How to use a semaphore and struct for multi threads - c

Question

I am trying to have a multithread program that work with struct and semaphore.
I am not sure how to pass those values. Therefore I decided to pass my struct by reference and put my semaphore as a global variable.
What I am trying to do is to make two threads count to 20
(where one would be t1 = 0, 2 , 4 ,6 ... 20 and t2 = 1, 3 , ... , 19)
Here is what I have so far:
#include <fcntl.h>
#include <semaphore.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <pthread.h>
#define SEM_sem;
#define MaxVal 20;
typedef struct {
int startVal;
int maxVal;
} countStruct;
sem_t *sem;
void *t1(void *param) {
countStruct *counting;
counting = (countStruct *) param;
int i = counting->startVal;
while(i <= counting->maxVal) {
printf("%d\n",i);
i = i + 2;
sem_post(sem);
sem_wait(sem);
}
pthread_exit(0);
}
void *t2(void *param) {
countStruct *counting;
counting = (countStruct *) param;
int i = counting->startVal;
while(i <= counting->maxVal) {
printf("%d\n",i);
i = i + 2;
sem_post(sem);
sem_wait(sem);
}
pthread_exit(0);
}
int main() {
sem = sem_open(SEM_sem, O_CREAT | O_EXCL, 0666, 0);
if (sem == SEM_FAILED) {
printf("sem_open() failed\n");
sem_unlink(SEM_mpoulinl);
return(8);
}
pthread_attr_t attr;
pthread_t tid1, tid2
countStruct array_struct[3];
int i = 0;
for(i ; i < 2 ; i++) {
array_struct[i].startVal = i;
array_struct[i].maxVal = MaxVal;
}
pthread_create(&tid1, NULL, t1, (void*)&array_struct[0]);
sleep(1);
pthread_create(&tid2, NULL, t=2, (void*)&array_struct[1]);
pthread_join(tid1, NULL);
pthread_join(tid2, NULL);
sem_close(sem);
sem_unlink(SEM_mpoulinl);
return(0);
}
Unfortunately my code print t1 then t2.
Why is it not working?
is it because my semaphore is global?
I tested taking out sem_post(sem) sem_wait(sem) and the code still work, therefore those two things does not seem to work or am I not using it appropriately?
Thank you!