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exit-main.md

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Exit in Main

Go programs use os.Exit or log.Fatal* to exit immediately. (Panicking is not a good way to exit programs, please don't panic.)

Call one of os.Exit or log.Fatal* only in main(). All other functions should return errors to signal failure.

BadGood
 
func main() {
  body := readFile(path)
  fmt.Println(body)
}

func readFile(path string) string {
  f, err := os.Open(path)
  if err != nil {
    log.Fatal(err)
  }

  b, err := io.ReadAll(f)
  if err != nil {
    log.Fatal(err)
  }

  return string(b)
}
 
func main() {
  body, err := readFile(path)
  if err != nil {
    log.Fatal(err)
  }
  fmt.Println(body)
}

func readFile(path string) (string, error) {
  f, err := os.Open(path)
  if err != nil {
    return "", err
  }

  b, err := io.ReadAll(f)
  if err != nil {
    return "", err
  }

  return string(b), nil
}

Rationale: Programs with multiple functions that exit present a few issues:

  • Non-obvious control flow: Any function can exit the program so it becomes difficult to reason about the control flow.
  • Difficult to test: A function that exits the program will also exit the test calling it. This makes the function difficult to test and introduces risk of skipping other tests that have not yet been run by go test.
  • Skipped cleanup: When a function exits the program, it skips function calls enqueued with defer statements. This adds risk of skipping important cleanup tasks.