Conditional compiling in golang.

While hacking around experimental builds of docker, I learned about conditional compiling in golang. As any project grew or ported to multiple architecture/platforms the need of conditional compiling is obvious.

Unlike C golang, do not have macros. Golang make use of build tags to achieve this.

Build Tags:

Build tags are basically annotation in source code. generally it is first line of file, where build tags are defined.

// +build <tags>

These tags are searched by go build tool before passing to go compiler. These tags can be passed while go build command like

$ go build -tags <tags>

Here are few points to remember regarding tags. - Tags specifies architecture and platforms are special. i.e. No need to pass such tags, go build is smart enough to pass them. - e.g. linux, windows, darwin, 386, arch etc - Tags can be passed with negation i.e. ! - A blank line must be present between build tags and code, else that line will be ignored. - OR condition for tags can be specified with space. - AND condition can be specified with ,

If some tag is defined in file and is not passed in go build` command, it will be ignored.

Lets see this with example with custom tag.

• Here we will take three files
• main.go
• include.go
• exclude.go

○ cat main.go
package main

import (
"fmt"
)

func init() {
        fmt.Println("Hello from Main init()")
}


func main() {
        fmt.Println("Hello from main")
}

○ cat include.go

package main

import "fmt"

func init() {
        fmt.Println("Hello from IncludeInit()")
}

○ cat exclude.go
// +build exclude

package main

import "fmt"

func init(){
        fmt.Println("Hello from exclude")
}

As you can see, here exclude file has build tag of exclude.

Now if do go build, It will not compile exclude.go.

○ go build

○ ./temp
Hello from IncludeInit()
Hello from Main init()
Hello from main

Now let try using buildtag exclude.

○ go build -tags exclude

○ ./temp
Hello from exclude
Hello from IncludeInit()
Hello from Main init()
Hello from main

You can see now, the exclude.go is part of binary.

One more interesting thing with golang is go list tool. Using the go list, without compiling, you can see which files will be compiled, with given gotags or default.

I found go list --help not great, but the summary of this tool is a go template can be used to check the list of files that will be included in the build. A simple example as below will help you to understand better.

○ go list -f '{{.GoFiles}}' --tags exclude
[exclude.go include.go main.go]

○ go list -f '{{.GoFiles}}'
[include.go main.go]

As you can see, if exclude tag is not used, only include.go and main.go files are used.

So it that all about conditional compiling in golang?

No, There is even simpler method available too :-) and it is file-name suffixes.

In golang, there are few go environment variables defined. GOOS and GOARCH are two of such. - GOOS defines the current OS - GOARCH defines the current machine architecture.

go build matches value of these two environment variables in filename and if it matches then only include for compiling.

So by naming files like exclude_linux.go, it will be included only if compiled in linux system. exclude_linux_amd64.go will ensure, it will be inlcuded only if OS is linux and architecture of system in 64 bit.

You may try these combination.

Which one should be used?

• For custom tags build tags are the only options.
• For platform and architecture specific, if you plan to use cross build on one systems, build tags will be preferable.
• Also, if same file does work for multiple platforms, build tags will be better choice.

For rest, file sufixes can be used.

Hope this info will be useful.