Hi all, I'm trying to test my concepts of how shared memory works and how to access it.

So I've got a basic program that sets up a structure and shared memory segment and then tries to write / read to the shared memory segment from two different processes by doing a fork...

This is my code:

#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <semaphore.h>

struct shared
{
pid_t sh_pid;
};

int main (void)
{

struct shared *shmPtr;

size_t size = 14336;
pid_t pid;
int shm_fd;

sem_t mutex;
sem_init(&mutex, 0, 1);

shm_fd = shm_open("/shared.seg", O_CREAT|O_EXCL, size);

shmPtr = mmap (NULL, 14336, PROT_READ|PROT_WRITE, MAP_SHARED, shm_fd, (off_t) 0 );

if ( (pid = fork()) < 0 )
{
printf("PID ERROR");
//CATCH ERROR HERE AND EXIT
}
else if ( pid > 0 )
{

sem_wait (&mutex);

printf("Inside parent semaphore\n");

//SEGMENTATION FAULT OCCURS HERE!!
int display_pid = shmPtr->sh_pid;

printf("pid is: %d\n", display_pid);

sem_post (&mutex);
}
else
{
sem_wait(&mutex);

printf("Inside child semaphore.\n");

shmPtr->sh_pid = pid;

sem_post(&mutex);
}

shm_unlink("/shared.seg");

return 0;
}


So it looks like I can store some data into the shared memory segment but when I try to retrieve it I'm getting a segmentation fault. I'm guessing that I'm incorrectly calling the shared memory segment but I can't figure out what I'm doing wrong, anyone able to point me in the right direction?