Do Deferred Functions Execute After `log.Fatalln` in Go?

Log.Fatalln and Deferred Function Execution

In Go programs, deferred functions are automatically called when the enclosing function returns. However, a common question arises: are deferred functions executed when the log.Fatalln function is invoked?

Understanding log.Fatalln

The log.Fatalln function is a member of the standard Go logging package. It logs the provided message and then immediately calls os.Exit(1) to terminate the program. The os.Exit function, in turn, prevents the execution of any deferred functions.

Deferred Function Execution in Case of log.Fatalln

Therefore, the answer to the question is no. Deferred functions are not executed when log.Fatalln is called. This is because log.Fatalln terminates the program abruptly, bypassing the execution of deferred functions.

This behavior is documented in the description of log.Fatal, the function equivalent to log.Fatalln:

Fatal is equivalent to Print() followed by a call to os.Exit(1).

Practical Demonstration

The following code demonstrates this behavior:

db, err := sql.Open("postgres", "…")
if err != nil {
    log.Fatalln(err)
}
defer db.Close()

tpl, err := template.ParseGlob("")
if err != nil {
    log.Fatalln(err)
}

If template.ParseGlob("") returns an error, the call to db.Close() will not be executed because log.Fatalln will terminate the program before reaching that line.

Alternative Approach

If you need to ensure that cleanup tasks are performed before the program terminates, you should consider using other logging functions, such as log.Print or log.Println, which do not terminate the program.

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