Reading two files - back and forth synchronization

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C

synchronization

I'm writing a program that creates two threads. Each thread is responsible for reading through one text file, with one char on each line.

The first is formatted like:

h
0
h
0
...

The second is formatted like:

0
i
0
i
0
i

Sometimes there can be multiple letters after each other, or multiple zeros after each other. However, the one certainty is that if there is a letter on one line of one file, the corresponding line of the second file will have a 0, and vice versa.

The threads are supposed to keep reading the file input into a global char array until they reach a zero. At this point, they allow the other thread to take over. And they keep going back and forth until both files are completely read.

At this point, when I run, I get variations of either (1) many h's followed by many i's or (2) (the correct answer) a continuous stream of hihihi's, or (3) sometimes many i's followed by many h's. So, I know that my synchronization methods are off.

Here is an example of one of my threads: (Note both threads are exactly the same, except for the file being opened.)

void *getMessage1()
{
FILE *studentOne = fopen("Student1", "r");

size_t howManyChars;
char *placeHolderChars; 
int count = 1;
while (count < 501)
{
    placeHolderChars = NULL;
    getline(&placeHolderChars, &howManyChars, studentOne);

    if(strcmp(placeHolderChars, "0\n") == 0) //if we've reached a zero
    {

         pthread_mutex_unlock(&lock); 
    }
    else
    {   while(1)
        {
            if(pthread_mutex_trylock(&lock) == 0)
            {

                break;
            }
        }

        if(strlen(placeHolderChars)>0)
        {
             placeHolderChars[1] = '\0';
        }

        strcat(message,placeHolderChars);
    }

    free(placeHolderChars);

    if(feof(studentOne))
    {

        pthread_mutex_unlock(&lock); //unlock
        fclose(studentOne);
        break;
    }
    count++;

 }

return 0;
}

Here is my main method:

int main(void)
{
pthread_t id1;
pthread_t id2;

pthread_create((&id1), NULL, getMessage1, NULL);
pthread_create((&id2), NULL, getMessage2, NULL);

pthread_join(id1, NULL);
pthread_join(id2, NULL);

int j;

for (j = 0; j < 1001; j++) 
{
     printf ("%c ",message[j]);
}

return 0;
}

What I have tried:

I have tried using various combinations of lock, unlock (mutex) and wait & signal. A user on stackoverflow told me that I should use semaphores instead of mutex.However, my professor told me that mutex would be the easiest/best way to do this.

I would appreciate any guidance on how I can better use lock, unlock, wait, and/or signal to create a working synchronization technique with consistent results.