Handling signals with Go

2016-07-18 · #golang #development #grupo-estudos-golang

Handling signals is good practice. It lets you shut your program down gracefully, releasing resources and closing databases, instead of just terminating. Handling operating system signals with Golang is simple, because the system delivers the signal to a channel. All you have to do is listen to that channel.

First, we create a channel:

sc := make(chan os.Signal, 1)

Next, we say which signal we care about. In this case ^C, that is, SIGINT.

signal.Notify(sc, os.Interrupt)

You can register more than one signal. Just add more arguments, for example: signal.Notify(sc, os.Interrupt, syscall.SIGTERM).

The signals I find most interesting to handle are:

• os.Interrupt: same as syscall.SIGINT, represents the ^C signal
• syscall.SIGTERM: termination signal, usually sent by the kill command
• syscall.SIGWINCH: window resize signal (when the terminal is resized)
• syscall.SIGUSR1: user signal 1 (you can create your own signals)
• syscall.SIGUSR2: user signal 2

Then you just wait for the channel to receive a signal.

<-sc

You need to handle the signals in a goroutine.

Example

go func() {
    sc := make(chan os.Signal, 1)
    signal.Notify(
        sc,
        os.Interrupt,
        syscall.SIGTERM,
        syscall.SIGWINCH,
        syscall.SIGUSR1,
        syscall.SIGUSR2)
    for {
        s := <-sc
        switch s {
        case os.Interrupt:
            fmt.Printf("\r\nYou pressed ^C\r\n")
            os.Exit(0)
        case syscall.SIGTERM:
            fmt.Printf("\r\nYou sent SIGTERM\r\n")
            os.Exit(0)
        case syscall.SIGUSR1:
            fmt.Printf("\r\nYou sent SIGUSR1\r\n")
        case syscall.SIGUSR2:
            fmt.Printf("\r\nYou sent SIGUSR2\r\n")
        case syscall.SIGWINCH:
            fmt.Printf("\r\nThe window was resized\r\n")
        }
    }
}()

c := make(chan struct{})
<-c

full source code

SIGUSR1 and SIGUSR2 are user signals. You can define your own meaning for them, for example to request information or to change the program’s behavior.

To send user signals, use the kill command with the -USR1 or -USR2 option.

kill -USR1 <pid>

The SIGWINCH signal is sent when the terminal window is resized. You can use it to capture the new dimensions and resize your program’s window.

SIGTERM and SIGINT are termination signals. The first is sent by the kill command, and the second is sent when the user presses ^C. You can use them to shut the program down gracefully, closing connections and releasing resources.

Video with the explanation:

Cesar Gimenes