where does go log the errors?

Question

I'm using the log packages and I'm wondering what's the default destination of the log data. I can't find it anywhere. Do I need a io writer and specifically call it after each log or how is it supposed to work ?

Answer 1

It should be stdErr, as in line 58 of log.go:

var std = New(os.Stderr, "", LstdFlags)

So package methods like Fatal() uses std by default:

// Fatal is equivalent to Print() followed by a call to os.Exit(1).
func Fatal(v ...interface{}) {
    std.Output(2, fmt.Sprint(v...))
    os.Exit(1)
}

The blog post "How to write Go packages coders will love " (by Baron Schwartz) references that technique:

One of the patterns I've found in the standard Go library is what I call package-and-object. (That's my own name; maybe I'm reinventing or naming something already known by another name.) You can see this idiom in several packages. It makes these packages a delight to use.

The essence of the idiom is that you design a type with methods as usual, and then you also place matching functions at the package level itself. These functions simply delegate to a default instance of the type that's a private package-level variable, created in an init() function.

To take a look at the default logging functionality, for example, you can just import the log package and then write things like log.Print(). It's super-concise and handy, and it does the right thing.
Want to customize it? Make your own log.Logger variable, and set its properties.

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Note that, with Go 1.13 (Q3 2019), the new Writer() function returns the output destination for the standard logger.