## 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.

Most users don't need to do this, and will instead install from precompiled binary packages as described in Getting Started, a much simpler process. If you want to help develop what goes into those precompiled packages, though, read on.

There are two official Go compiler tool chains. This document focuses on the `gc` Go compiler and tools (`6g`, `8g` etc.). For information on how to work on `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`
  A 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`
  Supports only Linux binaries. Less widely used than the other ports and therefore not as thoroughly tested.

Except for things like low-level operating system interface code, the run-time support is the same in all ports and includes a mark-and-sweep garbage collector, efficient array and string slicing, and support for efficient goroutines, such as stacks that grow and shrink on demand.

The compilers can target the FreeBSD, Linux, NetBSD, OpenBSD, OS X (Darwin), and Windows operating systems. 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 a C compiler installed.

On OS X, a C compiler is bundled in the command line tools for Xcode, and you don't need to install the whole Xcode to compile Go. If you have already installed Xcode 4.3+, you can install command line tools from the Components tab of the Downloads preferences panel. To verify you have a working compiler, just invoke `gcc` in a freshly created Terminal window, unless you see the "`gcc: command not found`" error, you are ready to go.

On Ubuntu/Debian, use `sudo apt-get install gcc libc6-dev`. If you want to build 32-bit binaries on a 64-bit system you'll also need the `libc6-dev-i386` package.

On Windows, install `gcc` with MinGW. (Make sure you add its `bin` subdirectory to your `PATH`.)

## Install Mercurial, if needed

To perform the next step you must have Mercurial installed. (Check that you have an `hg` command.)

If you do not have a working Mercurial installation, follow the instructions on the Mercurial downloads page.

Mercurial versions 1.7.x and up require the configuration of Certification Authorities (CAs). Error messages of the form:

    warning: code.google.com certificate with fingerprint b1:af: ... bc not verified (check hostfingerprints or web.cacerts config setting)

when using Mercurial indicate that the CAs are missing. Check your Mercurial version (`hg --version`) and configure the CAs if necessary.

## Fetch the repository

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 -u release https://code.google.com/p/go

## Install Go

To build the Go distribution, run

    $ cd go/src
    $ ./all.bash

(To build under Windows use `all.bat`.)

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

    ALL TESTS PASSED

    ---
    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. ***

where the details on the last few lines 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.

## Testing your installation

Check that Go is installed correctly by building a simple program.

Create a file named `hello.go` and put the following program in it:

    package main

    import "fmt"

    func main() {
        fmt.Printf("hello, world\n")
    }

Then run it with the `go` tool:

    $ go run hello.go
    hello, world

If you see the "hello, world" message then Go is installed correctly.

## Community resources

The usual community resources such as `#go-nuts` on the Freenode IRC server and the Go Nuts mailing list have active developers that can help you with problems with your installation or your development work. For those who wish to keep up to date, there is another mailing list, golang-checkins, that receives a message summarizing each checkin to the Go repository.

Bugs can be reported using the Go issue tracker.

## Keeping up with releases

The Go project maintains a stable tag in its Mercurial repository: `release`.

The `release` tag refers to the current stable release of Go. Most Go users should use this version. New releases are announced on the golang-announce mailing list.

To update an existing tree to the latest release, you can run:

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

## Optional environment variables

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

> `$GOROOT`

> The root of the Go tree, often `$HOME/go`. Its value is built into the tree when it is compiled, and defaults to the parent of the directory where `all.bash` was run. There is no need to set this unless you want to switch between multiple local copies of the repository.

> `$GOROOT_FINAL`

> The value assumed by installed binaries and scripts when `$GOROOT` is not set explicitly. It defaults to the value of `$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 values of `$GOHOSTOS` and `$GOHOSTARCH` respectively (described below).

> Choices for `$GOOS` are `darwin` (Mac OS X 10.6 and above), `freebsd`, `linux`, `netbsd`, `openbsd`, `plan9`, and `windows`. Choices for `$GOARCH` are `amd64` (64-bit x86, the most mature port), `386` (32-bit x86), and `arm` (32-bit ARM). The valid combinations of `$GOOS` and `$GOARCH` are:

> | `$GOOS` | `$GOARCH` |
> --- | --- | --- | ---
> | `darwin` | `386`
> | `darwin` | `amd64`
> | `freebsd` | `386`
> | `freebsd` | `amd64`
> | `linux` | `386`
> | `linux` | `amd64`
> | `linux` | `arm`
> | `netbsd` | `386`
> | `netbsd` | `amd64`
> | `openbsd` | `386`
> | `openbsd` | `amd64`
> | `plan9` | `386`
> | `windows` | `386`
> | `windows` | `amd64`

> `$GOHOSTOS` and `$GOHOSTARCH`

> The name of the host operating system and compilation architecture. These default to the local system's operating system and architecture.

> Valid choices are the same as for `$GOOS` and `$GOARCH`, listed above. The specified values must be compatible with the local system. For example, you should not set `$GOHOSTARCH` to `arm` on an x86 system.

> `$GOBIN`

> The location where Go 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. If `$GOBIN` is set, the go command installs all commands there.

> `$GOARM` (arm, default=6)

> The ARM architecture version the run-time libraries should target. Setting `$GOARM` to 5 causes the linker to emit calls to a software floating point implementation instead of using hardware floating point support.

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=amd64
    export GOOS=linux

although, to reiterate, none of these variables needs to be set to build, install, and develop the Go tree.