## Introduction

Go is an open source project, distributed under a BSD-style license. This document explains how to check out the sources, build them on your own machine, and run them.

There are two distinct ways to experiment with Go. This document focuses on the `gc` Go compiler and tools (`6g`, `8g` etc.). For information on how to use `gccgo`, a more traditional compiler using the GCC back end, see Setting up and using gccgo.

The Go compilers support three instruction sets. There are important differences in the quality of the compilers for the different architectures.

`amd64` (a.k.a. `x86-64`); `6g,6l,6c,6a`
  The most mature implementation. The compiler has an effective optimizer (registerizer) and generates good code (although `gccgo` can do noticeably better sometimes).
`386` (a.k.a. `x86` or `x86-32`); `8g,8l,8c,8a`
  Comparable to the `amd64` port.
`arm` (a.k.a. `ARM`); `5g,5l,5c,5a`
  Still a work in progress. It only supports Linux binaries, floating point is weak, and the optimizer is not enabled. Tested against QEMU and an android phone.

Except for things like low-level operating system interface code, the runtime support is the same in all ports and includes a mark-and-sweep garbage collector (a fancier one is in the works), efficient array and string slicing, support for segmented stacks, and a strong goroutine implementation.

The compilers can target the FreeBSD, Linux, Native Client, and OS X (a.k.a. Darwin) operating systems. (A port to Microsoft Windows is in progress but incomplete.) The full set of supported combinations is listed in the discussion of environment variables below.

## Install C tools, if needed

The Go tool chain is written in C. To build it, you need to have GCC, the standard C libraries, the parser generator Bison, `make`, `awk`, and the text editor `ed` installed.

On OS X, they can be installed as part of Xcode.

On Linux, use `sudo apt-get install bison ed gawk gcc libc6-dev make` (or the equivalent on your Linux distribution).

## Fetch the repository

If you do not have Mercurial installed (you do not have an `hg` command), `sudo easy_install mercurial` works on most systems. (On Ubuntu/Debian, you might try `apt-get install python-setuptools python-dev build-essential gcc` first.) If that fails, visit the Mercurial Download page.

Go will install to a directory named `go`. Change to the directory that will be its parent and make sure the `go` directory does not exist. Then check out the repository:

    $ hg clone -r release https://go.googlecode.com/hg/ go

## Install Go

To build the Go distribution, run

    $ cd go/src
    $ ./all.bash

If all goes well, it will finish by printing output like:

    --- cd ../test
    N known bugs; 0 unexpected bugs

    ---
    Installed Go for linux/amd64 in /home/you/go.
    Installed commands in /home/you/go/bin.
    *** You need to add /home/you/go/bin to your $PATH. ***
    The compiler is 6g.

where N is a number that varies from release to release and the details on the last few lines will reflect the operating system, architecture, and root directory used during the install.

For more information about ways to control the build, see the discussion of environment variables below.

## Writing programs

Given a file `file.go`, compile it using

    $ 6g file.go

`6g` is the Go compiler for `amd64`; it will write the output in `file.6`. The ‘`6`’ identifies files for the `amd64` architecture. The identifier letters for `386` and `arm` are ‘`8`’ and ‘`5`’. That is, if you were compiling for `386`, you would use `8g` and the output would be named `file.8`.

To link the file, use

    $ 6l file.6

and to run it

    $ ./6.out

A complete example:

    $ cat >hello.go <<EOF
    package main

    import "fmt"

    func main() {
    	fmt.Printf("hello, world\n")
    }
    EOF
    $ 6g hello.go
    $ 6l hello.6
    $ ./6.out
    hello, world
    $

There is no need to list `hello.6`'s package dependencies (in this case, package `fmt`) on the `6l` command line. The linker learns about them by reading `hello.6`.

To build more complicated programs, you will probably want to use a `Makefile`. There are examples in places like `go/src/cmd/godoc/Makefile` and `go/src/pkg/*/Makefile`. The document about contributing to the Go project gives more detail about the process of building and testing Go programs.

## Keeping up with releases

New releases are announced on the Go Nuts mailing list. To update an existing tree to the latest release, you can run:

    $ cd go/src
    $ hg pull
    $ hg update release
    $ ./all.bash

## Community resources

For real-time help, there may be users or developers on `#go-nuts` on the Freenode IRC server.

The official mailing list for discussion of the Go language is Go Nuts.

Bugs can be reported using the Go issue tracker.

For those who wish to keep up with development, there is another mailing list, golang-checkins, that receives a message summarizing each checkin to the Go repository.

## Environment variables

The Go compilation environment can be customized by five environment variables. None are required by the build, but you may wish to set them to override the defaults.

`$GOROOT`
  The root of the Go tree, often `$HOME/go`. This defaults to the parent of the directory where `all.bash` is run. If you choose not to set `$GOROOT`, you must run `gomake` instead of `make` or `gmake` when developing Go programs using the conventional makefiles.
`$GOROOT_FINAL`
  The value assumed by installed binaries and scripts when `$GOROOT` is not set. It defaults to the value used for `$GOROOT`. If you want to build the Go tree in one location but move it elsewhere after the build, set `$GOROOT_FINAL` to the eventual location.
`$GOOS` and `$GOARCH`
  The name of the target operating system and compilation architecture. These default to the local system's operating system and architecture.

  Choices for `$GOOS` are `linux`, `freebsd`, `darwin` (Mac OS X 10.5 or 10.6), and `nacl` (Native Client, an incomplete port). Choices for `$GOARCH` are `amd64` (64-bit x86, the most mature port), `386` (32-bit x86), and `arm` (32-bit ARM, an incomplete port). The valid combinations of `$GOOS` and `$GOARCH` are:

  | `$GOOS` | `$GOARCH` |
  --- | --- | --- | ---
  | `darwin` | `386`
  | `darwin` | `amd64`
  | `freebsd` | `386`
  | `freebsd` | `amd64`
  | `linux` | `386`
  | `linux` | `amd64`
  | `linux` | `arm` | incomplete
  | `nacl` | `386`
  | `windows` | `386` | incomplete

`$GOBIN`
  The location where binaries will be installed. The default is `$GOROOT/bin`. After installing, you will want to arrange to add this directory to your `$PATH`, so you can use the tools.
`$GOARM` (arm, default=6)
  The ARM architecture version the runtime libraries should target. ARMv6 cores have more efficient synchronization primitives. Setting `$GOARM` to 5 will compile the runtime libraries using just SWP instructions that work on older architectures as well. Running v6 code on an older core will cause an illegal instruction trap.

Note that `$GOARCH` and `$GOOS` identify the target environment, not the environment you are running on. In effect, you are always cross-compiling. By architecture, we mean the kind of binaries that the target environment can run: an x86-64 system running a 32-bit-only operating system must set `GOARCH` to `386`, not `amd64`.

If you choose to override the defaults, set these variables in your shell profile (`$HOME/.bashrc`, `$HOME/.profile`, or equivalent). The settings might look something like this:

    export GOROOT=$HOME/go
    export GOARCH=386
    export GOOS=linux