unix - Why are these two addresses not the same?

Question

shmget.c:

#include<sys/types.h>
#include<string.h>
#include<sys/ipc.h>
#include<sys/shm.h>
#include<stdio.h>
main()
{
    key_t key;
    int shmid;
    char* addr1;
    key = ftok("/home/tamil/myc/pws.c",'T');
    shmid = shmget(key,128*1024,IPC_CREAT|SHM_R|SHM_W);

    addr1 = shmat(shmid,0,0);

    printf("
IPC SHARED MEMORY");
    printf("
 SENDER ADDRESS");
    printf("
THE ADDRESS IS %p",addr1);
    printf("
ENTER THE MESSAGE:");
    scanf("%s",addr1);
    printf("
MESSAGE STORED IN %p IS %s",addr1,addr1);  
}

shmget2.c:

#include<sys/types.h>
#include<string.h>
#include<sys/ipc.h>
#include<sys/shm.h>
#include<stdio.h>

main()
{
    int shmid;
    char* addr1;
    key_t key;


    key = ftok("/home/tamil/myc/pws.c",'T');
    shmid = shmget(key,128*1024,SHM_R|SHM_W);

    addr1 = shmat(shmid,0,0);


    printf("
IPC SHARED MEMORY");
    printf("
 SENDER ADDRESS");
    printf("
THE ADDRESSS IS %p",addr1);
    printf("
MESSAGE STORED IN %p IS %s",addr1,addr1);

}

Output:

tamil@ubuntu:~/myc$ cc shmget.c
tamil@ubuntu:~/myc$ ./a.out

IPC SHARED MEMORY
 SENDER ADDRESS
**THE ADDRESS IS **0xb786c000****
ENTER THE MESSAGE:helloworld

MESSAGE STORED IN **0xb786c000** IS helloworldtamil@ubuntu:~/myc$ cc shmget2.c
tamil@ubuntu:~/myc$ ./a.out

IPC SHARED MEMORY
 SENDER ADDRESS
**THE ADDRESSS IS **0xb7706000****
MESSAGE STORED IN **0xb7706000** IS helloworldtamil@ubuntu:~/myc$ 

This all works well. But the address is not the same. Why is this?

Answer 1

Shared memory can be mapped to different regions in the address space of different processes. This is perfectly normal. But if you're storing pointers inside the shared memory to other parts of the shared memory, then you're in trouble. You'll need to use something like an offset pointer in that case.