Setting Golang log output to file does not persist outside a function declaration

Question

I have the following code:

func main() {
    initSetLogOutput()

    log.Println("Another log")
}

func initSetLogOutput() {
    f, err := os.OpenFile("errors.log", os.O_RDWR | os.O_CREATE | os.O_APPEND, 0666)
    if err != nil {
        log.Fatalf("error opening file: %v", err)
    }
    defer f.Close()

    log.SetOutput(f)
    log.Println("This is a test log entry")
}

After compilation I run the application and I get the the first log This is a test log entry but the second log does not write to the log file. What is causing this? Is the declaration of log.SetOutput limited to the scope of a function? How do I get the log output option to persist throughout the entire application?

My output log looks like below:

2019/01/10 15:53:36 This is a test log entry
2019/01/10 15:54:27 This is a test log entry
2019/01/10 15:55:43 This is a test log entry
2019/01/10 15:57:40 This is a test log entry
2019/01/10 16:02:27 This is a test log entry

Your initSetLogOutput closes f on return (defer f.Close()), so once it returns and main resumes, the file pointer is closed and the logger has nowhere to write to. — Adrian
– Adrian, Commented Jan 10, 2019 at 16:07
Ok, I had wondered if that was the cause, but I wasn't sure what would be best practice. So I should write some code close the file when the application closes. — Samuel Hawksby-Robinson
– Samuel Hawksby-Robinson, Commented Jan 10, 2019 at 16:08

icza · Accepted Answer · 2019-01-10 16:07:56Z

5

Inside initSetLogOutput() you have a defer f.Close() line, which means before initSetLogOutput() returns, the file will be closed.

Instead close it at the end of main(), like this:

func main() {
    initSetLogOutput()
    log.Println("Another log")
    closeLogOutput()
}

var logFile *os.File

func initSetLogOutput() {
    var err error
    logFile, err = os.OpenFile("errors.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
    if err != nil {
        log.Fatalf("error opening file: %v", err)
    }

    log.SetOutput(logFile)
    log.Println("This is a test log entry")
}

func closeLogOutput() {
    logFile.Close()
}

answered Jan 10, 2019 at 16:07

icza

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1 Comment

Samuel Hawksby-Robinson Over a year ago

Thank you for helping me better understand the way golang's process flow works

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