Exit with error code in go?

Question

What's the idiomatic way to exit a program with some error code?

The documentation for Exit says "The program terminates immediately; deferred functions are not run.", and log.Fatal just calls Exit. For things that aren't heinous errors, terminating the program without running deferred functions seems extreme.

Am I supposed to pass around some state that indicate that there's been an error, and then call Exit(1) at some point where I know that I can exit safely, with all deferred functions having been run?

Solution 1

I do something along these lines in most of my real main packages, so that the return err convention is adopted as soon as possible, and has a proper termination:

func main() {
    if err := run(); err != nil {
        fmt.Fprintf(os.Stderr, "error: %v\n", err)
        os.Exit(1)
    }
}

func run() error {
    err := something()
    if err != nil {
        return err
    }
    // etc
}

Solution 2

In python I commonly use pattern which converted to go looks like this:

func run() int {
    // here goes
    // the code

    return 1
}

func main() {
    os.Exit(run())
}

Solution 3

I think the most clear way to do it is to set the exitCode at the top of main, then defer closing as the next step. That lets you change exitCode anywhere in main, and it's last value will be exited with:

package main

import (
    "fmt"
    "os"
)

func main() {
    exitCode := 0
    defer func() { os.Exit(exitCode) }()

    // Do whatever, including deferring more functions

    defer func() {
        fmt.Printf("Do some cleanup\n")
    }()

    func() {
        fmt.Printf("Do some work\n")
    }()

    // But let's say something went wrong
    exitCode = 1

    // Do even more work/cleanup if you want

    // At the end, os.Exit will be called with the last value of exitCode
}

Output:

Do some work
Do some cleanup

Program exited: status 1.

Go Playgroundhttps://play.golang.org/p/AMUR4m_A9Dw

Note that an important disadvantage of this is that you don't exit the process as soon as you set the error code.

Solution 4

As mentioned by fas, you have func Exit(exitcode int) from the os package.

However, if you need the defered function to be applied, you always can use the defer keyword like this:

http://play.golang.org/p/U-hAS88Ug4

You perform all your operation, affect a error variable and at the very end, when everything is cleaned up, you can exit safely.

Otherwise, you could also use panic/recover: http://play.golang.org/p/903e76GnQ-

When you have an error, you panic, end you cleanup where you catch (recover) it.

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Comments

[topskip]

I am pretty sure @dan knows about it.

[d1str0]

Maybe I misread or he edited but I thought he was talking about something else.

[marczoid]

I think I understand what you mean in the first approach, but the example is a little confusing to me. Why defer fct1() and fct2()? This mean that they will be executed in reverse order! It seems you intend something more like this, or not?

[Loupax]

I consider this the best solution. You can return an error from the run() method and have main() deal with it without how it sees fit.

[Ben]

Leaving this up, but after writing more Go code I think stackoverflow.com/a/18969976/2958070 (answer by Gustavo Niemeyer) is clearer and easier to use in basically all cases. Keeping track of function calls is a lot easier than keeping track of defers

[sprut]

What's the best method to test this code?

[Gustavo Niemeyer]

Run the binary in a functional test.

[emlai]

_, _ = fmt.Fprintf(…) to explicitly ignore the error returned by Fprintf.

[Mitar]

A downside of this approach is that if your code panics, it will never be printed out, because before panic propagates to the top, you exit the program in defer.