This is regarding shared memory in LINUX

Question

#include<sys/shm.h>
#include<sys/stat.h>
#include<stdio.h>
int main(void)
{
int segment_id;
char *shared_memory;
const int size=4069;
segment_id=shmget(IPC_PRIVATE,size,S_IRUSR|S_IWUSR);
printf("segment ID=%d\n",segment_id);
shared_memory=(char *)shmat(segment_id,NULL,0);
sprintf(shared_memory,"Hi There!");
while(1){
}
return 0; 

}

If the same segment_id entered in the input of below program, will it work?

#include<sys/shm.h>
#include<sys/stat.h>
#include<stdio.h>
int main(void)
{
int segment_id;
char *shared_memory;
const int size=4069;
printf("please input segment id\n");
scanf("%d",&segment_id);
shared_memory=(char *)shmat(segment_id,NULL,0);
printf("%s\n",shared_memory );
return 0;
}

It's working here, but please explain how it works, means how the same segment_id is working in below program also?

Answer 1

You are using System V shared memory, a facility provided by the operating system.

The first line of the shmget (2) manual says,

shmget() returns the identifier of the System V shared memory segment associated with the value of the argument key.

The first line of the shmat (2) manual says,

shmat() attaches the System V shared memory segment identified by shmid to the address space of the calling process.

With shmget , you provide a key and it returns an identifier. With shmat , you provide that identifier, and it returns an address you can use. What's confusing about that?

I suspect you're surprised for the same reason I was when I first encountered it. Unlike most system resources -- file descriptors, sockets, etc. -- shared memory segments aren't freed when the program terminates. They remain there, ready to be reused whenever a program provides the appropriate identifier.

You don't even need to write a program to see them. Have a look at ipcs (1) and ipcrm (1).

If you think about it, for some applications they have to be persistent. The Berkeley alternative, mmap (2), uses filenames for the same purpose.