## 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`
  Incomplete. It only supports Linux binaries, the optimizer is incomplete, and floating point uses the VFP unit. However, all tests pass. Work on the optimizer is continuing. Tested against a Nexus One.

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 (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, and OS X (a.k.a. Darwin) operating systems. (A port to Microsoft Windows is in progress but incomplete. See the Windows Port page for details.) 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 these programs installed:

- GCC,
- the standard C libraries,
- the parser generator Bison,
- GNU `make` (version 3.81 or later), and
- `awk`.

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

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

## Install Mercurial, if needed

To perform the next step you must have Mercurial installed. (Check that you have an `hg` command.) This suffices to install Mercurial on most systems:

    sudo easy_install mercurial

(On Ubuntu/Debian, you might try `apt-get install python-setuptools python-dev build-essential` first. The Mercurial in your distribution's package repository will most likely be old and broken.)

If that fails, try installing manually from the Mercurial Download page.

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

    warning: go.googlecode.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://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:

    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. ***
    The compiler is 6g.

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.

## 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.

## What's next

Start by reading the Go Tutorial.

Build a web application by following the Wiki Codelab.

Read Effective Go to learn about writing idiomatic Go code.

For the full story, consult Go's extensive documentation.

## Keeping up with releases

The Go project maintains two stable tags in its Mercurial repository: `release` and `weekly`. The `weekly` tag is updated about once a week, and should be used by those who want to track the project's development. The `release` tag is given, less often, to those weekly releases that have proven themselves to be robust.

Most Go users will want to keep their Go installation at the latest `release` tag. 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

To use the `weekly` tag run `hg update weekly` instead.

## 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 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 `$GOARCH`
  The name of the target operating system and compilation architecture. These default to the values of `$GOHOSTOS` and `$GOHOSTARCH` respectively (described below).

  Choices for `$GOOS` are `linux`, `freebsd`, `darwin` (Mac OS X 10.5 or 10.6), and `windows` (Windows, an incomplete port). Choices for `$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
  | `windows` | `386` | incomplete

`$GOHOSTOS` and `$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.

`$GOBIN`
  The location where binaries will be installed. The default is `$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 run-time libraries should target. ARMv6 cores have more efficient synchronization primitives. Setting `$GOARM` to 5 will compile the run-time 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