c - how to use shared memory to communicate between two processes

Question

I am trying to communicate between two processes. I am trying to save data(like name, phone number, address) to shared memory in one process and trying to print that data through other process.

process1.c

#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>
int main ()
{
  int segment_id;
  char* shared_memory[3];
  int segment_size;
  key_t shm_key;
  int i=0;
  const int shared_segment_size = 0x6400;
  /* Allocate a shared memory segment. */
  segment_id = shmget (shm_key, shared_segment_size,
            IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR);
  /* Attach the shared memory segment. */
  shared_memory[3] = (char*) shmat (segment_id, 0, 0);
  printf ("shared memory attached at address %p
", shared_memory);
  /* Write a string to the shared memory segment. */
   sprintf(shared_memory[i], "maddy 
");
   sprintf(shared_memory[i+1], "73453916
");
   sprintf(shared_memory[i+2], "america
");

  /*calling the other process*/
  system("./process2");

  /* Detach the shared memory segment. */
  shmdt (shared_memory);
  /* Deallocate the shared memory segment.*/
  shmctl (segment_id, IPC_RMID, 0);

  return 0;
}

---

process2.c

#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>
int main ()
{
  int segment_id;
  char* shared_memory[3];
  int segment_size;
  int i=0;
  key_t shm_key;
  const int shared_segment_size = 0x6400;
  /* Allocate a shared memory segment. */
  segment_id = shmget (shm_key, shared_segment_size,
              S_IRUSR | S_IWUSR);
  /* Attach the shared memory segment. */
  shared_memory[3] = (char*) shmat (segment_id, 0, 0);
  printf ("shared memory22 attached at address %p
", shared_memory);
   printf ("name=%s
", shared_memory[i]);
   printf ("%s
", shared_memory[i+1]);
   printf ("%s
", shared_memory[i+2]);
  /* Detach the shared memory segment. */
  shmdt (shared_memory);
   return 0;
}

But I am not getting the desired output. the output which i got is:

shared memory attached at address 0x7fff0fd2d460
Segmentation fault

Anyone can please help me with this. Is this the correct way of initializing shared_memory[3].

Thank you.

Answer 1

char* shared_memory[3];
...
shared_memory[3] = (char*) shmat (segment_id, 0, 0);

You declare shared_memory as an array capable of holding three pointers to char, but what you actually do with it is to write a pointer one place behind the end of the array. Since there is no telling what the memory there is otherwise used for, what happens next is generally unpredictable.

Things go conclusively bad afterwards when you try to make use of the pointers in shared_memory[0] through shared_memory[2], because those pointers have never been initialized. They are filled with meaningless garbage from the stack -- thus the segmentation fault.

It seems, in general, that you're failing to distinguish between the array and its elements. You should go and make yourself a lot more comfortable with arrays and pointers in sequential code before you try your hand at shared-memory IPC.

Note that shared memory is one of the more easy-to-get-wrong ways of doing IPC out there. Unless you have rigid efficiency constraints and are going to exchange a lot of data, it's much easier to work with pipes, named pipes, or sockets.