go

Documentation

Overview

Go is a tool for managing Go source code.

Usage:

go command [arguments]

The commands are:

build       compile packages and dependencies
clean       remove object files
doc         show documentation for package or symbol
env         print Go environment information
bug         start a bug report
fix         run go tool fix on packages
fmt         run gofmt on package sources
generate    generate Go files by processing source
get         download and install packages and dependencies
install     compile and install packages and dependencies
list        list packages
run         compile and run Go program
test        test packages
tool        run specified go tool
version     print Go version
vet         run go tool vet on packages

Use "go help [command]" for more information about a command.

Additional help topics:

c           calling between Go and C
buildmode   description of build modes
filetype    file types
gopath      GOPATH environment variable
environment environment variables
importpath  import path syntax
packages    description of package lists
testflag    description of testing flags
testfunc    description of testing functions

Use "go help [topic]" for more information about that topic.

Compile packages and dependencies

Usage:

go build [-o output] [-i] [build flags] [packages]

Build compiles the packages named by the import paths, along with their dependencies, but it does not install the results.

If the arguments to build are a list of .go files, build treats them as a list of source files specifying a single package.

When compiling a single main package, build writes the resulting executable to an output file named after the first source file ('go build ed.go rx.go' writes 'ed' or 'ed.exe') or the source code directory ('go build unix/sam' writes 'sam' or 'sam.exe'). The '.exe' suffix is added when writing a Windows executable.

When compiling multiple packages or a single non-main package, build compiles the packages but discards the resulting object, serving only as a check that the packages can be built.

When compiling packages, build ignores files that end in '_test.go'.

The -o flag, only allowed when compiling a single package, forces build to write the resulting executable or object to the named output file, instead of the default behavior described in the last two paragraphs.

The -i flag installs the packages that are dependencies of the target.

The build flags are shared by the build, clean, get, install, list, run, and test commands:

-a
	force rebuilding of packages that are already up-to-date.
-n
	print the commands but do not run them.
-p n
	the number of programs, such as build commands or
	test binaries, that can be run in parallel.
	The default is the number of CPUs available.
-race
	enable data race detection.
	Supported only on linux/amd64, freebsd/amd64, darwin/amd64 and windows/amd64.
-msan
	enable interoperation with memory sanitizer.
	Supported only on linux/amd64,
	and only with Clang/LLVM as the host C compiler.
-v
	print the names of packages as they are compiled.
-work
	print the name of the temporary work directory and
	do not delete it when exiting.
-x
	print the commands.

-asmflags 'flag list'
	arguments to pass on each go tool asm invocation.
-buildmode mode
	build mode to use. See 'go help buildmode' for more.
-compiler name
	name of compiler to use, as in runtime.Compiler (gccgo or gc).
-gccgoflags 'arg list'
	arguments to pass on each gccgo compiler/linker invocation.
-gcflags 'arg list'
	arguments to pass on each go tool compile invocation.
-installsuffix suffix
	a suffix to use in the name of the package installation directory,
	in order to keep output separate from default builds.
	If using the -race flag, the install suffix is automatically set to race
	or, if set explicitly, has _race appended to it.  Likewise for the -msan
	flag.  Using a -buildmode option that requires non-default compile flags
	has a similar effect.
-ldflags 'flag list'
	arguments to pass on each go tool link invocation.
-linkshared
	link against shared libraries previously created with
	-buildmode=shared.
-pkgdir dir
	install and load all packages from dir instead of the usual locations.
	For example, when building with a non-standard configuration,
	use -pkgdir to keep generated packages in a separate location.
-tags 'tag list'
	a space-separated list of build tags to consider satisfied during the
	build. For more information about build tags, see the description of
	build constraints in the documentation for the go/build package.
-toolexec 'cmd args'
	a program to use to invoke toolchain programs like vet and asm.
	For example, instead of running asm, the go command will run
	'cmd args /path/to/asm <arguments for asm>'.

All the flags that take a list of arguments accept a space-separated list of strings. To embed spaces in an element in the list, surround it with either single or double quotes.

For more about specifying packages, see 'go help packages'. For more about where packages and binaries are installed, run 'go help gopath'. For more about calling between Go and C/C++, run 'go help c'.

Note: Build adheres to certain conventions such as those described by 'go help gopath'. Not all projects can follow these conventions, however. Installations that have their own conventions or that use a separate software build system may choose to use lower-level invocations such as 'go tool compile' and 'go tool link' to avoid some of the overheads and design decisions of the build tool.

See also: go install, go get, go clean.

Remove object files

Usage:

go clean [-i] [-r] [-n] [-x] [build flags] [packages]

Clean removes object files from package source directories. The go command builds most objects in a temporary directory, so go clean is mainly concerned with object files left by other tools or by manual invocations of go build.

Specifically, clean removes the following files from each of the source directories corresponding to the import paths:

_obj/            old object directory, left from Makefiles
_test/           old test directory, left from Makefiles
_testmain.go     old gotest file, left from Makefiles
test.out         old test log, left from Makefiles
build.out        old test log, left from Makefiles
*.[568ao]        object files, left from Makefiles

DIR(.exe)        from go build
DIR.test(.exe)   from go test -c
MAINFILE(.exe)   from go build MAINFILE.go
*.so             from SWIG

In the list, DIR represents the final path element of the directory, and MAINFILE is the base name of any Go source file in the directory that is not included when building the package.

The -i flag causes clean to remove the corresponding installed archive or binary (what 'go install' would create).

The -n flag causes clean to print the remove commands it would execute, but not run them.

The -r flag causes clean to be applied recursively to all the dependencies of the packages named by the import paths.

The -x flag causes clean to print remove commands as it executes them.

For more about build flags, see 'go help build'.

For more about specifying packages, see 'go help packages'.

