NET-2: Multiplexed Sqrt Server

Requirements

• Write a C/C++ TCP server program that:

  • Creates a TCP socket and binds it to a specified port (default: 8080)
  • Listens for incoming client connections
  • Uses epoll to handle multiple simultaneous clients in a single thread — no fork(), no threads
  • All sockets (listening and client) must be set to non-blocking mode (O_NONBLOCK)
  • For each connected client:
    • Reads exactly 4 bytes representing a signed 32-bit integer (int32_t) in network byte order
    • Computes f(x) = sqrt(abs(x)) and sends the result back as an 8-byte IEEE 754 double in network byte order
    • Handles the case where the client sends multiple requests over the same connection
    • Detects client disconnection and removes the descriptor from the epoll instance
  • Prints a log line when a client connects or disconnects, and for each request processed
  • Runs indefinitely until terminated with Ctrl+C

• Write a C/C++ TCP client program that:

  • Connects to the server at a specified IP address and port
  • Reads an integer from stdin, sends it to the server as a 4-byte network-order int32_t
  • Receives the 8-byte double response and prints the result
  • Repeats until EOF (Ctrl+D) or the user types exit

• Both programs must use the BSD socket API and proper byte order conversion

• Handle all system call errors — check every return value

Expected result

The resulting applications should be able to build and execute from command line as follows:

make
./sqrt-server 8080

In separate terminals, run multiple clients simultaneously:

./sqrt-client 127.0.0.1 8080

Server terminal output:

[Server] Listening on port 8080 (epoll, non-blocking)
[Server] Client connected: 127.0.0.1:54321 (fd=5)
[Server] Client connected: 127.0.0.1:54322 (fd=6)
[Server] fd=5 recv: -16 -> sqrt(abs(-16)) = 4.000000
[Server] fd=6 recv: 225  -> sqrt(abs(225))  = 15.000000
[Server] fd=5 recv: 0    -> sqrt(abs(0))    = 0.000000
[Server] Client disconnected: fd=5

Client terminal output:

[Client] Connected to 127.0.0.1:8080
> -16
[Server]: sqrt(abs(-16)) = 4.000000
> 225
[Server]: sqrt(abs(225)) = 15.000000
> exit
[Client] Disconnecting...

Notes

• Use epoll_create1(EPOLL_CLOEXEC) to create the epoll instance
• Use level-triggered mode (default) — no need for edge-triggered for this task
• double has no standard network byte order representation — use a byte-swap approach or send as raw bytes with a defined endianness (document your choice)
• sqrt and fabs are in <math.h> / <cmath>; link with -lm if needed

Bonus (Optional)

• Switch to edge-triggered mode (EPOLLET) and implement the full draining loop for reads
• Support an arbitrary number of requests per connection with a per-client receive buffer to handle partial reads correctly
• Add a timeout: disconnect clients that send no request within 30 seconds (use epoll_wait timeout or timerfd)
• Report the number of active connections when receiving SIGUSR1

Deliverables

The final solution should contain a Makefile for building both programs. The Makefile should contain targets:

• all — build both server and client
• sqrt-server — build only the server
• sqrt-client — build only the client
• clean — remove compiled binaries

It’s recommended to have the compiler and compiler flags declared as Makefile variables. Alternatively, cmake could also be used instead of make.