1(void *arg)
{
int j;
int s=0;
printf("first THREADDDDDDDDD \n");
for(j=0; j<100; j++)
s= s + j;
sum = s;
sem_post(&evt);
return NULL;
}
void *tache2(void *arg)
{
sem_wait(&evt);
printf("second THREADDDDDDDDD \n");
moyenne= sum/10;
return NULL;
}
int main()
{
pthread_t th1, th2;
sem_init(&evt, 0,0);
printf("start main\n") ;
pthread_create(&th1, NULL, tache1, NULL);
pthread_create(&th2, NULL, tache2, NULL);
pthread_join(th1, NULL);
pthread_join(th2, NULL);
printf("the sum in the main is %d.\n", sum);
printf("AVG %d.\n", moyenne);
printf("End main\n") ;
return 0;
}