Show documentation for package or symbol

Usage:

go doc [-u] [-c] [package|[package.]symbol[.method]]

Doc prints the documentation comments associated with the item identified by its arguments (a package, const, func, type, var, or method) followed by a one-line summary of each of the first-level items "under" that item (package-level declarations for a package, methods for a type, etc.).

Doc accepts zero, one, or two arguments.

Given no arguments, that is, when run as

go doc

it prints the package documentation for the package in the current directory. If the package is a command (package main), the exported symbols of the package are elided from the presentation unless the -cmd flag is provided.

When run with one argument, the argument is treated as a Go-syntax-like representation of the item to be documented. What the argument selects depends on what is installed in GOROOT and GOPATH, as well as the form of the argument, which is schematically one of these:

go doc <pkg>
go doc <sym>[.<method>]
go doc [<pkg>.]<sym>[.<method>]
go doc [<pkg>.][<sym>.]<method>

The first item in this list matched by the argument is the one whose documentation is printed. (See the examples below.) However, if the argument starts with a capital letter it is assumed to identify a symbol or method in the current directory.

For packages, the order of scanning is determined lexically in breadth-first order. That is, the package presented is the one that matches the search and is nearest the root and lexically first at its level of the hierarchy. The GOROOT tree is always scanned in its entirety before GOPATH.

If there is no package specified or matched, the package in the current directory is selected, so "go doc Foo" shows the documentation for symbol Foo in the current package.

The package path must be either a qualified path or a proper suffix of a path. The go tool's usual package mechanism does not apply: package path elements like . and ... are not implemented by go doc.

When run with two arguments, the first must be a full package path (not just a suffix), and the second is a symbol or symbol and method; this is similar to the syntax accepted by godoc:

go doc <pkg> <sym>[.<method>]

In all forms, when matching symbols, lower-case letters in the argument match either case but upper-case letters match exactly. This means that there may be multiple matches of a lower-case argument in a package if different symbols have different cases. If this occurs, documentation for all matches is printed.

Examples:

go doc
	Show documentation for current package.
go doc Foo
	Show documentation for Foo in the current package.
	(Foo starts with a capital letter so it cannot match
	a package path.)
go doc encoding/json
	Show documentation for the encoding/json package.
go doc json
	Shorthand for encoding/json.
go doc json.Number (or go doc json.number)
	Show documentation and method summary for json.Number.
go doc json.Number.Int64 (or go doc json.number.int64)
	Show documentation for json.Number's Int64 method.
go doc cmd/doc
	Show package docs for the doc command.
go doc -cmd cmd/doc
	Show package docs and exported symbols within the doc command.
go doc template.new
	Show documentation for html/template's New function.
	(html/template is lexically before text/template)
go doc text/template.new # One argument
	Show documentation for text/template's New function.
go doc text/template new # Two arguments
	Show documentation for text/template's New function.

At least in the current tree, these invocations all print the
documentation for json.Decoder's Decode method:

go doc json.Decoder.Decode
go doc json.decoder.decode
go doc json.decode
cd go/src/encoding/json; go doc decode

Flags:

-c
	Respect case when matching symbols.
-cmd
	Treat a command (package main) like a regular package.
	Otherwise package main's exported symbols are hidden
	when showing the package's top-level documentation.
-u
	Show documentation for unexported as well as exported
	symbols and methods.

Print Go environment information

Usage:

go env [var ...]

Env prints Go environment information.

By default env prints information as a shell script (on Windows, a batch file). If one or more variable names is given as arguments, env prints the value of each named variable on its own line.

Start a bug report

Usage:

go bug

Bug opens the default browser and starts a new bug report. The report includes useful system information.

Run go tool fix on packages

Usage:

go fix [packages]

Fix runs the Go fix command on the packages named by the import paths.

For more about fix, see 'go doc cmd/fix'. For more about specifying packages, see 'go help packages'.

To run fix with specific options, run 'go tool fix'.

See also: go fmt, go vet.

Run gofmt on package sources

Usage:

go fmt [-n] [-x] [packages]

Fmt runs the command 'gofmt -l -w' on the packages named by the import paths. It prints the names of the files that are modified.

For more about gofmt, see 'go doc cmd/gofmt'. For more about specifying packages, see 'go help packages'.

The -n flag prints commands that would be executed. The -x flag prints commands as they are executed.

To run gofmt with specific options, run gofmt itself.

See also: go fix, go vet.

Generate Go files by processing source

Usage:

go generate [-run regexp] [-n] [-v] [-x] [build flags] [file.go... | packages]

Generate runs commands described by directives within existing files. Those commands can run any process but the intent is to create or update Go source files.

Go generate is never run automatically by go build, go get, go test, and so on. It must be run explicitly.

Go generate scans the file for directives, which are lines of the form,

//go:generate command argument...

(note: no leading spaces and no space in "//go") where command is the generator to be run, corresponding to an executable file that can be run locally. It must either be in the shell path (gofmt), a fully qualified path (/usr/you/bin/mytool), or a command alias, described below.

Note that go generate does not parse the file, so lines that look like directives in comments or multiline strings will be treated as directives.

The arguments to the directive are space-separated tokens or double-quoted strings passed to the generator as individual arguments when it is run.

Quoted strings use Go syntax and are evaluated before execution; a quoted string appears as a single argument to the generator.

Go generate sets several variables when it runs the generator:

$GOARCH
	The execution architecture (arm, amd64, etc.)
$GOOS
	The execution operating system (linux, windows, etc.)
$GOFILE
	The base name of the file.
$GOLINE
	The line number of the directive in the source file.
$GOPACKAGE
	The name of the package of the file containing the directive.
$DOLLAR
	A dollar sign.

Other than variable substitution and quoted-string evaluation, no special processing such as "globbing" is performed on the command line.

