minitalk

Inter-process communication using UNIX signals — a tiny client/server messaging system.

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About

minitalk implements a small communication program between a server and a client using only the UNIX signals SIGUSR1 and SIGUSR2. The client encodes each character of a message into individual bits and sends them one by one to the server, which reconstructs the original string and prints it.

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How It Works

Each character (8 bits) is transmitted bit by bit, LSB first:

• SIGUSR1 represents a 0 bit
• SIGUSR2 represents a 1 bit

The client sends each bit with kill(pid, signal) and shifts the character right after each send. The server accumulates bits in a static char, setting the appropriate bit position on each SIGUSR2 received. After 8 bits, it writes the reconstructed character and resets.

Client                           Server
  │                                │
  │─── SIGUSR1 (bit 0) ──────────► │
  │─── SIGUSR2 (bit 1) ──────────► │
  │─── SIGUSR1 (bit 0) ──────────► │  ... (8 signals = 1 char)
  │                                │──► write(1, &letter, 1)

A usleep(100) between each signal gives the server time to process before the next arrives.

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Structure

minitalk/
├── minitalk.h    # Shared header — includes signal.h and bundled libft
├── client.c      # Reads PID and message, sends bits signal by signal
├── server.c      # Handles incoming signals, reconstructs and prints characters
└── libft/        # Bundled libft for ft_printf, ft_atoi, ft_strlen

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Usage

Build

make

Produces two binaries: server and client.

Step 1 — Start the server

./server

Output:

Server PID: 12345
Waiting for messages...

Step 2 — Send a message

In a second terminal:

./client 12345 "Hello, 42!"

The server prints:

Hello, 42!

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Error Handling

• Wrong number of arguments → usage message to stderr and exit
• Invalid or unreachable PID → detected via kill(pid, 0) before sending
• Signal send failure → error message to stderr and exit

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Makefile Targets

Target	Description
make / make all	Build server and client
make clean	Remove object files
make fclean	Remove object files and binaries
make re	fclean + all

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Notes

• Uses signal() for signal handling — sufficient for the mandatory part.
• The server supports one client at a time; simultaneous clients would corrupt the static state.
• Bundled libft used for output and argument parsing — no external dependencies.
• Written in compliance with the 42 Norm.

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42 Heilbronn — Core Curriculum