Project 5: System V Message Queue Multi-Client Server

A server using System V message queues where clients multiplex on a single queue using message types.

Quick Reference

Attribute Value
Primary Language C
Alternative Languages N/A
Difficulty Level 3 (Advanced)
Time Estimate See main guide
Knowledge Area IPC, Multiplexing
Tooling See main guide
Prerequisites See main guide

What You Will Build

A server using System V message queues where clients multiplex on a single queue using message types.

Why It Matters

This project builds core skills that appear repeatedly in real-world systems and tooling.

Core Challenges

  • ftok collisions → Understanding key generation
  • Message type protocol → Designing the multiplexing scheme
  • Cleanup on crash → Orphaned queues persist!

Key Concepts

  • Map the project to core concepts before you code.

Real-World Outcome

# Server uses one queue for all clients
$ ./sysv_server
Server starting, key=0x12345678, msqid=100
Waiting for requests (msgtyp=1)...

# Client 1 (PID 5001) sends request
$ ./sysv_client "HELLO"
Sending to server (msgtyp=1)
Waiting for response (msgtyp=5001)
Response: HELLO_PROCESSED

# Client 2 (PID 5002) sends simultaneously
$ ./sysv_client "WORLD"
Response: WORLD_PROCESSED

# Both used the SAME queue, but different message types!
$ ipcs -q
------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages
0x12345678 100        user       644        0            0

Implementation Guide

  1. Reproduce the simplest happy-path scenario.
  2. Build the smallest working version of the core feature.
  3. Add input validation and error handling.
  4. Add instrumentation/logging to confirm behavior.
  5. Refactor into clean modules with tests.

Milestones

  • Milestone 1: Minimal working program that runs end-to-end.
  • Milestone 2: Correct outputs for typical inputs.
  • Milestone 3: Robust handling of edge cases.
  • Milestone 4: Clean structure and documented usage.

Validation Checklist

  • Output matches the real-world outcome example
  • Handles invalid inputs safely
  • Provides clear errors and exit codes
  • Repeatable results across runs

References

  • Main guide: UNIX_IPC_STEVENS_VOL2_MASTERY.md
  • Primary references are listed in the main guide