Goroutine IDs

Dec 16, 2015 · 3 minute read · Comments
Go Hacks

Do Goroutine IDs Exist?

Yes, of course. The runtime has to have some way to track them.

Should I use them?

Are There Packages I Can Use?

There exist packages from people on the Go team, with colorful descriptions like “If you use this package, you will go straight to hell.“

There are also packages to do goroutine local storage built on top of goroutine IDs, like github.com/jtolds/gls and github.com/tylerb/gls that use dirty knowledge from the above to create an environment contrary to Go design principles.

Minimal Code

So you like pain? Here’s how to get the current goroutine ID:

The Hacky Way in “Pure” Go

This is adapted from Brad Fitzpatrick’s http/2 library that is being rolled into Go 1.6. It was used solely for debugging purposes, and is not used during regular operation

package main

import (
    "bytes"
    "fmt"
    "runtime"
    "strconv"
)

func main() {
    fmt.Println(getGID())
}

func getGID() uint64 {
    b := make([]byte, 64)
    b = b[:runtime.Stack(b, false)]
    b = bytes.TrimPrefix(b, []byte("goroutine "))
    b = b[:bytes.IndexByte(b, ' ')]
    n, _ := strconv.ParseUint(string(b), 10, 64)
    return n
}

How does this work?

It is possible to parse debug information (meant for human consumption, mind you) and retrieve the goroutine ID. There’s even debug code in the http/2 library that uses this to track ownership of connections. It was used sparingly, however, for debugging purposes only. It’s not meant to be a high-performance operation.

The debug information itself was retrieved by calling runtime.Stack(buf []byte, all bool) int which will print a stacktrace into the buffer it’s given in textual form. The very first line in the stacktrace is the text “goroutine #### […” where #### is the actual goroutine ID. The rest is just text manipulation to extract and parse the number. I took out some error handling, but if you want the goroutine ID in the first place, I assume you want to live dangerously already.

The Legitimate CGo Version

The C version is from github.com/davecheney/junk/id where the C code directly accesses the goid property of the current goroutine and just returns that. The code below was copied verbatim from Dave Cheney’s repo, so all credit to him.

File id.c

#include "runtime.h"

int64 ·Id(void) {
	return g->goid;
}

File id.go

package id

func Id() int64

Where to go from here?

Running screaming away from goroutine IDs. Forget they exist, and you will be much happier. The use of them is a red flag from a design standpoint, because nearly all uses (speaking from research, not tacit knowledge) try to create something to do goroutine-local state, which violates the “Share Memory By Communicating” tenet of Go programming.

Scott Mansfield