Pthreads stack priority - c

Question

i have little trouble with pthread priority.I have set scheduler FIFO. I have 2 thread, each have diffretnt priority. On my project when i push A or B, one thread start working. Thats fine, but when one thread working and push again B(priority15) and then A(priority 20) my assumption is that thread A jumps before B in stack queue and after first thread will by next thread A with priority 20 and then thread B (15). Is that problem FIFO or something else?
PS: I dont want use semaphore, i want to solve it just with priority and scheduler. Thanks
#include <stdio.h>
#include<pthread.h>
#include<semaphore.h>
#include<unistd.h>
#include <time.h>
#include <sched.h>
sem_t mutex;
void* levelOnefunction(void *a)
{
sem_wait(&mutex);
int* b = (int *) a;
printf("Thread! next main!\n");
printf("sched prior:%d\n",*b);
sleep(3);
sem_post(&mutex);
return 0;
}
int main()
{
char s;
pthread_t t;
pthread_attr_t tattr;
struct sched_param param;
sem_init(&mutex,0,1);
pthread_attr_init(&tattr);
if(pthread_attr_setschedpolicy(&tattr,SCHED_FIFO)!=0)
printf("ERROR FIFO!\n");
//pthread_setschedparam(t,SCHED_FIFO,&param);
if(pthread_attr_getschedparam(&tattr,&param)!=0)
printf("ERROR attr get sheduler!\n");
/* int k =pthread_attr_setinheritsched(&tattr,PTHREAD_EXPLICIT_SCHED);
if (k!=0)
printf("ERROR\n"); */
printf("Initial priority is %d \n",param.sched_priority);
int min = sched_get_priority_min(SCHED_FIFO);
int max = sched_get_priority_max(SCHED_FIFO);
printf("MIN - %d ---> MAX - %d\n",min,max);
scanf("%c",&s);
while (s != '\0')
{
if(s=='a')
{
printf("a\n");
param.sched_priority=20;
if(pthread_attr_setschedparam(&tattr,&param)!=0)
printf("ERROR attr_setschedul!\n");
printf("update priority is:%d\n",param.sched_priority);
if(pthread_create(&t,&tattr,levelOnefunction,(void *)&param.sched_priority)!=0)
printf("ERROR thread1\n");
pthread_join(t,NULL);
printf("main finish!\n");
printf("%c end\n\n",s);
}
else if(s=='b')
{
printf("b\n");
param.sched_priority=15;
if(pthread_attr_setschedparam(&tattr,&param)!=0)
printf("ERROR attr_setschedul!\n");
if(pthread_create(&t,&tattr,levelOnefunction,(void *)&param.sched_priority)!=0)
printf("ERROR Thread 2 ");
pthread_join(t,NULL);
printf("main finish!\n");
printf("%c end\n\n",s);
}
else if(s=='\n')
{
}
else if (s!='a' && s!='b')
{
printf("bad key\n");
}
scanf("%c",&s);
}
sem_destroy(&mutex);
return 0;
}

Answer 1

You are calling pthread_join() in the main thread immediately after creating each child thread.
This means that you never have more than one child thread created at a time - after creating one child thread, the main thread will then block in pthread_join() until that child thread is complete. Only then does it call scanf() again and potentially create another child thread.
Thread priorities don't come into it.