How to build against a modified version of Go

This week I read an article about the vertical scaling limits of a Go application under heavy I/O load, that linked to a Go issue in which the syscall.EpollWait bottleneck is investigated. One particular comment in this issue thread sparked my interest, it says:

Note that is possible to simply edit the runtime source in GOROOT and rebuild your program (no special steps required for the runtime, it is treated like any other package).

What this means is, that any import of standard library packages is handled as if they're user packages, just that the standard library packages are resolved with $GOROOT/src as the base directory. Also, it's a little bit more complicated than the comment makes it look like, but we'll come to that. With this knowledge, let's try what is needed to run a Go program with a custom Go build.

The following steps assume that a Go distribution is already available on your system. Let's create a simple hello-world Go app:

$ mkdir app && cd app
$ go mod init

Where hello.go contains the following lines:

package main

import "fmt"

func main() {

Running go run hello.go will, obviously, print World! to the console.

My contrived example will modify fmt.Println in a way that it always puts a Hello, before everything it prints. I know, a horrible idea, but nobody says you should ship something like this in production 😛.

We need to clone a source copy of Go into some local folder, in this case I use _go, because the go tool ignores folders that are prefixed with either _ or ..

$ git clone --depth 1 --branch go1.21.6 _go

Now we need to build the go tool from the cloned source:

pushd _go/src/go && ./make.bash; popd

If you forget to do this you'll see an error like

go: no such tool "compile"

when trying to compile against your modified Go version. A slight environment modifications is necessary so that the custom go build is picked up:

export GOROOT=$PWD/_go
export PATH=$GOROOT/bin:$PATH

With this done let's modify $GOROOT/src/fmt/print.go as shown in this diff:

diff --git a/src/fmt/print.go b/src/fmt/print.go
index 9c3bd3e..3a83a99 100644
--- a/src/fmt/print.go
+++ b/src/fmt/print.go
@@ -301,6 +301,7 @@ func Append(b []byte, a ...any) []byte {
 // It returns the number of bytes written and any write error encountered.
 func Fprintln(w io.Writer, a ...any) (n int, err error) {
 	p := newPrinter()
+	p.doPrint([]any{"Hello,"})
 	n, err = w.Write(p.buf)

Another run of go run hello.go will now print Hello,World!. Success 🎉!