Golang compilation constraint / conditional compilation (/ / +build < tags>)

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Usually when compiling a program , There may be some conditions （ Such as different platforms 、 The architecture has different code implementations ）,

Let the compiler compile only the code that meets the conditions , Discard code that doesn't meet the criteria , This is conditional compilation .

golang in , Call it compilation constraint , The methods of compiling constraints are 2 Species difference ：

• Compile tags （build tag）
• file extension

1、 Compile tags （build tag）

Add comments at the top of the source file , To determine whether the file is involved in compilation

// +build <tags>

explain ：

• Separated by spaces AND
• Separated by commas OR
• ! Express NOT

Labels can be specified as the following ：

• operating system , In the environment variables GOOS Value , Such as ：linux、darwin、windows wait . Can pass go env see GOOS The value of the environment variable .
• The architecture of the operating system , In the environment variables GOARCH Value , Such as ：arch64、x86、i386 wait . Can pass go env see GOARCH The value of the environment variable .
• The compiler used ,gc perhaps gccgo.
• Open or not CGO,cgo.
• golang Version number ： such as Go Version 1.1 by go1.1,Go Version 1.12 Version is go1.12, And so on .
• Other custom labels , adopt go build -tags The specified value .

for example , The compilation condition is (linux AND 386) OR (darwin AND (NOT cgo))

// +build linux,386 darwin,!cgo

Another file can have multiple compilation constraints , For example, the condition is (linux OR darwin) AND amd64

// +build linux darwin
// +build amd64

You can also exclude a file from compilation , Use ignore label .

// +build ignore

Be careful ：// +build The next line of must be empty .

// +build linux

// main package comment
package main

to glance at , The real case

The following expression will not be recognized as build tag, It will be resolved to package comments ：

// +build linux
package main

2、 file extension

In addition to compiling tags , The compiler will also automatically select the compiled file according to the file suffix , The format is as follows :

$filename_$GOOS.go
$filename_$GOARCH.go
$filename_$GOOS_$GOARCH.go

• $filename: Source file name .
• $GOOS: Represents the operating system , Get... From the environment variable .
• $GOARCH: Represents the system architecture , Get... From the environment variable .

If there is in the project tcp.go and tcp_linux_x86.go Two documents , perform :

GOOS=linux GOARCH=x86 go build

Will choose tcp_linux_x86.go Compile , And perform :

GOOS=linux GOARCH=x86 go build

choice tcp.go Compile .

3、 utilize ldflags Assign values to variables during compilation

This section is for additional instructions , It does not belong to the category of conditional compilation .

Sometimes we need to assign values to variables during compilation , You can use ldflags Parameters complete .

ldflags yes go build A parameter of , Use as follows ：

go build -ldflags "-w -s -X main.Version=${VERSION} -X github.com/demo/version.BuildNo=${BUILD_NO}"

Parameter description ：

• -w Delete DWARF Information ： The compiled program cannot be used gdb debug .

• -s Delete symbol table ：panic Of stack trace No file name / Line number information , Equivalent to C/C++ The program is strip.

• -X Replace the value of the variable in the package .

add -w -s It can effectively reduce the size of the compiled program , But it is not conducive to debugging and log tracking .