Question

Hi i am trying to create an application1 which takes input data “hello world”. I am creating a new process using system() and I want to access data of application1 in this process using shared memory(interprocess communication). I tried to run this program but couldn’t get the output “hello world”. How to attach the shared memory in application1 and process1 to the same address location.
please help me with this.

Application1.c

#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>
int main ()
{
int segment_id;
char* shared_memory;
struct shmid_ds shmbuffer;
int segment_size;
const int shared_segment_size = 0x6400;
/* Allocate a shared memory segment. */
segment_id = shmget (IPC_PRIVATE, shared_segment_size,
                        IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR);
/* Attach the shared memory segment. */
shared_memory = (char*) shmat (segment_id, 0, 0);
printf ("shared memory attached at address %p\n", shared_memory);
/* Determine the segment’s size. */
shmctl (segment_id, IPC_STAT, &shmbuffer);
segment_size = shmbuffer.shm_segsz;
printf ("segment size: %d\n", segment_size);
/* Write a string to the shared memory segment. */
sprintf (shared_memory, "Hello, world.");
/* Detach the shared memory segment. */
system("./process1");
shmdt (shared_memory);
shmctl (segment_id, IPC_RMID, 0);

return 0;
}

process1.c

#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>
int main ()
{
int segment_id;
char* shared_memory;
struct shmid_ds shmbuffer;
int segment_size;
const int shared_segment_size = 0x6400;
/* Allocate a shared memory segment. */
segment_id = shmget (IPC_PRIVATE, shared_segment_size,
                        IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR);
/* Attach the shared memory segment. */
shared_memory = (char*) shmat (segment_id, 0, 0);
printf ("shared memory2 attached at address %p\n", shared_memory);
printf ("%s\n", shared_memory);
/* Detach the shared memory segment. */
shmdt (shared_memory);
return 0;
}

output:

shared memory attached at address 0x7f616e4f2000
segment size: 25600
shared memory22 attached at address 0x7f8746d17000

The output is not printing data in the shared memory. I want the output to print “hello, world”.

Thank you

Answer 1

A couple of things:

1) The first argument to shmget is the key. You’re using IPC_PRIVATE in both processes, which means that it’ll allocate a “new” piece of shared memory in both processes. What you want to do is to make arrangements so that both processes use the same key, but not the IPC_PRIVATE key.

2) The shared_memory pointer in both process DO NOT need to be the same value for things to work. Yes, the memory is shared, but that doesn’t mean that the pointers will have the same value. The shared memory can be mapped to different memory locations in each process.