Question

Sometimes this program errors with a segmentation fault. What is a segmentation fault? Why is it happening? And how do I fix it?

I am expecting the output as:

I am consumer
I am producer 
I am consumer
I am producer
(etc.)

However, this is not the case. Can anyone explain this to me?

#include<stdio.h>
#include<semaphore.h>
#include<sys/stat.h>
#include<fcntl.h>

sem_t* mutex;
sem_t* null;

main()
{
 int temp;
 int pid;
 pid = fork();

 sem_unlink("/mutex");
 sem_unlink("/null");

 null = (sem_t*)sem_open("/null",O_CREAT,S_IWUSR|S_IWGRP|S_IWOTH,0);
 mutex =(sem_t*)sem_open("/mutex",O_CREAT,S_IWUSR|S_IWGRP|S_IWOTH,1);


 if (pid != 0)
 while(1)
 {
   sem_post(null);
   sem_wait(mutex);
   printf("\nIam In Producer\n");
   scanf("%d",&temp); // just  for my verification that where i am during execution
   sem_post(mutex);
   sem_wait(null);
 }

 else
 while(1)
 {
   sem_post(null);
   sem_wait(mutex);
   printf("\nIam In consumer\n");
   scanf("%d",&temp); // just for my verification that where i am during execution
   sem_post(mutex);
   sem_wait(null);
 }

}

---

Answer 1

I think you need to move the sem_unlink() and sem_open() calls to before you call fork(). You’re probably unlinking things you don’t want to.

Explanation – you call fork(), now you have two processes running. Let’s say the parent gets through the sem_unlink() and sem_open() calls before the child gets any processor time. Now the child starts running, and immediately unlinks the parent’s semaphores!