creating threads using pthread.c - c

Question

I am trying to learn how to create threads in c using the pthread library, I am using the following code:
#include <stdlib.h>
#include <stdio.h>
#include <semaphore.h>
#include <pthread.h>
static int glob = 0;
static sem_t sem;
static void *threadFunc(void *arg) {
int loops = *((int *) arg);
int loc, j;
for (j = 0; j < loops; j++) {
if (sem_wait(&sem) == -1)
exit(2);
loc = glob;
loc++;
glob = loc;
if (sem_post(&sem) == -1)
exit(2);
}
printf("\n%d %d\n",glob/20,glob);
return NULL;
}
int main(int argc, char *argv[]) {
pthread_t t1, t2, t3, t4;
int s;
int loops = 20;
if (sem_init(&sem, 0, 1) == -1) {
printf("Error, init semaphore\n");
exit(1);
}
s = pthread_create(&t1, NULL, threadFunc, &loops);
if (s != 0) {
printf("Error, creating threads\n");
exit(1);
}
s = pthread_create(&t2, NULL, threadFunc, &loops);
if (s != 0) {
printf("Error, creating threads\n");
exit(1);
}
s = pthread_create(&t3, NULL, threadFunc, &loops);
if (s != 0) {
printf("Error, creating threads\n");
exit(1);
}
s = pthread_create(&t4, NULL, threadFunc, &loops);
if (s != 0) {
printf("Error, creating threads\n");
exit(1);
}
s = pthread_join(t1, NULL);
if (s != 0) {
printf("Error, creating threads\n");
exit(1);
}
s = pthread_join(t2, NULL);
if (s != 0) {
printf("Error, creating threads\n");
exit(1);
}
s = pthread_join(t3, NULL);
if (s != 0) {
printf("Error, creating threads\n");
exit(1);
}
s = pthread_join(t4, NULL);
if (s != 0) {
printf("Error, creating threads\n");
exit(1);
}
printf("glob value %d \n", glob);
exit(0);
}
What are the expected values of glob when I try to print them using the print statement in threadFunc? Shuold they be 20,40,60 and 80? When I execute the above program I get different values for glob like, 61, 50, 73 and 80!! or 29,76,78,80? How come? EVerytime I execute I get different values for glob. I think it has something to do with the semaphore but then how can the value for glob decrease like in the first output example I gave you?
Furthermore, what is the purpose for a thread_initiate given to pthread_create? Not threadFunc specifically but in general what do programmers dealing with threads in c generally do using the thread_initiate function passed to pthread_create?

Answer 1

I figured it out, I didn't think about the code properly. The threads are running concurrently so there is no way to decide what the value of glob will be. If two threads are running, the first one might be 5 values into the loop and the second thread might be 2 values which will mean the value of glob is 7. When glob is printed the value will always be greater than a multiple of 20 (for this particular problem).
As for the second part I believe that the starting routine is the code that the thread is going to run.
Thanks to #WhozCraig and #JoachimPileborg for the help!