Question

I wrote a simple program solving the Readers-Writers problem using semaphores. It runs perfectly on Linux os, but when I run it on my Mac osX I get unexpected results and I can’t figure out why.

My Program:

#include <semaphore.h>
#include <sys/types.h>
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>

void* function1(void* val);
void* function2(void* val);

// shared values
volatile int X;
volatile int Y;

// declare semaphores
sem_t s1;
sem_t s2;

main()
{
void* status;

pthread_t thread1;
pthread_t thread2;
srand(time(NULL));

// initialize semaphores to zero
sem_init(&s1, 0, 0);
sem_init(&s2, 0, 0);

pthread_create(&thread1, NULL, function1, NULL);
pthread_create(&thread2, NULL, function2, NULL);

pthread_join(thread1, &status);
pthread_join(thread2, &status);

sem_destroy(&s1);
sem_destroy(&s2);

}

void* function1(void* val)
{
   while(1)
   {
   X = rand()%1000; // write 
   printf("After thread ID A writes to X, X = %d\n", X);
   sem_post(&s1); // signal
   sem_wait(&s2); // wait
   printf("After thread ID A reads from Y, Y = %d\n", Y); // read
   sleep(3);
   }   
}

void* function2(void* val)
{
   while(1)
   {
    sem_wait(&s1); // wait
    printf("After thread ID B reads from X, X = %d\n", X); // read
    Y = rand()%1000; // write
    printf("After thread ID B write to Y, Y = %d\n", Y);
    sem_post(&s2); // signal
    sleep(3);
   }
}

The output I receive on Linux (what it’s supposed to look like):

After thread ID A writes to X, X = 100
After thread ID B reads from X, X = 100
After thread ID B write to Y, Y = 234
After thread ID A reads from Y, Y = 234
...

The output on Mac osX (unexpected):

After thread ID A writes to X, X = 253
After thread ID A reads from Y, Y = 0
After thread ID B reads from X, X = 253
After thread ID B write to Y, Y = 728
...

Answer 1

Check the error return on the sem_init calls; I bet you’ll find the OS X version returning a “Function not implemented” error.

This is because unnamed POSIX semaphores are not implemented on OS X. You need to use named semaphores, or pthread mutex/condition variables.