As a last step before running the command, any invocations of any environment variables with alphanumeric names, such as $GOFILE or $HOME, are expanded throughout the command line. The syntax for variable expansion is $NAME on all operating systems. Due to the order of evaluation, variables are expanded even inside quoted strings. If the variable NAME is not set, $NAME expands to the empty string.

A directive of the form,

//go:generate -command xxx args...

specifies, for the remainder of this source file only, that the string xxx represents the command identified by the arguments. This can be used to create aliases or to handle multiword generators. For example,

//go:generate -command foo go tool foo

specifies that the command "foo" represents the generator "go tool foo".

Generate processes packages in the order given on the command line, one at a time. If the command line lists .go files, they are treated as a single package. Within a package, generate processes the source files in a package in file name order, one at a time. Within a source file, generate runs generators in the order they appear in the file, one at a time.

If any generator returns an error exit status, "go generate" skips all further processing for that package.

The generator is run in the package's source directory.

Go generate accepts one specific flag:

-run=""
	if non-empty, specifies a regular expression to select
	directives whose full original source text (excluding
	any trailing spaces and final newline) matches the
	expression.

It also accepts the standard build flags including -v, -n, and -x. The -v flag prints the names of packages and files as they are processed. The -n flag prints commands that would be executed. The -x flag prints commands as they are executed.

For more about build flags, see 'go help build'.

For more about specifying packages, see 'go help packages'.

Download and install packages and dependencies

Usage:

go get [-d] [-f] [-fix] [-insecure] [-t] [-u] [build flags] [packages]

Get downloads the packages named by the import paths, along with their dependencies. It then installs the named packages, like 'go install'.

The -d flag instructs get to stop after downloading the packages; that is, it instructs get not to install the packages.

The -f flag, valid only when -u is set, forces get -u not to verify that each package has been checked out from the source control repository implied by its import path. This can be useful if the source is a local fork of the original.

The -fix flag instructs get to run the fix tool on the downloaded packages before resolving dependencies or building the code.

The -insecure flag permits fetching from repositories and resolving custom domains using insecure schemes such as HTTP. Use with caution.

The -t flag instructs get to also download the packages required to build the tests for the specified packages.

The -u flag instructs get to use the network to update the named packages and their dependencies. By default, get uses the network to check out missing packages but does not use it to look for updates to existing packages.

The -v flag enables verbose progress and debug output.

Get also accepts build flags to control the installation. See 'go help build'.

When checking out a new package, get creates the target directory GOPATH/src/<import-path>. If the GOPATH contains multiple entries, get uses the first one. For more details see: 'go help gopath'.

When checking out or updating a package, get looks for a branch or tag that matches the locally installed version of Go. The most important rule is that if the local installation is running version "go1", get searches for a branch or tag named "go1". If no such version exists it retrieves the most recent version of the package.

When go get checks out or updates a Git repository, it also updates any git submodules referenced by the repository.

Get never checks out or updates code stored in vendor directories.

For more about specifying packages, see 'go help packages'.

For more about how 'go get' finds source code to download, see 'go help importpath'.

See also: go build, go install, go clean.

Compile and install packages and dependencies

Usage:

go install [build flags] [packages]

Install compiles and installs the packages named by the import paths, along with their dependencies.

For more about the build flags, see 'go help build'. For more about specifying packages, see 'go help packages'.

See also: go build, go get, go clean.

List packages

Usage:

go list [-e] [-f format] [-json] [build flags] [packages]

List lists the packages named by the import paths, one per line.

The default output shows the package import path:

bytes
encoding/json
github.com/gorilla/mux
golang.org/x/net/html

The -f flag specifies an alternate format for the list, using the syntax of package template. The default output is equivalent to -f '{{.ImportPath}}'. The struct being passed to the template is:

type Package struct {
    Dir           string // directory containing package sources
    ImportPath    string // import path of package in dir
    ImportComment string // path in import comment on package statement
    Name          string // package name
    Doc           string // package documentation string
    Target        string // install path
    Shlib         string // the shared library that contains this package (only set when -linkshared)
    Goroot        bool   // is this package in the Go root?
    Standard      bool   // is this package part of the standard Go library?
    Stale         bool   // would 'go install' do anything for this package?
    StaleReason   string // explanation for Stale==true
    Root          string // Go root or Go path dir containing this package
    ConflictDir   string // this directory shadows Dir in $GOPATH
    BinaryOnly    bool   // binary-only package: cannot be recompiled from sources

    // Source files
    GoFiles        []string // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
    CgoFiles       []string // .go sources files that import "C"
    IgnoredGoFiles []string // .go sources ignored due to build constraints
    CFiles         []string // .c source files
    CXXFiles       []string // .cc, .cxx and .cpp source files
    MFiles         []string // .m source files
    HFiles         []string // .h, .hh, .hpp and .hxx source files
    FFiles         []string // .f, .F, .for and .f90 Fortran source files
    SFiles         []string // .s source files
    SwigFiles      []string // .swig files
    SwigCXXFiles   []string // .swigcxx files
    SysoFiles      []string // .syso object files to add to archive
    TestGoFiles    []string // _test.go files in package
    XTestGoFiles   []string // _test.go files outside package

    // Cgo directives
    CgoCFLAGS    []string // cgo: flags for C compiler
    CgoCPPFLAGS  []string // cgo: flags for C preprocessor
    CgoCXXFLAGS  []string // cgo: flags for C++ compiler
    CgoFFLAGS    []string // cgo: flags for Fortran compiler
    CgoLDFLAGS   []string // cgo: flags for linker
    CgoPkgConfig []string // cgo: pkg-config names

    // Dependency information
    Imports      []string // import paths used by this package
    Deps         []string // all (recursively) imported dependencies
    TestImports  []string // imports from TestGoFiles
    XTestImports []string // imports from XTestGoFiles

    // Error information
    Incomplete bool            // this package or a dependency has an error
    Error      *PackageError   // error loading package
    DepsErrors []*PackageError // errors loading dependencies
}

Packages stored in vendor directories report an ImportPath that includes the path to the vendor directory (for example, "d/vendor/p" instead of "p"), so that the ImportPath uniquely identifies a given copy of a package. The Imports, Deps, TestImports, and XTestImports lists also contain these expanded imports paths. See golang.org/s/go15vendor for more about vendoring.

