Do Deferred Functions Execute After a `log.Fatalln()` Call in Go?

Deferred Function Calls and log.Fatalln()

In Go, deferred functions are executed when a function returns, even if the function panics. However, what happens to deferred functions when the program terminates due to a call to log.Fatalln()?

Deferred Function Behavior in log.Fatalln()

The log.Fatalln() function is used to log an error and then terminate the program. It is equivalent to calling log.Print() diikuti by a call to os.Exit(1).

os.Exit() and Deferred Functions

os.Exit() causes the program to exit immediately. This is important because:

log.Fatal calls os.Exit
os.Exit does not run deferred functions

Therefore, any deferred functions that are defined after a call to log.Fatalln() are not called.

Example

Here's an example to demonstrate this behavior:

import (
    "log"
    "os"
)

func main() {
    f, _ := os.Create("foo.txt")
    defer f.Close()

    log.Fatalln("Error")
}

In this example, the program attempts to open and close a file using a deferred function. However, the file is never closed because the program is terminated by the call to log.Fatalln().

Conclusion

If you need to properly close resources or perform cleanup actions before the program exits, avoid using log.Fatalln(). Instead, handle errors gracefully and ensure that deferred functions are executed properly.

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