Mastering Linux Shared Memory: IPC Basics, APIs, and Sample Code

Shared Memory IPC Principles

Shared memory is a separate memory segment with its own data structures, including permissions, size, and timestamps. The structure is defined as:

from /usr/include/linux/shm.h
struct shmid_ds {
    struct ipc_perm shm_perm; /* operation perms */
    int shm_segsz;           /* size of segment (bytes) */
    __kernel_time_t shm_atime; /* last attach time */
    __kernel_time_t shm_dtime; /* last detach time */
    __kernel_time_t shm_ctime; /* last change time */
    __kernel_ipc_pid_t shm_cpid; /* creator pid */
    __kernel_ipc_pid_t shm_lpid; /* last operator pid */
    unsigned short shm_nattch;   /* number of current attaches */
    unsigned short shm_unused;   /* compatibility */
    void *shm_unused2;           /* used by DIPC */
    void *shm_unused3;           /* unused */
};

Two processes must attach the shared memory segment to their address spaces before using it.

The system imposes the following limits:

#define SHMMAX 0x2000000   /* max shared segment size (bytes) */
#define SHMMIN 1           /* min shared segment size (bytes) */
#define SHMMNI 4096        /* max number of segments system‑wide */
#define SHMALL (SHMMAX/getpagesize()*(SHMMNI/16))
#define SHMSEG SHMMNI      /* max shared segments per process */

Linux Shared Memory Management

1. Create shared memory

#include <sys/ipc.h>
#include <sys/shm.h>

int shmget(key_t key, size_t size, int shmflg);

2. Control shared memory

#include <sys/ipc.h>
#include <sys/shm.h>

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

3. Attach shared memory

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

4. Detach shared memory

int shmdt(const void *shmaddr);

Conventions Between Parent and Child Processes

After fork(), the child inherits the parent's attached shared memory.

If exec() is called, all attached shared memory is automatically detached.

When a process calls exit(), its attached shared memory is detached.

Example Program

The following C program allocates a shared memory segment, the parent writes an integer, and the child reads it.

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

#define SHM_SIZE 1024

int main() {
    int shm_id, pid;
    int *ptr = NULL;
    // Create shared memory
    shm_id = shmget((key_t)1004, SHM_SIZE, IPC_CREAT | 0600);
    // Attach shared memory to process address space
    ptr = (int*)shmat(shm_id, 0, 0);
    printf("Attach addr is %p 
", ptr);
    *ptr = 1004;
    printf("The Value of Parent is : %d 
", *ptr);
    if ((pid = fork()) == -1) {
        perror("fork Err");
        exit(0);
    } else if (!pid) {
        printf("The Value of Child is : %d 
", *ptr);
        exit(0);
    } else {
        sleep(1);
    }
    // Detach shared memory
    shmdt(ptr);
    // Delete shared memory
    shmctl(shm_id, IPC_RMID, 0);
    return 0;
}

Running the program produces output showing the same integer value in both parent and child processes.