27.c

/*
============================================================================
Name : 27
Author : Piyush Singh
Description : Write a program to receive messages from the message queue.
a. with 0 as a flag
b. with IPC_NOWAIT as a flag
Date: 26th Sep, 2025.
============================================================================
*/

// Program: Receive messages from System V message queue (blocking and non-blocking)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <errno.h>

// Define message structure
struct msgbuf 
{
    long mtype;
    char mtext[100];
};

int main() 
{
    key_t key;
    int msgid;
    struct msgbuf message;

    // Step 1: Generate key
    key = ftok("msgKeyFile", 'A');
    if (key == -1) 
    {
        perror("❌ ftok failed");
        exit(1);
    }

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

    // Step 3a: Blocking receive (flag = 0)
    printf("⏳ Waiting for message (blocking)...\n");
    if (msgrcv(msgid, &message, sizeof(message.mtext), 0, 0) == -1)
        perror("❌ msgrcv (blocking) failed");
    else
        printf("📥 Received (blocking): %s\n", message.mtext);

    // Step 3b: Non-blocking receive (flag = IPC_NOWAIT)
    printf("⚡ Trying non-blocking receive...\n");
    if (msgrcv(msgid, &message, sizeof(message.mtext), 0, IPC_NOWAIT) == -1) 
        if (errno == ENOMSG)
            printf("🚫 No message available (non-blocking).\n");
        else
            perror("❌ msgrcv (non-blocking) failed");

    else
        printf("📥 Received (non-blocking): %s\n", message.mtext);

    return 0;
}

/*
Output:
╰─ ./26                                                                       ─╯
✅ Message sent to queue (ID: 65539): 📨 Hello from macOS sender!

╰─ ./27                                                                       ─╯
⏳ Waiting for message (blocking)...
📥 Received (blocking): 📨 Hello from macOS sender!
⚡ Trying non-blocking receive...
🚫 No message available (non-blocking).

*/