The error information, if any, is

type PackageError struct {
    ImportStack   []string // shortest path from package named on command line to this one
    Pos           string   // position of error (if present, file:line:col)
    Err           string   // the error itself
}

The template function "join" calls strings.Join.

The template function "context" returns the build context, defined as:

type Context struct {
	GOARCH        string   // target architecture
	GOOS          string   // target operating system
	GOROOT        string   // Go root
	GOPATH        string   // Go path
	CgoEnabled    bool     // whether cgo can be used
	UseAllFiles   bool     // use files regardless of +build lines, file names
	Compiler      string   // compiler to assume when computing target paths
	BuildTags     []string // build constraints to match in +build lines
	ReleaseTags   []string // releases the current release is compatible with
	InstallSuffix string   // suffix to use in the name of the install dir
}

For more information about the meaning of these fields see the documentation for the go/build package's Context type.

The -json flag causes the package data to be printed in JSON format instead of using the template format.

The -e flag changes the handling of erroneous packages, those that cannot be found or are malformed. By default, the list command prints an error to standard error for each erroneous package and omits the packages from consideration during the usual printing. With the -e flag, the list command never prints errors to standard error and instead processes the erroneous packages with the usual printing. Erroneous packages will have a non-empty ImportPath and a non-nil Error field; other information may or may not be missing (zeroed).

For more about build flags, see 'go help build'.

For more about specifying packages, see 'go help packages'.

Compile and run Go program

Usage:

go run [build flags] [-exec xprog] gofiles... [arguments...]

Run compiles and runs the main package comprising the named Go source files. A Go source file is defined to be a file ending in a literal ".go" suffix.

By default, 'go run' runs the compiled binary directly: 'a.out arguments...'. If the -exec flag is given, 'go run' invokes the binary using xprog:

'xprog a.out arguments...'.

If the -exec flag is not given, GOOS or GOARCH is different from the system default, and a program named go_$GOOS_$GOARCH_exec can be found on the current search path, 'go run' invokes the binary using that program, for example 'go_nacl_386_exec a.out arguments...'. This allows execution of cross-compiled programs when a simulator or other execution method is available.

For more about build flags, see 'go help build'.

See also: go build.

Test packages

Usage:

go test [build/test flags] [packages] [build/test flags & test binary flags]

'Go test' automates testing the packages named by the import paths. It prints a summary of the test results in the format:

ok   archive/tar   0.011s
FAIL archive/zip   0.022s
ok   compress/gzip 0.033s
...

followed by detailed output for each failed package.

'Go test' recompiles each package along with any files with names matching the file pattern "*_test.go". Files whose names begin with "_" (including "_test.go") or "." are ignored. These additional files can contain test functions, benchmark functions, and example functions. See 'go help testfunc' for more. Each listed package causes the execution of a separate test binary.

Test files that declare a package with the suffix "_test" will be compiled as a separate package, and then linked and run with the main test binary.

The go tool will ignore a directory named "testdata", making it available to hold ancillary data needed by the tests.

By default, go test needs no arguments. It compiles and tests the package with source in the current directory, including tests, and runs the tests.

The package is built in a temporary directory so it does not interfere with the non-test installation.

In addition to the build flags, the flags handled by 'go test' itself are:

-args
    Pass the remainder of the command line (everything after -args)
    to the test binary, uninterpreted and unchanged.
    Because this flag consumes the remainder of the command line,
    the package list (if present) must appear before this flag.

-c
    Compile the test binary to pkg.test but do not run it
    (where pkg is the last element of the package's import path).
    The file name can be changed with the -o flag.

-exec xprog
    Run the test binary using xprog. The behavior is the same as
    in 'go run'. See 'go help run' for details.

-i
    Install packages that are dependencies of the test.
    Do not run the test.

-o file
    Compile the test binary to the named file.
    The test still runs (unless -c or -i is specified).

The test binary also accepts flags that control execution of the test; these flags are also accessible by 'go test'. See 'go help testflag' for details.

For more about build flags, see 'go help build'. For more about specifying packages, see 'go help packages'.

See also: go build, go vet.

Run specified go tool

Usage:

go tool [-n] command [args...]

Tool runs the go tool command identified by the arguments. With no arguments it prints the list of known tools.

The -n flag causes tool to print the command that would be executed but not execute it.

For more about each tool command, see 'go tool command -h'.

Print Go version

Usage:

go version

Version prints the Go version, as reported by runtime.Version.

Run go tool vet on packages

Usage:

go vet [-n] [-x] [build flags] [packages]

Vet runs the Go vet command on the packages named by the import paths.

For more about vet, see 'go doc cmd/vet'. For more about specifying packages, see 'go help packages'.

To run the vet tool with specific options, run 'go tool vet'.

The -n flag prints commands that would be executed. The -x flag prints commands as they are executed.

For more about build flags, see 'go help build'.

See also: go fmt, go fix.

Calling between Go and C

There are two different ways to call between Go and C/C++ code.

The first is the cgo tool, which is part of the Go distribution. For information on how to use it see the cgo documentation (go doc cmd/cgo).

The second is the SWIG program, which is a general tool for interfacing between languages. For information on SWIG see http://swig.org/. When running go build, any file with a .swig extension will be passed to SWIG. Any file with a .swigcxx extension will be passed to SWIG with the -c++ option.

When either cgo or SWIG is used, go build will pass any .c, .m, .s, or .S files to the C compiler, and any .cc, .cpp, .cxx files to the C++ compiler. The CC or CXX environment variables may be set to determine the C or C++ compiler, respectively, to use.

