Question

I have a problem with semaphore and fork in Linux, here is my code:

#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <stdlib.h>
#include <unistd.h>

#define KEY 464

void debug(int semafor_id) {
    printf("Value of sem 0 is: %d \n",  semctl (semafor_id, 0 , GETVAL , 0));
}

void main()
{
   int id;  
   struct sembuf operations[1]; 


   // Sreate semaphore
   id = semget(KEY, 1, 0777 | IPC_CREAT );


   // set value
    operations[0].sem_num = 0;
    operations[0].sem_op = 10;  
    operations[0].sem_flg = 0;

    semop(id, operations, 1);  

   // show what is the value 

    debug(id);


   // do the same with the child process 
    if(!fork()) {
        printf("IN child process \n");
        debug(id);
        exit(0);
    }

    semctl (id, 0 ,  IPC_RMID , 0);

}

The output is:

• Value of sem 0 is: 10
• IN child process:
• value of sem 0 is: -1

So it seems I can’t use semaphore in child process. I don’t think I need to use shared memory. Help?

Answer 1

You have a race condition there.

If the parent continues executing after the fork, before the child gets a chance to run, then the parent will destroy the semaphore before the child can inspect its value.

Add a pause of some sort, or better use wait, in the parent before you destroy the semaphore and/or use strerror to figure out what the exact error is in the child.