Shared Memory Example

shm.c

/*
 * Example 1: POSIX Shared Memory
 * This example shows how to create, write to, and read from a POSIX shared memory segment
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>

#define SHM_NAME "/my_shm"
#define SHM_SIZE 1024

// Structure to be shared between processes
typedef struct {
    int count;
    char message[256];
} SharedData;

// Writer process
void posix_writer() {
    // Create and open shared memory segment
    int fd = shm_open(SHM_NAME, O_CREAT | O_RDWR, 0666);
    if (fd == -1) {
        perror("shm_open");
        exit(1);
    }

    // Set the size of the shared memory segment
    if (ftruncate(fd, SHM_SIZE) == -1) {
        perror("ftruncate");
        exit(1);
    }

    // Map shared memory to process address space
    SharedData* shared_data = (SharedData*)mmap(NULL, SHM_SIZE, 
                                              PROT_READ | PROT_WRITE, 
                                              MAP_SHARED, fd, 0);
    if (shared_data == MAP_FAILED) {
        perror("mmap");
        exit(1);
    }

    // Write to shared memory
    shared_data->count = 42;
    strcpy(shared_data->message, "Hello from POSIX shared memory!");

    // Cleanup
    munmap(shared_data, SHM_SIZE);
    close(fd);
}

// Reader process
void posix_reader() {
    // Open existing shared memory segment
    int fd = shm_open(SHM_NAME, O_RDONLY, 0666);
    if (fd == -1) {
        perror("shm_open");
        exit(1);
    }

    // Map shared memory to process address space
    SharedData* shared_data = (SharedData*)mmap(NULL, SHM_SIZE, 
                                              PROT_READ, 
                                              MAP_SHARED, fd, 0);
    if (shared_data == MAP_FAILED) {
        perror("mmap");
        exit(1);
    }

    // Read from shared memory
    printf("Count: %d\n", shared_data->count);
    printf("Message: %s\n", shared_data->message);

    // Cleanup
    munmap(shared_data, SHM_SIZE);
    close(fd);
    shm_unlink(SHM_NAME);  // Remove shared memory segment
}

/*
 * Example 2: System V Shared Memory
 * This example demonstrates the older System V IPC shared memory interface
 */

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

#define SHM_KEY 12345

void sysv_writer() {
    // Create shared memory segment
    int shmid = shmget(SHM_KEY, SHM_SIZE, IPC_CREAT | 0666);
    if (shmid == -1) {
        perror("shmget");
        exit(1);
    }

    // Attach shared memory segment
    SharedData* shared_data = (SharedData*)shmat(shmid, NULL, 0);
    if (shared_data == (void*)-1) {
        perror("shmat");
        exit(1);
    }

    // Write to shared memory
    shared_data->count = 100;
    strcpy(shared_data->message, "Hello from System V shared memory!");

    // Detach shared memory
    shmdt(shared_data);
}

void sysv_reader() {
    // Get existing shared memory segment
    int shmid = shmget(SHM_KEY, SHM_SIZE, 0666);
    if (shmid == -1) {
        perror("shmget");
        exit(1);
    }

    // Attach shared memory segment
    SharedData* shared_data = (SharedData*)shmat(shmid, NULL, 0);
    if (shared_data == (void*)-1) {
        perror("shmat");
        exit(1);
    }

    // Read from shared memory
    printf("Count: %d\n", shared_data->count);
    printf("Message: %s\n", shared_data->message);

    // Cleanup
    shmdt(shared_data);
    shmctl(shmid, IPC_RMID, NULL);  // Remove shared memory segment
}

/*
 * Example 3: Using shared memory with memory mapped files
 */

void mmap_example() {
    // Create a file to back the shared memory
    int fd = open("shared_file.dat", O_CREAT | O_RDWR, 0666);
    if (fd == -1) {
        perror("open");
        exit(1);
    }

    // Set file size
    if (ftruncate(fd, SHM_SIZE) == -1) {
        perror("ftruncate");
        exit(1);
    }

    // Map file into memory
    SharedData* shared_data = (SharedData*)mmap(NULL, SHM_SIZE,
                                              PROT_READ | PROT_WRITE,
                                              MAP_SHARED, fd, 0);
    if (shared_data == MAP_FAILED) {
        perror("mmap");
        exit(1);
    }

    // Use the mapped memory
    shared_data->count = 200;
    strcpy(shared_data->message, "Hello from memory mapped file!");

    // Ensure changes are written to disk
    msync(shared_data, SHM_SIZE, MS_SYNC);

    // Cleanup
    munmap(shared_data, SHM_SIZE);
    close(fd);
}

int main() {
    // Example usage of POSIX shared memory
    if (fork() == 0) {
        posix_writer();
        exit(0);
    } else {
        sleep(1);  // Ensure writer goes first
        posix_reader();
    }

    // Example usage of System V shared memory
    if (fork() == 0) {
        sysv_writer();
        exit(0);
    } else {
        sleep(1);
        sysv_reader();
    }

    // Example usage of memory mapped files
    mmap_example();

    return 0;
}