Description of build modes

The 'go build' and 'go install' commands take a -buildmode argument which indicates which kind of object file is to be built. Currently supported values are:

-buildmode=archive
	Build the listed non-main packages into .a files. Packages named
	main are ignored.

-buildmode=c-archive
	Build the listed main package, plus all packages it imports,
	into a C archive file. The only callable symbols will be those
	functions exported using a cgo //export comment. Requires
	exactly one main package to be listed.

-buildmode=c-shared
	Build the listed main packages, plus all packages that they
	import, into C shared libraries. The only callable symbols will
	be those functions exported using a cgo //export comment.
	Non-main packages are ignored.

-buildmode=default
	Listed main packages are built into executables and listed
	non-main packages are built into .a files (the default
	behavior).

-buildmode=shared
	Combine all the listed non-main packages into a single shared
	library that will be used when building with the -linkshared
	option. Packages named main are ignored.

-buildmode=exe
	Build the listed main packages and everything they import into
	executables. Packages not named main are ignored.

-buildmode=pie
	Build the listed main packages and everything they import into
	position independent executables (PIE). Packages not named
	main are ignored.

-buildmode=plugin
	Build the listed main packages, plus all packages that they
	import, into a Go plugin. Packages not named main are ignored.

File types

The go command examines the contents of a restricted set of files in each directory. It identifies which files to examine based on the extension of the file name. These extensions are:

.go
	Go source files.
.c, .h
	C source files.
	If the package uses cgo or SWIG, these will be compiled with the
	OS-native compiler (typically gcc); otherwise they will
	trigger an error.
.cc, .cpp, .cxx, .hh, .hpp, .hxx
	C++ source files. Only useful with cgo or SWIG, and always
	compiled with the OS-native compiler.
.m
	Objective-C source files. Only useful with cgo, and always
	compiled with the OS-native compiler.
.s, .S
	Assembler source files.
	If the package uses cgo or SWIG, these will be assembled with the
	OS-native assembler (typically gcc (sic)); otherwise they
	will be assembled with the Go assembler.
.swig, .swigcxx
	SWIG definition files.
.syso
	System object files.

Files of each of these types except .syso may contain build constraints, but the go command stops scanning for build constraints at the first item in the file that is not a blank line or //-style line comment. See the go/build package documentation for more details.

Non-test Go source files can also include a //go:binary-only-package comment, indicating that the package sources are included for documentation only and must not be used to build the package binary. This enables distribution of Go packages in their compiled form alone. See the go/build package documentation for more details.

GOPATH environment variable

The Go path is used to resolve import statements. It is implemented by and documented in the go/build package.

The GOPATH environment variable lists places to look for Go code. On Unix, the value is a colon-separated string. On Windows, the value is a semicolon-separated string. On Plan 9, the value is a list.

If the environment variable is unset, GOPATH defaults to a subdirectory named "go" in the user's home directory ($HOME/go on Unix, %USERPROFILE%\go on Windows), unless that directory holds a Go distribution. Run "go env GOPATH" to see the current GOPATH.

See https://golang.org/wiki/SettingGOPATH to set a custom GOPATH.

Each directory listed in GOPATH must have a prescribed structure:

The src directory holds source code. The path below src determines the import path or executable name.

The pkg directory holds installed package objects. As in the Go tree, each target operating system and architecture pair has its own subdirectory of pkg (pkg/GOOS_GOARCH).

If DIR is a directory listed in the GOPATH, a package with source in DIR/src/foo/bar can be imported as "foo/bar" and has its compiled form installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a".

The bin directory holds compiled commands. Each command is named for its source directory, but only the final element, not the entire path. That is, the command with source in DIR/src/foo/quux is installed into DIR/bin/quux, not DIR/bin/foo/quux. The "foo/" prefix is stripped so that you can add DIR/bin to your PATH to get at the installed commands. If the GOBIN environment variable is set, commands are installed to the directory it names instead of DIR/bin. GOBIN must be an absolute path.

Here's an example directory layout:

GOPATH=/home/user/go

/home/user/go/
    src/
        foo/
            bar/               (go code in package bar)
                x.go
            quux/              (go code in package main)
                y.go
    bin/
        quux                   (installed command)
    pkg/
        linux_amd64/
            foo/
                bar.a          (installed package object)

Go searches each directory listed in GOPATH to find source code, but new packages are always downloaded into the first directory in the list.

See https://golang.org/doc/code.html for an example.

Internal Directories

Code in or below a directory named "internal" is importable only by code in the directory tree rooted at the parent of "internal". Here's an extended version of the directory layout above:

/home/user/go/
    src/
        crash/
            bang/              (go code in package bang)
                b.go
        foo/                   (go code in package foo)
            f.go
            bar/               (go code in package bar)
                x.go
            internal/
                baz/           (go code in package baz)
                    z.go
            quux/              (go code in package main)
                y.go

The code in z.go is imported as "foo/internal/baz", but that import statement can only appear in source files in the subtree rooted at foo. The source files foo/f.go, foo/bar/x.go, and foo/quux/y.go can all import "foo/internal/baz", but the source file crash/bang/b.go cannot.

See https://golang.org/s/go14internal for details.

Vendor Directories

Go 1.6 includes support for using local copies of external dependencies to satisfy imports of those dependencies, often referred to as vendoring.

Code below a directory named "vendor" is importable only by code in the directory tree rooted at the parent of "vendor", and only using an import path that omits the prefix up to and including the vendor element.

Here's the example from the previous section, but with the "internal" directory renamed to "vendor" and a new foo/vendor/crash/bang directory added:

/home/user/go/
    src/
        crash/
            bang/              (go code in package bang)
                b.go
        foo/                   (go code in package foo)
            f.go
            bar/               (go code in package bar)
                x.go
            vendor/
                crash/
                    bang/      (go code in package bang)
                        b.go
                baz/           (go code in package baz)
                    z.go
            quux/              (go code in package main)
                y.go

The same visibility rules apply as for internal, but the code in z.go is imported as "baz", not as "foo/vendor/baz".

