init function

There is a init() function, as the name suggests, it will do some initialization work, such as initializing variables which may not be expressed easily, or calibrating program state. A file can contain one or more init() functions, as shown here:

package main

import "fmt"

var global int = 0

func init() {
    global++
    fmt.Println("In first Init(), global is: ", global)
}

func init() {
    global++
    fmt.Println("In Second Init(), global is: ", global)
}

func main() {
    fmt.Println("In main(), global is: ", global)
}

The execution result is like this:

In first Init(), global is:  1
In Second Init(), global is:  2
In main(), global is:  2

Since one package can contain multiple files, there may be many init() functions. You should not presume which file's init() functions are executed first. The only thing which is guaranteed is that the variables declared in package will be evaluated before all init() functions are executed in this package.

See another example. The $GOROOT/src directory is like this:

# tree
.
├── foo
│   └── foo.go
└── play
    └── main.go

There are 2 simple packages: foo and play. The foo/foo.go is here:

package foo

import "fmt"

var Global int

func init() {
        Global++
        fmt.Println("foo init() is called, Global is: ", Global)
}

While the play/main.go is:

package main

import "foo"


func main() {
}

Build play command:

# go install play
# play
src/play/main.go:3: imported and not used: "foo"

The cause of this error is that main.go doesn't use any functions or variables exported by foo package. So if you just want an imported package's init() function is executed, and don't want to use package's other stuff, you should modify "import "foo"" to "import _ "foo"":

 package main

import _ "foo"


func main() {
}

Now the build process will success, and the output of play command is like this:

# play
foo init() is called, Global is:  1

References:
Effective Go;
When is the init() function in go (golang) run?.