## 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 Linux, FreeBSD and NetBSD binaries. Less widely used than the other ports.

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), Plan 9, 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. Please refer to the InstallFromSource page on the Go community Wiki for operating system specific instructions.

## 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

## (Optional) Switch to the default branch

If you intend to modify the go source code, and contribute your changes to the project, then move your repository off the release branch, and onto the default (development) branch. Otherwise, skip this step.

    $ hg update default

## 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.

## Set up your work environment

You're almost done. You just need to do a little more setup.

How to Write Go Code Learn how to set up and use the Go tools

The How to Write Go Code document provides essential setup instructions for using the Go tools.

## Install additional tools

The source code for several Go tools (including godoc) is kept in the go.tools repository. To install all of them, run the `go` `get` command:

    $ go get code.google.com/p/go.tools/cmd/...

Or if you just want to install a specific command (`godoc` in this case):

    $ go get code.google.com/p/go.tools/cmd/godoc

To install these tools, the `go` `get` command requires that Mercurial be installed locally.

You must also have a workspace (`GOPATH`) set up; see How to Write Go Code for the details.

Note: The `go` command will install the `godoc` binary to `$GOROOT/bin` (or `$GOBIN`) and the `cover` and `vet` binaries to `$GOROOT/pkg/tool/$GOOS_$GOARCH`. You can access the latter commands with "`go` `tool` `cover`" and "`go` `tool` `vet`".

## 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`
  | `freebsd` | `arm`
  | `linux` | `386`
  | `linux` | `amd64`
  | `linux` | `arm`
  | `netbsd` | `386`
  | `netbsd` | `amd64`
  | `netbsd` | `arm`
  | `openbsd` | `386`
  | `openbsd` | `amd64`
  | `plan9` | `386`
  | `plan9` | `amd64`
  | `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.

- `$GO386` (for `386` only, default is auto-detected if built natively, `387` if not)

  This controls the code generated by 8g to use either the 387 floating-point unit (set to `387`) or SSE2 instructions (set to `sse2`) for floating point computations.

  - `GO386=387`: use x87 for floating point operations; should support all x86 chips (Pentium MMX or later).
  - `GO386=sse2`: use SSE2 for floating point operations; has better performance than 387, but only available on Pentium 4/Opteron/Athlon 64 or later.
- `$GOARM` (for `arm` only; default is auto-detected if building on the target processor, 6 if not)

  This sets the ARM floating point co-processor architecture version the run-time should target. If you are compiling on the target system, its value will be auto-detected.

  - `GOARM=5`: use software floating point; when CPU doesn't have VFP co-processor
  - `GOARM=6`: use VFPv1 only; default if cross compiling; usually ARM11 or better cores (VFPv2 or better is also supported)
  - `GOARM=7`: use VFPv3; usually Cortex-A cores

  If in doubt, leave this variable unset, and adjust it if required when you first run the Go executable. The GoARM page on the Go community wiki contains further details regarding Go's ARM 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.