Code in vendor directories deeper in the source tree shadows code in higher directories. Within the subtree rooted at foo, an import of "crash/bang" resolves to "foo/vendor/crash/bang", not the top-level "crash/bang".

Code in vendor directories is not subject to import path checking (see 'go help importpath').

When 'go get' checks out or updates a git repository, it now also updates submodules.

Vendor directories do not affect the placement of new repositories being checked out for the first time by 'go get': those are always placed in the main GOPATH, never in a vendor subtree.

See https://golang.org/s/go15vendor for details.

Environment variables

The go command, and the tools it invokes, examine a few different environment variables. For many of these, you can see the default value of on your system by running 'go env NAME', where NAME is the name of the variable.

General-purpose environment variables:

GCCGO
	The gccgo command to run for 'go build -compiler=gccgo'.
GOARCH
	The architecture, or processor, for which to compile code.
	Examples are amd64, 386, arm, ppc64.
GOBIN
	The directory where 'go install' will install a command.
GOOS
	The operating system for which to compile code.
	Examples are linux, darwin, windows, netbsd.
GOPATH
	For more details see: 'go help gopath'.
GORACE
	Options for the race detector.
	See https://golang.org/doc/articles/race_detector.html.
GOROOT
	The root of the go tree.

Environment variables for use with cgo:

CC
	The command to use to compile C code.
CGO_ENABLED
	Whether the cgo command is supported.  Either 0 or 1.
CGO_CFLAGS
	Flags that cgo will pass to the compiler when compiling
	C code.
CGO_CPPFLAGS
	Flags that cgo will pass to the compiler when compiling
	C or C++ code.
CGO_CXXFLAGS
	Flags that cgo will pass to the compiler when compiling
	C++ code.
CGO_FFLAGS
	Flags that cgo will pass to the compiler when compiling
	Fortran code.
CGO_LDFLAGS
	Flags that cgo will pass to the compiler when linking.
CXX
	The command to use to compile C++ code.
PKG_CONFIG
	Path to pkg-config tool.

Architecture-specific environment variables:

GOARM
	For GOARCH=arm, the ARM architecture for which to compile.
	Valid values are 5, 6, 7.
GO386
	For GOARCH=386, the floating point instruction set.
	Valid values are 387, sse2.

Special-purpose environment variables:

GOROOT_FINAL
	The root of the installed Go tree, when it is
	installed in a location other than where it is built.
	File names in stack traces are rewritten from GOROOT to
	GOROOT_FINAL.
GO_EXTLINK_ENABLED
	Whether the linker should use external linking mode
	when using -linkmode=auto with code that uses cgo.
	Set to 0 to disable external linking mode, 1 to enable it.
GIT_ALLOW_PROTOCOL
	Defined by Git. A colon-separated list of schemes that are allowed to be used
	with git fetch/clone. If set, any scheme not explicitly mentioned will be
	considered insecure by 'go get'.

Import path syntax

An import path (see 'go help packages') denotes a package stored in the local file system. In general, an import path denotes either a standard package (such as "unicode/utf8") or a package found in one of the work spaces (For more details see: 'go help gopath').

Relative import paths

An import path beginning with ./ or ../ is called a relative path. The toolchain supports relative import paths as a shortcut in two ways.

First, a relative path can be used as a shorthand on the command line. If you are working in the directory containing the code imported as "unicode" and want to run the tests for "unicode/utf8", you can type "go test ./utf8" instead of needing to specify the full path. Similarly, in the reverse situation, "go test .." will test "unicode" from the "unicode/utf8" directory. Relative patterns are also allowed, like "go test ./..." to test all subdirectories. See 'go help packages' for details on the pattern syntax.

Second, if you are compiling a Go program not in a work space, you can use a relative path in an import statement in that program to refer to nearby code also not in a work space. This makes it easy to experiment with small multipackage programs outside of the usual work spaces, but such programs cannot be installed with "go install" (there is no work space in which to install them), so they are rebuilt from scratch each time they are built. To avoid ambiguity, Go programs cannot use relative import paths within a work space.

Remote import paths

Certain import paths also describe how to obtain the source code for the package using a revision control system.

A few common code hosting sites have special syntax:

Bitbucket (Git, Mercurial)

	import "bitbucket.org/user/project"
	import "bitbucket.org/user/project/sub/directory"

GitHub (Git)

	import "github.com/user/project"
	import "github.com/user/project/sub/directory"

Launchpad (Bazaar)

	import "launchpad.net/project"
	import "launchpad.net/project/series"
	import "launchpad.net/project/series/sub/directory"

	import "launchpad.net/~user/project/branch"
	import "launchpad.net/~user/project/branch/sub/directory"

IBM DevOps Services (Git)

	import "hub.jazz.net/git/user/project"
	import "hub.jazz.net/git/user/project/sub/directory"

For code hosted on other servers, import paths may either be qualified with the version control type, or the go tool can dynamically fetch the import path over https/http and discover where the code resides from a <meta> tag in the HTML.

To declare the code location, an import path of the form

repository.vcs/path

specifies the given repository, with or without the .vcs suffix, using the named version control system, and then the path inside that repository. The supported version control systems are:

Bazaar      .bzr
Git         .git
Mercurial   .hg
Subversion  .svn

For example,

import "example.org/user/foo.hg"

denotes the root directory of the Mercurial repository at example.org/user/foo or foo.hg, and

import "example.org/repo.git/foo/bar"

denotes the foo/bar directory of the Git repository at example.org/repo or repo.git.

When a version control system supports multiple protocols, each is tried in turn when downloading. For example, a Git download tries https://, then git+ssh://.

