Multiple shared memory creation from one process

Question

Is it not possible to create two IPC shared memory segments from one process?

I am trying to create two shared memory from a single process one for sharing data with seperate process and other for shared with its child process. I am using shmget, i also tried to get the shared memory information in the system with ipcs. Output of which is something like this:

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x0beefbaf 0          root       666        225544     4                       
0x00000000 98305      root       666        4          0

Above output showed that two shared segment do get created, but shmget return only 0 as its output. Hence one shared memory segment is getting attached twice.

What is the problem with shmget or it is not possible to create two shared memory from one process.

Petesh Petesh · Accepted Answer · 2012-07-26T13:00:08

You're probably using the same key for the shared memory segment - if you want more than one shared memory segments then you need to use different keys. See the man page for ftok for the standard way of obtaining a key.

There are system level limits in place to prevent the use of too much memory - kernel.shmmax and the related sysctl properties.

This is a very simple example app that does exactly what the question asks:

#include <sys/shm.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>

int main(int argc, char **argv) {
     key_t key1;
     key_t key2;
     if (-1 != open("/tmp/foo", O_CREAT, 0777)) {
        key1 = ftok("/tmp/foo", 0);
     } else {
        perror("open");
        exit(1);
    }
    if (-1 != open("/tmp/foo2", O_CREAT, 0777)) {
        key2 = ftok("/tmp/foo2", 0);
    } else {
        perror("open");
        exit(1);
    }
    printf("%x %x\n", key1, key2);
    int id1 = shmget(key1, 0x1000, IPC_CREAT | SHM_R | SHM_W);
    int id2 = shmget(key2, 0x2000, IPC_CREAT | SHM_R | SHM_W);
    printf("%x %x\n", id1, id2);
    void *addr1 = shmat(id1, 0, 0);
    if (addr1 == (void *)-1) perror("shmat1");
    void *addr2 = shmat(id2, 0, 0);
    if (addr2 == (void *)-1) perror("shmat2");
    printf("%p %p\n", addr1, addr2);
    shmctl(id1, IPC_RMID, NULL);
    shmctl(id2, IPC_RMID, NULL);
}

Multiple shared memory creation from one process

1 Answers