Pthreads program with mutex - printing same string every time - c

Question

I have a school project that requires me to wrote a program printing this: <ONE><TWO><THREE><ONE><TWO><THREE><ONE><TWO><THREE>….............. using 3 threads and mutex. I have tried to do it with some help of the class, but it just keeps printing only <ONE>. Can you help me solve my problem and understand what have i wrong?
#include <pthread.h>
#include <stdio.h>
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
void *func(void *arg)
{
pthread_mutex_lock(&mutex);
while (1) {
printf ("<ONE>");
}
pthread_mutex_unlock(&mutex);
pthread_exit(NULL);
}
void *func2(void *arg)
{
pthread_mutex_lock(&mutex);
while (1) {
printf ("<TWO>");
}
pthread_mutex_unlock(&mutex);
pthread_exit(NULL);
}
void *func3(void *arg)
{
pthread_mutex_lock(&mutex);
while (1) {
printf ("<THREE>");
}
pthread_mutex_unlock(&mutex);
pthread_exit(NULL);
}
main()
{
pthread_t mythread1,mythread2,mythread3;
pthread_create( &mythread1, NULL, func, (void *) 1);
pthread_create( &mythread2, NULL, func2, (void *) 2);
pthread_create( &mythread3, NULL, func3, (void *) 3);
pthread_join ( mythread1, NULL);
pthread_join ( mythread2, NULL);
pthread_join ( mythread3, NULL);
exit(0);
}

Answer 1

As I made clear in the comments, this will get stuck in an infinite loop because you are doing the locking and unlocking outside the loop. First step is to move them inside.
void *func(void *arg)
{
while (1) {
pthread_mutex_lock(&mutex);
printf ("<ONE>");
pthread_mutex_unlock(&mutex);
}
pthread_exit(NULL);
}
Next, we need to add synchronization. An easy way to do that is to declare a global variable:
int next = 1;
Then we modify the function like this:
void *func(void *arg)
{
while (1) {
while(1) {
pthread_mutex_lock(&mutex);
if(next == 1) break;
pthread_mutex_unlock(&mutex);
}
printf ("<ONE>");
next = 2;
pthread_mutex_unlock(&mutex);
}
pthread_exit(NULL);
}
In func2 and func3 you need to modify if(next == 1) and next = 2 to appropriate values. func2 should have 2 and 3 while func3 should have 3 and 1.
This method is called busy waiting and is often not the best choice since it's quite cpu intense. A better alternative would be to look into pthread_cond_wait(). You can read about it here: http://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_cond_wait.html