By default, downloads are restricted to known secure protocols (e.g. https, ssh). To override this setting for Git downloads, the GIT_ALLOW_PROTOCOL environment variable can be set (For more details see: 'go help environment').

If the import path is not a known code hosting site and also lacks a version control qualifier, the go tool attempts to fetch the import over https/http and looks for a <meta> tag in the document's HTML <head>.

The meta tag has the form:

<meta name="go-import" content="import-prefix vcs repo-root">

The import-prefix is the import path corresponding to the repository root. It must be a prefix or an exact match of the package being fetched with "go get". If it's not an exact match, another http request is made at the prefix to verify the <meta> tags match.

The meta tag should appear as early in the file as possible. In particular, it should appear before any raw JavaScript or CSS, to avoid confusing the go command's restricted parser.

The vcs is one of "git", "hg", "svn", etc,

The repo-root is the root of the version control system containing a scheme and not containing a .vcs qualifier.

For example,

import "example.org/pkg/foo"

will result in the following requests:

https://example.org/pkg/foo?go-get=1 (preferred)
http://example.org/pkg/foo?go-get=1  (fallback, only with -insecure)

If that page contains the meta tag

<meta name="go-import" content="example.org git https://code.org/r/p/exproj">

the go tool will verify that https://example.org/?go-get=1 contains the same meta tag and then git clone https://code.org/r/p/exproj into GOPATH/src/example.org.

New downloaded packages are written to the first directory listed in the GOPATH environment variable (For more details see: 'go help gopath').

The go command attempts to download the version of the package appropriate for the Go release being used. Run 'go help get' for more.

Import path checking

When the custom import path feature described above redirects to a known code hosting site, each of the resulting packages has two possible import paths, using the custom domain or the known hosting site.

A package statement is said to have an "import comment" if it is immediately followed (before the next newline) by a comment of one of these two forms:

package math // import "path"
package math /* import "path" */

The go command will refuse to install a package with an import comment unless it is being referred to by that import path. In this way, import comments let package authors make sure the custom import path is used and not a direct path to the underlying code hosting site.

Import path checking is disabled for code found within vendor trees. This makes it possible to copy code into alternate locations in vendor trees without needing to update import comments.

See https://golang.org/s/go14customimport for details.

Description of package lists

Many commands apply to a set of packages:

go action [packages]

Usually, [packages] is a list of import paths.

An import path that is a rooted path or that begins with a . or .. element is interpreted as a file system path and denotes the package in that directory.

Otherwise, the import path P denotes the package found in the directory DIR/src/P for some DIR listed in the GOPATH environment variable (For more details see: 'go help gopath').

If no import paths are given, the action applies to the package in the current directory.

There are four reserved names for paths that should not be used for packages to be built with the go tool:

- "main" denotes the top-level package in a stand-alone executable.

- "all" expands to all package directories found in all the GOPATH trees. For example, 'go list all' lists all the packages on the local system.

- "std" is like all but expands to just the packages in the standard Go library.

- "cmd" expands to the Go repository's commands and their internal libraries.

Import paths beginning with "cmd/" only match source code in the Go repository.

An import path is a pattern if it includes one or more "..." wildcards, each of which can match any string, including the empty string and strings containing slashes. Such a pattern expands to all package directories found in the GOPATH trees with names matching the patterns. As a special case, x/... matches x as well as x's subdirectories. For example, net/... expands to net and packages in its subdirectories.

An import path can also name a package to be downloaded from a remote repository. Run 'go help importpath' for details.

Every package in a program must have a unique import path. By convention, this is arranged by starting each path with a unique prefix that belongs to you. For example, paths used internally at Google all begin with 'google', and paths denoting remote repositories begin with the path to the code, such as 'github.com/user/repo'.

Packages in a program need not have unique package names, but there are two reserved package names with special meaning. The name main indicates a command, not a library. Commands are built into binaries and cannot be imported. The name documentation indicates documentation for a non-Go program in the directory. Files in package documentation are ignored by the go command.

As a special case, if the package list is a list of .go files from a single directory, the command is applied to a single synthesized package made up of exactly those files, ignoring any build constraints in those files and ignoring any other files in the directory.

Directory and file names that begin with "." or "_" are ignored by the go tool, as are directories named "testdata".

Description of testing flags

The 'go test' command takes both flags that apply to 'go test' itself and flags that apply to the resulting test binary.

Several of the flags control profiling and write an execution profile suitable for "go tool pprof"; run "go tool pprof -h" for more information. The --alloc_space, --alloc_objects, and --show_bytes options of pprof control how the information is presented.

The following flags are recognized by the 'go test' command and control the execution of any test:

-bench regexp
    Run (sub)benchmarks matching a regular expression.
    The given regular expression is split into smaller ones by
    top-level '/', where each must match the corresponding part of a
    benchmark's identifier.
    By default, no benchmarks run. To run all benchmarks,
    use '-bench .' or '-bench=.'.

-benchtime t
    Run enough iterations of each benchmark to take t, specified
    as a time.Duration (for example, -benchtime 1h30s).
    The default is 1 second (1s).

-count n
    Run each test and benchmark n times (default 1).
    If -cpu is set, run n times for each GOMAXPROCS value.
    Examples are always run once.

-cover
    Enable coverage analysis.

-covermode set,count,atomic
    Set the mode for coverage analysis for the package[s]
    being tested. The default is "set" unless -race is enabled,
    in which case it is "atomic".
    The values:
	set: bool: does this statement run?
	count: int: how many times does this statement run?
	atomic: int: count, but correct in multithreaded tests;
		significantly more expensive.
    Sets -cover.

-coverpkg pkg1,pkg2,pkg3
    Apply coverage analysis in each test to the given list of packages.
    The default is for each test to analyze only the package being tested.
    Packages are specified as import paths.
    Sets -cover.

