Inconsistent counter value when used in multi-threaded program - c

Question

I'm doing an assignment in C where we have a narrow road that can only support up to three cars at a time. The road is one-way so cars are allowed to go either East or West. Starvation should also be avoided. My implementation so far seems to work generally but I'm having a hard time understanding why the numOfCarsOnRoad variable has inconsistent values (i.e. printing 'Number of cars on the road = 1' twice in the output instead of '.. = 3', '.. = 2', '.. = 1') despite each thread incrementing/decreasing it each time.
Furthermore, I've noticed that if for example three threads "start" one after another (thus calling sleep(1)), they may finish in a different order. I understand that each thread runs in a separate lightweight process but shouldn't the sleep(1) last shorter for the thread that went first? Is there a way to avoid this?
The number of cars (represented by threads) is passed via the cmd args (i.e. '-c 5').
header:
void * crossBridge(void *i);
void parseCarArg(int argc, char *argv[]);
C:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>
#include <string.h>
#include "ask2.h"
sem_t sem;
int numOfCarsOnRoad = 0;
int carsGoingW = 0;
int carsGoingE = 0;
long numOfCars = 0; // used for thread initializations
char currActiveDir = '_'; // either W or E ( _ initially)
void *crossBridge(void *i)
{
int id = *((int *)i);
char direction[5];
if (rand() % 2 == 0) {
strcpy(direction, "West");
carsGoingW++;
}
else {
strcpy(direction, "East");
carsGoingE++;
}
if (currActiveDir == '_')
currActiveDir = direction[0];
printf("Car #%d waiting to pass to the %s...\n", id, direction);
while (currActiveDir != direction[0] || numOfCarsOnRoad == 3)
{
sleep(2);
}
sem_wait(&sem); // enter critical region
numOfCarsOnRoad++;
printf("Car #%d going to the %s. Number of cars on the road = %d\n", id, direction, numOfCarsOnRoad);
sleep(1);
numOfCarsOnRoad--;
sem_post(&sem);
printf("Car #%d crossed to the %s! Number of cars on the road = %d\n", id, direction, numOfCarsOnRoad);
if(direction[0] == 'W') carsGoingW--;
else carsGoingE--;
// helps to avoid starvation
if (numOfCarsOnRoad == 0)
{
if (currActiveDir == 'W' && carsGoingE > 0)
currActiveDir = 'E';
else if (currActiveDir == 'E' && carsGoingW > 0)
currActiveDir = 'W';
}
pthread_exit(NULL);
}
void parseCarArg(int argc, char *argv[])
{
int i;
for (i = 0; i < argc; i++)
{
if (strcmp(argv[i], "-c") == 0)
{
if (++i < argc && strlen(argv[i]) > 0)
numOfCars = strtol(argv[i], NULL, 10); // convert to long
if (numOfCars == 0)
{
perror("You must enter a number of cars > 0!\n");
exit(EXIT_FAILURE);
}
break;
}
}
}
int main(int argc, char *argv[])
{
if (argc == 0)
exit(EXIT_FAILURE);
parseCarArg(argc, argv);
srand(time(NULL)); // seed the generator using epoch time in millis
if (sem_init(&sem, 0, 3) == -1)
{
perror("Failed to initialize semaphore!\n");
exit(EXIT_FAILURE);
}
pthread_t cars[numOfCars];
int i;
for (i = 0; i < numOfCars; i++)
{
if (pthread_create(&cars[i], NULL, crossBridge, &i) != 0)
{
perror("Failed to create threads for the cars!\n");
exit(EXIT_FAILURE);
}
}
// wait for all threads to finish
for (i = 0; i < numOfCars; i++)
pthread_join(cars[i], NULL);
sem_destroy(&sem);
return 0;
}