28.c

/*
============================================================================
Name : 28
Author : Piyush Singh
Description : Write a program to change the exiting message queue permission. (use msqid_ds structure)
Date: 26th Sep, 2025.
============================================================================
*/

// Program: Change permissions of a System V message queue
// On macOS, run with sudo and ensure msgq.key exists from earlier questions.
#include <stdio.h>        // For printf(), perror()
#include <stdlib.h>       // For exit()
#include <sys/ipc.h>      // For ftok(), key_t
#include <sys/msg.h>      // For msgget(), msgctl()
#include <sys/types.h>    // For uid_t, gid_t

int main() 
{
    key_t key;
    int msgid;
    struct msqid_ds info;

    // Step 1: Generate key from a real file
    key = ftok("msgKeyFile", 65);  // Ensure this file exists
    if (key == -1) 
    {
        perror("❌ ftok failed");
        exit(1);
    }

    // Step 2: Access existing message queue
    msgid = msgget(key, 0666);
    if (msgid == -1) 
    {
        perror("❌ msgget failed");
        exit(1);
    }

    // Step 3: Get current attributes
    if (msgctl(msgid, IPC_STAT, &info) == -1) 
    {
        perror("❌ msgctl IPC_STAT failed");
        exit(1);
    }

    // Step 4: Modify access permissions (e.g., 0644)
    info.msg_perm.mode = 0644;

    // Step 5: Apply changes
    if (msgctl(msgid, IPC_SET, &info) == -1) 
    {
        perror("❌ msgctl IPC_SET failed");
        exit(1);
    }

    printf("✅ Message queue permissions updated to: %o\n", info.msg_perm.mode);
    return 0;
}

/*
Output:
╰─ ./26                                                                       ─╯
✅ Message sent to queue (ID: 65540): 📨 Hello from macOS sender!

╰─ ./28                                                                       ─╯
✅ Message queue permissions updated to: 644
*/