Inter-Process Communication Using Message Queue

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1️⃣ What is a Message Queue?

A message queue is an IPC mechanism that allows processes to exchange messages in the form of structured data.

Unlike pipes:

• Processes need not be related

• Messages are stored in the kernel

• Communication is asynchronous

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2️⃣ Types of Message Queues in Linux

• System V Message Queues (commonly taught in OS labs)

• POSIX Message Queues (advanced)

👉 This example uses System V message queues.

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3️⃣ System Calls Used

System Call	Purpose
msgget()	Create or access message queue
msgsnd()	Send a message
msgrcv()	Receive a message
msgctl()	Control / delete queue

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4️⃣ Message Structure

Every message must begin with a long integer message type.

struct msg_buffer {
    long msg_type;
    char msg_text[100];
};

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5️⃣ Communication Model (Example)

• Sender process sends a message

• Receiver process receives and prints it

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6️⃣ Small Example: Message Queue IPC

🔹 Sender Program

#include <stdio.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <string.h>

struct msg_buffer {
    long msg_type;
    char msg_text[100];
};

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

    // Generate unique key
    key = ftok("progfile", 65);

    // Create message queue
    msgid = msgget(key, 0666 | IPC_CREAT);

    message.msg_type = 1;
    strcpy(message.msg_text, "Hello from sender process");

    // Send message
    msgsnd(msgid, &message, sizeof(message.msg_text), 0);

    printf("Message sent: %s\n", message.msg_text);

    return 0;
}

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🔹 Receiver Program

#include <stdio.h>
#include <sys/ipc.h>
#include <sys/msg.h>

struct msg_buffer {
    long msg_type;
    char msg_text[100];
};

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

    // Generate same key
    key = ftok("progfile", 65);

    // Access message queue
    msgid = msgget(key, 0666 | IPC_CREAT);

    // Receive message
    msgrcv(msgid, &message, sizeof(message.msg_text), 1, 0);

    printf("Message received: %s\n", message.msg_text);

    // Destroy the message queue
    msgctl(msgid, IPC_RMID, NULL);

    return 0;
}

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7️⃣ How to Run 

gcc sender.c -o sender
gcc receiver.c -o receiver
./sender
./receiver

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8️⃣ Sample Output

Sender:

Message sent: Hello from sender process

Receiver:

Message received: Hello from sender process

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9️⃣ Important Points 

• Message queues allow structured communication

• Messages have types

• Kernel stores messages until received

• Processes need same key

🟦 Explanation of Functions Used in Message Queue Program

We will explain the functions used in sender and receiver programs:

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1️⃣ ftok() — Generate Unique Key

Prototype

key_t ftok(const char *pathname, int proj_id);

Purpose

• Generates a unique IPC key

• Same key must be used by both sender and receiver

Arguments

Argument	Meaning
pathname	Existing file path
proj_id	Project identifier (integer/character)

Example

key = ftok("progfile", 65);

Important Notes

• File must exist

• Used to ensure both processes refer to the same message queue

📌 Exam Tip:

ftok() converts a filename and project ID into a unique key.

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2️⃣ msgget() — Create or Access Message Queue

Prototype

int msgget(key_t key, int msgflg);

Purpose

• Creates a new message queue or

• Accesses an existing message queue

Arguments

Argument	Meaning
key	Key generated by ftok()
msgflg	Permission + creation flags

Example

msgid = msgget(key, 0666 | IPC_CREAT);

Flags Explained

• 0666 → Read/write permission

• IPC_CREAT → Create queue if it does not exist

📌 Return Value

• Returns message queue ID

• -1 on failure

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3️⃣ msgsnd() — Send Message to Queue

Prototype

int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

Purpose

• Sends a message to the queue

Arguments

Argument	Meaning
msqid	Message queue ID
msgp	Pointer to message structure
msgsz	Size of message text (excluding long)
msgflg	Control flags

Example

msgsnd(msgid, &message, sizeof(message.msg_text), 0);

Important Notes

• msg_type must be first field

• msgsz excludes long msg_type

📌 Common Mistake

sizeof(struct msg_buffer)   ❌

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4️⃣ msgrcv() — Receive Message from Queue

Prototype

ssize_t msgrcv(int msqid, void *msgp, size_t msgsz,
               long msgtyp, int msgflg);

Purpose

• Receives a message from the queue

Arguments

Argument	Meaning
msqid	Message queue ID
msgp	Pointer to buffer
msgsz	Size of message text
msgtyp	Type of message to receive
msgflg	Control flags

Example

msgrcv(msgid, &message, sizeof(message.msg_text), 1, 0);

Message Type Rules

msgtyp	Meaning
>0	Receive specific type
0	Receive first message
<0	Receive lowest type ≤ abs

📌 Blocking Behavior

• Blocks until message arrives (unless IPC_NOWAIT used)

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5️⃣ msgctl() — Control Message Queue

Prototype

int msgctl(int msqid, int cmd, struct msqid_ds *buf);

Purpose

• Control or delete the message queue

Arguments

Argument	Meaning
msqid	Message queue ID
cmd	Control command
buf	Optional structure

Example

msgctl(msgid, IPC_RMID, NULL);

Common Commands

Command	Description
IPC_RMID	Remove message queue
IPC_STAT	Get status
IPC_SET	Set permissions

📌 Exam Tip

Always remove the message queue to avoid resource leakage.

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6️⃣ struct msg_buffer — Message Structure

Definition

struct msg_buffer {
    long msg_type;
    char msg_text[100];
};

Rules

• msg_type must be long

• First field is mandatory

• Used to identify message category

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7️⃣ Supporting Library Headers

Header	Purpose
<sys/ipc.h>	IPC key definitions
<sys/msg.h>	Message queue functions
<string.h>	String operations
<stdio.h>	Input/output