Signals in Racket

Our program will demonstrate how to handle signals in Racket. We’ll create a program that waits for a signal (like SIGINT or SIGTERM) and then gracefully shuts down.

#lang racket

(require racket/system)

(define (handle-signal sig)
  (printf "\nReceived signal: ~a\n" sig)
  (exit 0))

(define (main)
  (printf "Awaiting signal...\n")
  
  ; Set up signal handlers
  (handle-signals 
   (list signal/int signal/term)
   handle-signal)
  
  ; Keep the program running
  (let loop ()
    (sleep 1)
    (loop)))

(main)

In this Racket program:

1. We import the racket/system module, which provides functions for handling system signals.

2. We define a handle-signal function that will be called when a signal is received. It prints the signal and exits the program.

3. In the main function:

   • We print a message indicating that we’re waiting for a signal.
   • We use handle-signals to set up handlers for SIGINT and SIGTERM. When either of these signals is received, our handle-signal function will be called.
   • We enter an infinite loop to keep the program running until a signal is received.

4. Finally, we call the main function to start the program.

When we run this program, it will block waiting for a signal. By typing ctrl-C (which the terminal shows as ^C), we can send a SIGINT signal, causing the program to print the signal received and then exit.

$ racket signals.rkt
Awaiting signal...
^C
Received signal: break

In Racket, we don’t need to explicitly create channels or use goroutines as in the original example. The handle-signals function sets up the signal handling for us, and our handler function is called directly when a signal is received.

This example demonstrates how to handle system signals in Racket, allowing for graceful shutdown or other appropriate actions when certain signals are received.