C - Semaphore not shared over all processes

Question

I am working on a project that requires process synchronization. The problem I have is that the samaphore I use does not seem to be shared across all processes. It behaves similar to local variable. This is a simplified code, but demonstrates the same problem:

#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <unistd.h>
#include <time.h>
#include <semaphore.h>
#include <fcntl.h>


sem_t sem_1;    //Global variable (semaphore).

int main(){
    sem_init(&sem_1, 1, 0);     //semaphore starts with 0           
    pid_t child_pid_or_zero = fork();   //process fork
    if (child_pid_or_zero < 0){     
        perror("fork() error");
        exit (2);
    }

    if (child_pid_or_zero != 0){
        sem_wait(&sem_1);           //decrement to -1 and waits 
        printf("I am the parent %d, my child is %d.\n",getpid(),child_pid_or_zero);

    }else{

        printf("i am child\n");
        sem_post(&sem_1);       //increments
    }   
return 0;
}

The parent process never gets over the wait signal. I tried adding mupltiple sem_post() to both processes and print the values with sem_getvalue() and the numbers printed seemed not to be shared (every processs incremented its own semaphore).

Thanks for help.

After a `fork` the parent and child have seperate address spaces. They are exact (more or less) copies of the address space immediately after the `fork` but any subsequent changes to one are not seen by the other. — kaylum, Apr 15 '16 at 11:24
Read the `sem_init` man page or see questions like [How to share semaphores between processes using shared memory](http://stackoverflow.com/questions/8359322/how-to-share-semaphores-between-processes-using-shared-memory?rq=1) — kaylum, Apr 15 '16 at 11:28

score 1 · Accepted Answer · answered Apr 15 '16 at 11:28

1

POSIX is quite unclear how the pshared argument (second argument) to sem_init works. The Linux man page explains it better:

If pshared is nonzero, then the semaphore is shared between processes, and should be located in a region of shared memory

Allocating that memory and putting the semaphore there is your responsibility. This is not something that the system will do for you.

answered Apr 15 '16 at 11:28

Art

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this does not fix the underlying problem with the posted code – user3629249 Apr 17 '16 at 00:49

score 0 · Answer 2 · answered Apr 17 '16 at 00:47

the semaphore needs to be in some shared memory, not in the file global area.

Therefore need to use: shm_open() or shm_get() or mmap() to have some shared memory.

The data passed to the child process is set as 'copyonwrite' so when the child process calls the function: sem_post(), the data is copied,

As I said, the program needs some shared memory and the semaphore needs to be located within that shared memory.

C - Semaphore not shared over all processes

2 Answers2