将Golang日志输出设置为文件不会在函数声明之外持续存在

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

Answer 1

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()
}