C Shared Memory

Linux has two standards:

• IPC SysV (from Unix System V)
• IPC POSIX

Methods

key = ftok("/etc/passwd", 'a')
We can use same key for semaphores, shared memory and etc.

int shmget(key, size_t size, int flg);

Creates shred memory.

• size - it rounds to page size*n
• flg - IPC_CREAT|IPC_EXCL|0644

void *shmat(int shmid, const void *shmaddr, int shmflg);
 
int shmdt(int shmid);

Attach memory/detach memory. It connects program to memory. Access to array that belongs to several processes.

• shmid
• shmaddr
  • NULL - system will decide, where to attach.
• shmflg
  • SHM_RDONLY
  • SHM_RND

shmctl(int shmid, int cmd, struct shmid_ds *buf);

• cmd:
  • IPC_STAT - read sheared memory parameters.
  • IPC_SET - set shared memory parameters.
  • IPC_RMID - delete for process. Deletes completely if all processes deleted it.

File Permissions

	Owner	Owner’s group	All
Permissions	rwx	rwx	rwx
Binary	110	110	100
Oct	6	6	4

Example

Program 1.

key_t key = ftok("/etc/passwd", 'Z');
 
int shmid = shmget(key, 4096, IPC_CREAT|0664);
 
char *buf = (char*) shmat(shmid, NULL, 0);
sprintf(buf, "Miumiu"); // not atomic btw
shmdt(shmid);
return 0;
 
/* instead of sprintf
char local_buf[100] = {"Miumiu"};
int len = strlen(local_buf);
for (int i = 0; i < len; i++) {
	buf[i] = local_buf[i];
}
buf[len] = '\0';
*/

Program 2.

key_t key = ftok("/etc/passwd", 'Z');
 
int shmid = shmget(key, 4096, 0);
 
char *buf = (char*) shmat(shmid, NULL, 0);
 
printf("%s\n", buf);
 
shmdt(shmid);
shmctl(shmid, IPC_RMID, NULL);
return 0;
 
/*
int len = strlen(buf);
write(1, buf, sizeof(char) * len);
*/

Example With Semaphores

Process 1 writes -> 2 reads -> 3 reads -> {2 times} -> end.
Semaphores:

• 1st - write, +2, wait for 0, write, +1
• 2d - -2, read, +3, -4, read, +5, return
• 3d - -3, read, -5, read, return

It’s not clear. Let’s make 3 binary.

semop.h

strct sembuf
	f_up = {0, 1, 0},
	f_down = {0, -1, 0},
	s_up = {1, 1, 0},
	s_down = {1, -1, 0},
	t_up = {2, 1, 0},
	t_down = {2, -1, 0};

Process 1.

#include <semop.h>
 
key_t key = ftok("/etc/passwd", 'a');
int shmid = shmget(key, 4096, IPC_CREAT|0664);
int semid = semget(key, 3, IPC_CREAT|0664);
 
char *buf = (char*) shmat(shmid, NULL, 0);
buf[0] = '2';
sprintf(buf + 1, "Miumiu for 2d process");
 
semop(semid, &s_up, 1);
semop(semid, &f_down, 1);
 
buf[0] = '3';
sprintf(buf + 1, "Miumiu for 3d process");
 
semop(semid, &s_up, 1);
semof(semid, &f_down, 1);
 
shmdt(shmid);
semctl(semid, 0, IPC_RMID, 0);
shmctl(shmid, IPC_RMID, NULL);
 
return 0;

Process 2.

#include <semop.h>
 
key_t key = ftok("/etc/passwd", 'a');
int shmid = shmget(key, 4096, IPC_CREAT|0664);
int semid = semget(key, 3, IPC_CREAT|0664);
key_t key = ftok("/etc/passwd", 'a');
int shmid = shmget(key, 4096, IPC_CREAT|0664);
int semid = semget(key, 3, IPC_CREAT|0664);
 
char *buf = (char*) shmat(shmid, NULL, 0);
 
for (int i = 0; i < 2; i++) {
	semop(semid, &s_down);
	if (buf[0] == '2') {
		printf("%s\n", buf + 1);
	}
	semop(semid, &t_up);
}
 
return 0;
char *buf = shmat(shmid, NULL, 0);
 
for (int i = 0; i < 2; i++) {
	semop(semid, &s_down);
	if (buf[0] == '2') {
		printf("%s\n", buf + 1);
	}
	semop(semid, &t_up);
}
 
shmdt(shmid);
return 0;

Process 3.

#include <semop.h>
 
key_t key = ftok("/etc/passwd", 'a');
int shmid = shmget(key, 4096, IPC_CREAT|0664);
int semid = semget(key, 3, IPC_CREAT|0664);
 
char *buf = shmat(shmid, NULL, 0);
 
for (int i = 0; i < 2; i++) {
	semop(semid, &t_down);
	if (buf[0] == '3') {
		printf("%s\n", buf + 1);
	}
	semop(semid, &f_up);
}
 
shmdt(shmid);
return 0;