go build vs go build file.go

What's the difference between go build and go build file.go?

I'm asking because when I run go build on a package that imports a local package, then I get this error message.

can't load package: C:\go\src\bug\main.go:3:8: local import "./local_file" in non-local package

However, when I specify a file name it works. Ex go build main.go

Console history on Windows XP.

C:\gopath\src\bug:>go version
go version go1.1 windows/386
C:\gopath\src\bug:>dir
...
<DIR>          local_file
				55 main.go
...

C:\gopath\src\bug:>type main.go
package main

import _ "./local_file"

func main() {
}

C:\gopath\src\bug:>type local_file\local_file.go
package local_file

import "fmt"

func init() {
		fmt.Println("Called: local_file.init()")
}

C:\gopath\src\bug:>go run main.go
Called: local_file.init()

C:\gopath\src\bug:>go build main.go

C:\gopath\src\bug:>dir
...
<DIR>          local_file
		 1,285,120 main.exe
				55 main.go	
...

C:\gopath\src\bug:>go build
can't load package: C:\gopath\src\bug\main.go:3:8: local import "./local_file" in non-local package

I asked this question on the google group site for golang and here's one of the responses.

> go build file.go (even if file.go is in GOPATH) isn't building a
> package in a workspace, so it follows doesn't follow that rule. The
> downside is that you have to name all the *.go files, you don't get
> some of the automatic behaviour of the go command (eg: testing &
> installing), and you can't build a package (which requires
> installing), only executables. Also, it's not go-gettable.
>
> The "go build file.go" should only ever be used for simple one source
> file binaries that you don't intend to ever distribute or re-use code
> from. Even then, it's often simpler and faster to follow the go tool's
> conventions: one package per folder (and vice versa) no relative
> imports (ever) "go run file.go" follows the same logic, and is even
> less recommended.
>
> Both patterns are meant for very specific uses, and should be avoided
> in regular programs and packages.
>
> By: Carlos Castillo

So it turns out that Google Go enforces the import path for packages to be relative to the src directory in the environmental variables GOROOT or GOPATH.

Thus here's the fix to make the program build with go build.

Note: GOPATH is set to C:\gopath

Change main.go from

package main

import _ "./local_file"

func main() {
}

To this

package main

import _ "bug/local_file"

func main() {
}