-cpu 1,2,4
    Specify a list of GOMAXPROCS values for which the tests or
    benchmarks should be executed.  The default is the current value
    of GOMAXPROCS.

-parallel n
    Allow parallel execution of test functions that call t.Parallel.
    The value of this flag is the maximum number of tests to run
    simultaneously; by default, it is set to the value of GOMAXPROCS.
    Note that -parallel only applies within a single test binary.
    The 'go test' command may run tests for different packages
    in parallel as well, according to the setting of the -p flag
    (see 'go help build').

-run regexp
    Run only those tests and examples matching the regular expression.
    For tests the regular expression is split into smaller ones by
    top-level '/', where each must match the corresponding part of a
    test's identifier.

-short
    Tell long-running tests to shorten their run time.
    It is off by default but set during all.bash so that installing
    the Go tree can run a sanity check but not spend time running
    exhaustive tests.

-timeout t
    If a test runs longer than t, panic.
    The default is 10 minutes (10m).

-v
    Verbose output: log all tests as they are run. Also print all
    text from Log and Logf calls even if the test succeeds.

The following flags are also recognized by 'go test' and can be used to profile the tests during execution:

-benchmem
    Print memory allocation statistics for benchmarks.

-blockprofile block.out
    Write a goroutine blocking profile to the specified file
    when all tests are complete.
    Writes test binary as -c would.

-blockprofilerate n
    Control the detail provided in goroutine blocking profiles by
    calling runtime.SetBlockProfileRate with n.
    See 'go doc runtime.SetBlockProfileRate'.
    The profiler aims to sample, on average, one blocking event every
    n nanoseconds the program spends blocked.  By default,
    if -test.blockprofile is set without this flag, all blocking events
    are recorded, equivalent to -test.blockprofilerate=1.

-coverprofile cover.out
    Write a coverage profile to the file after all tests have passed.
    Sets -cover.

-cpuprofile cpu.out
    Write a CPU profile to the specified file before exiting.
    Writes test binary as -c would.

-memprofile mem.out
    Write a memory profile to the file after all tests have passed.
    Writes test binary as -c would.

-memprofilerate n
    Enable more precise (and expensive) memory profiles by setting
    runtime.MemProfileRate.  See 'go doc runtime.MemProfileRate'.
    To profile all memory allocations, use -test.memprofilerate=1
    and pass --alloc_space flag to the pprof tool.

-mutexprofile mutex.out
    Write a mutex contention profile to the specified file
    when all tests are complete.
    Writes test binary as -c would.

-mutexprofilefraction n
    Sample 1 in n stack traces of goroutines holding a
    contended mutex.

-outputdir directory
    Place output files from profiling in the specified directory,
    by default the directory in which "go test" is running.

-trace trace.out
    Write an execution trace to the specified file before exiting.

Each of these flags is also recognized with an optional 'test.' prefix, as in -test.v. When invoking the generated test binary (the result of 'go test -c') directly, however, the prefix is mandatory.

The 'go test' command rewrites or removes recognized flags, as appropriate, both before and after the optional package list, before invoking the test binary.

For instance, the command

go test -v -myflag testdata -cpuprofile=prof.out -x

will compile the test binary and then run it as

pkg.test -test.v -myflag testdata -test.cpuprofile=prof.out

(The -x flag is removed because it applies only to the go command's execution, not to the test itself.)

The test flags that generate profiles (other than for coverage) also leave the test binary in pkg.test for use when analyzing the profiles.

When 'go test' runs a test binary, it does so from within the corresponding package's source code directory. Depending on the test, it may be necessary to do the same when invoking a generated test binary directly.

The command-line package list, if present, must appear before any flag not known to the go test command. Continuing the example above, the package list would have to appear before -myflag, but could appear on either side of -v.

To keep an argument for a test binary from being interpreted as a known flag or a package name, use -args (see 'go help test') which passes the remainder of the command line through to the test binary uninterpreted and unaltered.

For instance, the command

go test -v -args -x -v

will compile the test binary and then run it as

pkg.test -test.v -x -v

Similarly,

go test -args math

will compile the test binary and then run it as

pkg.test math

In the first example, the -x and the second -v are passed through to the test binary unchanged and with no effect on the go command itself. In the second example, the argument math is passed through to the test binary, instead of being interpreted as the package list.

Description of testing functions

The 'go test' command expects to find test, benchmark, and example functions in the "*_test.go" files corresponding to the package under test.

A test function is one named TestXXX (where XXX is any alphanumeric string not starting with a lower case letter) and should have the signature,

func TestXXX(t *testing.T) { ... }

A benchmark function is one named BenchmarkXXX and should have the signature,

func BenchmarkXXX(b *testing.B) { ... }

An example function is similar to a test function but, instead of using *testing.T to report success or failure, prints output to os.Stdout. If the last comment in the function starts with "Output:" then the output is compared exactly against the comment (see examples below). If the last comment begins with "Unordered output:" then the output is compared to the comment, however the order of the lines is ignored. An example with no such comment is compiled but not executed. An example with no text after "Output:" is compiled, executed, and expected to produce no output.

Godoc displays the body of ExampleXXX to demonstrate the use of the function, constant, or variable XXX. An example of a method M with receiver type T or *T is named ExampleT_M. There may be multiple examples for a given function, constant, or variable, distinguished by a trailing _xxx, where xxx is a suffix not beginning with an upper case letter.

Here is an example of an example:

func ExamplePrintln() {
	Println("The output of\nthis example.")
	// Output: The output of
	// this example.
}

Here is another example where the ordering of the output is ignored:

func ExamplePerm() {
	for _, value := range Perm(4) {
		fmt.Println(value)
	}

	// Unordered output: 4
	// 2
	// 1
	// 3
	// 0
}

The entire test file is presented as the example when it contains a single example function, at least one other function, type, variable, or constant declaration, and no test or benchmark functions.

See the documentation of the testing package for more information.