using ldflags for giving binary file a version not working in golang

The main.go as follows:

package main

func main() {
	println("hello world")
}

The Makefile as follows:

flags=-X main.version=1.5.0

build:
	go build -ldflags "$(flags)" -o main main.go

Then I run make, and got main file.

After I runing ./main -v, I just got:

hello world

Why it does not display 1.5.0?

go version is go version go1.18 darwin/arm64, when I run go version -m main, I got:

main: go1.18
        path    command-line-arguments
        build   -compiler=gc
        build   -ldflags="-X main.version=1.5.0"
        build   CGO_ENABLED=1
        build   CGO_CFLAGS=
        build   CGO_CPPFLAGS=
        build   CGO_CXXFLAGS=
        build   CGO_LDFLAGS=
        build   GOARCH=arm64
        build   GOOS=darwin

I had a misunderstanding of ldflags. It works as follows:

package main

import (
	"fmt"
	"os"
)

var (
	version string
)

func main() {
	args := os.Args
	if len(args) == 2 && (args[1] == "--version" || args[1] == "-v") {
		fmt.Printf("project version: %s \n", version)
		return
	}
}

$ ./main -v 
project version: 1.5.0

The variable you're setting with ldflags (version) must be declared in the package at the package level. The handling of -v you yourself must implement.

package main

import "flag"

var version string

func main() {
    var vFlag bool
    flag.BoolVar(&vFlag, "v", false, "show version")
    flag.Parse()
    
    if vFlag {
        println(version)
    } else {
        println("hello world")
    }
}

go build -ldflags "-X main.version=1.5.0" -o main main.go
./main -v
# 1.5.0
./main
# hello world

If one builds a package (as in go build github.com/xxx/yyy/zzz then the symbol must be prefixed accordingly. For example:

go build \
    -ldflags "-X github.com/xxx/yyy/zzz/main.version=1.5.0" \
    -o my-awesome-app \
    github.com/xxx/yyy/zzz

B.T.W. Another way to figure out which symbol ID to use is to run nm and grep for the variable you are looking for.

This comment is not an answer for the actual question but is related to the title. Therefore, I think it is worth sharing here.

One reason why ldflags may seem not to be working is that it works only for string type variables.

Example:

package main

import "fmt"

var sss = "FOO"
var ccc = 2

func main() {
        fmt.Println("variables:", sss, ccc)
}

• Executing this code with go run main.go displays variables: FOO 2
• Executing it with go run -ldflags="-X main.sss=BAR" main.go displays variables: BAR 2
• Executing it with go run -ldflags="-X main.ccc=5" main.go displays main.ccc: cannot set with -X: not a var of type string (type.int)

Another gotcha is you get no error message if -X is missing. It just doesn't do what you expect:

• Executing it with go run -ldflags="main.sss=BAR" main.go displays variables: FOO 2