"Train crossing mutex problem" - Getting NULL on last 2 lines - c

Question

Hey I'm having trouble solving a bug that causes some unwanted NULL's to be displayed in my final output. When you run the program enter a string such as 'nnseeeewwwew' into the command line before running.
Here is the code
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <string.h>
/* mutex */
pthread_mutex_t thisMutex;
/* Condition Variables */
sem_t nQueue, eQueue, sQueue, wQueue;
int nFirst, sFirst, eFirst, wFirst;
int done = 0;
/* Thread Prototypes */
void *bats(void *);
void *checking(void *);
/* Prototypes */
void arrive(int num, char *dir);
void cross();
void leave(int num, char *dir);
void check();
/* Global pointer to argv */
char *directions = NULL;
int main(int argc, char *argv[])
{
int i = 0,
n = 0,
s = 0,
e = 0,
w = 0;
directions = argv[1];
while( argv[1][i] == 'n' || argv[1][i] == 'e' ||
argv[1][i] == 's' || argv[1][i] == 'w' )
{
if(argv[1][i] == 'n'){n++;}
if(argv[1][i] == 's'){s++;}
if(argv[1][i] == 'e'){e++;}
if(argv[1][i] == 'w'){w++;}
i++;
}
pthread_mutex_init(&thisMutex, NULL);
sem_init(&nQueue, 0, n);
sem_init(&sQueue, 0, s);
sem_init(&eQueue, 0, e);
sem_init(&wQueue, 0, w);
nFirst = sFirst = eFirst = wFirst = 0;
pthread_t tid[i];
pthread_t checker;
pthread_create(&checker, NULL, checking, NULL);
for(int j = 0; j < i; j++)
{ pthread_create(&tid[j], NULL, bats, (void*) &j); }
for(int j = 0; j < i; j++)
{ pthread_join(tid[j], NULL); }
done = 1;
pthread_join(checker, NULL);
pthread_mutex_destroy(&thisMutex);
sem_destroy(&nQueue);
sem_destroy(&sQueue);
sem_destroy(&wQueue);
sem_destroy(&eQueue);
return 0;
}
void *checking(void *arg)
{
while( done == 0 )
{
if( nFirst == 1 || sFirst == 1 ||
eFirst == 1 || wFirst == 1 )
check();
}
exit(0);
}
void *bats(void *arg)
{
int index = *(int *)arg;
char *dir;
switch (directions[index])
{
case 'n':
dir = "North";
break;
case 's':
dir = "South";
break;
case 'e':
dir = "East";
break;
case'w':
dir = "West";
break;
}
arrive(index, dir);
leave(index, dir);
_exit(0);
}
/* Functions */
void arrive(int num, char *dir)
{
printf("BAT %d from %s arrives at crossing\n", num, dir);
if(strcmp(dir,"North")== 0)
{
sem_wait(&nQueue);
nFirst = 1;
while(eFirst == 1){ /* do nothing */ }
cross();
nFirst = 0;
sem_post(&nQueue);
}
else if(strcmp(dir,"West")== 0)
{
sem_wait(&wQueue);
wFirst = 1;
while(nFirst == 1){ /* do nothing */ }
cross();
wFirst = 0;
sem_post(&wQueue);
}
else if(strcmp(dir,"South")== 0)
{
sem_wait(&sQueue);
sFirst = 1;
while(wFirst == 1){ /* do nothing */ }
cross();
sFirst = 0;
sem_post(&sQueue);
}
else if(strcmp(dir,"East")== 0)
{
sem_wait(&eQueue);
eFirst = 1;
while(sFirst == 1){ /* do nothing */ }
cross();
eFirst= 0;
sem_post(&eQueue);
}
}
void cross()
{
pthread_mutex_lock(&thisMutex);
sleep(1);
pthread_mutex_unlock(&thisMutex);;
}
void leave(int num, char *dir)
{
printf("BAT %d from %s leaving crossing\n", num, dir);
}
void check()
{
if( nFirst == 1 && sFirst == 1 &&
eFirst == 1 && wFirst == 1 )
{
eFirst = 0;
sleep(1);
eFirst = 1;
}
}
The goal of this assignment is to
BATs arriving from the same direction line up behind the first BAT already at the crossing;
BATs arriving from the right always have the right of way (unless the waiting BAT receives a signal to go);
Deadlock has to be prevented
Starvation has to be prevented
Once you run the code, you'll see the last two lines have 'NULL' for the direction the BAT is arriving from. Need help to solve that issue
Thanks!

Answer 1

Your program has undefined behaviour becasue you are passing pointer to local variable into your thread function. In bats function when dereferencing pointer you may get garbage value:
for(int j = 0; j < i; j++)
{
pthread_create(&tid[j], NULL, bats, (void*) &j);
}
j is local inside for loop.
You could allocate memory to hold value for int
variable:
for(int j = 0; j < i; j++)
{
int* var = malloc(sizeof(int));
*var = j;
pthread_create(&tid[j], NULL, bats, var);
}
remember to free this memory in bats function:
void *bats(void *arg)
{
int index = *(int *)arg;
//...
leave(index, dir);
free (arg); // <--
pthread_exit(NULL); // instead of _exit(0);
}

Answer 2

dir is a char pointer. So you cannot directly use an equality sign like if( dir == "North") What you need is if ((strcmp(dir,"North")== 0)
in the loop below,
while( argv[1][i] == 'n' || argv[1][i] == 'e' ||
argv[1][i] == 's' || argv[1][i] == 'w' )
{
i++;
if(argv[1][i] == 'n'){n++;}
if(argv[1][i] == 's'){s++;}
if(argv[1][i] == 'e'){e++;}
if(argv[1][i] == 'w'){w++;}
}
You are incrementing i first inside the loop. So, for the last element in the array argv[1][i] you are reading junk values. The increment of i should be the last statement in the loop. i.e. after if(argv[1][i] == 'w'){w++;}