creating threads using pthread.c

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?