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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package help
import "cmd/go/internal/base"
var HelpC = &base.Command{
UsageLine: "c",
Short: "calling between Go and C",
Long: `
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 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.
var HelpPackages = &base.Command{
UsageLine: "packages",
Short: "package lists and patterns",
Long: `
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 packages found in all the GOPATH
trees. For example, 'go list all' lists all the packages on the local
system. When using modules, "all" expands to all packages in
the main module and their dependencies, including dependencies
needed by tests of any of those.
- "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
To make common patterns more convenient, there are two special cases.
First, /... at the end of the pattern can match an empty string,
so that net/... matches both net and packages in its subdirectories, like net/http.
Second, any slash-separated pattern element containing a wildcard never
participates in a match of the "vendor" element in the path of a vendored
package, so that ./... does not match packages in subdirectories of
./vendor or ./mycode/vendor, but ./vendor/... and ./mycode/vendor/... do.
Note, however, that a directory named vendor that itself contains code
is not a vendored package: cmd/vendor would be a command named vendor,
and the pattern cmd/... matches it.
See for more about vendoring.
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 ''.
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".
var HelpImportPath = &base.Command{
UsageLine: "importpath",
Short: "import path syntax",
Long: `
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 ""
import ""
GitHub (Git)
import ""
import ""
Launchpad (Bazaar)
import ""
import ""
import ""
import ""
import ""
IBM DevOps Services (Git)
import ""
import ""
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
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
Fossil .fossil
Git .git
Mercurial .hg
Subversion .svn
For example,
import ""
denotes the root directory of the Mercurial repository at or foo.hg, and
import ""
denotes the foo/bar directory of the Git repository at 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
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 "bzr", "fossil", "git", "hg", "svn".
The repo-root is the root of the version control system
containing a scheme and not containing a .vcs qualifier.
For example,
import ""
will result in the following requests: (preferred) (fallback, only with -insecure)
If that page contains the meta tag
<meta name="go-import" content=" git">
the go tool will verify that contains the
same meta tag and then git clone into
When using GOPATH, downloaded packages are written to the first directory
listed in the GOPATH environment variable.
(See 'go help gopath-get' and 'go help gopath'.)
When using modules, downloaded packages are stored in the module cache.
(See 'go help modules-get' and 'go help goproxy'.)
When using modules, an additional variant of the go-import meta tag is
recognized and is preferred over those listing version control systems.
That variant uses "mod" as the vcs in the content value, as in:
<meta name="go-import" content=" mod">
This tag means to fetch modules with paths beginning with
from the module proxy available at the URL
See 'go help goproxy' for details about the proxy protocol.
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.
Import path checking is also disabled when using modules.
Import path comments are obsoleted by the go.mod file's module statement.
See for details.
var HelpGopath = &base.Command{
UsageLine: "gopath",
Short: "GOPATH environment variable",
Long: `
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 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
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:
bar/ (go code in package bar)
quux/ (go code in package main)
quux (installed command)
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 for an example.
GOPATH and Modules
When using modules, GOPATH is no longer used for resolving imports.
However, it is still used to store downloaded source code (in GOPATH/src/mod)
and compiled commands (in GOPATH/bin).
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:
bang/ (go code in package bang)
foo/ (go code in package foo)
bar/ (go code in package bar)
baz/ (go code in package baz)
quux/ (go code in package main)
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 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:
bang/ (go code in package bang)
foo/ (go code in package foo)
bar/ (go code in package bar)
bang/ (go code in package bang)
baz/ (go code in package baz)
quux/ (go code in package main)
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 for details.
var HelpEnvironment = &base.Command{
UsageLine: "environment",
Short: "environment variables",
Long: `
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:
The gccgo command to run for 'go build -compiler=gccgo'.
The architecture, or processor, for which to compile code.
Examples are amd64, 386, arm, ppc64.
The directory where 'go install' will install a command.
The directory where the go command will store cached
information for reuse in future builds.
The operating system for which to compile code.
Examples are linux, darwin, windows, netbsd.
For more details see: 'go help gopath'.
URL of Go module proxy. See 'go help goproxy'.
Options for the race detector.
The root of the go tree.
The directory where the go command will write
temporary source files, packages, and binaries.
The directory where the go tools (compile, cover, doc, etc...)
are installed. This is printed by go env, but setting the
environment variable has no effect.
Environment variables for use with cgo:
The command to use to compile C code.
Whether the cgo command is supported. Either 0 or 1.
Flags that cgo will pass to the compiler when compiling
C code.
A regular expression specifying additional flags to allow
to appear in #cgo CFLAGS source code directives.
Does not apply to the CGO_CFLAGS environment variable.
A regular expression specifying flags that must be disallowed
from appearing in #cgo CFLAGS source code directives.
Does not apply to the CGO_CFLAGS environment variable.
but for the C preprocessor.
but for the C++ compiler.
but for the Fortran compiler.
but for the linker.
The command to use to compile C++ code.
Path to pkg-config tool.
Architecture-specific environment variables:
For GOARCH=arm, the ARM architecture for which to compile.
Valid values are 5, 6, 7.
For GOARCH=386, the floating point instruction set.
Valid values are 387, sse2.
For GOARCH=mips{,le}, whether to use floating point instructions.
Valid values are hardfloat (default), softfloat.
For GOARCH=mips64{,le}, whether to use floating point instructions.
Valid values are hardfloat (default), softfloat.
Special-purpose environment variables:
If set, where to find gccgo tools, such as cgo.
The default is based on how gccgo was configured.
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
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.
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'.
var HelpFileType = &base.Command{
UsageLine: "filetype",
Short: "file types",
Long: `
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 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.
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.
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. Even binary-only packages require
accurate import blocks listing required dependencies, so that
those dependencies can be supplied when linking the resulting
var HelpBuildmode = &base.Command{
UsageLine: "buildmode",
Short: "build modes",
Long: `
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
Build the listed non-main packages into .a files. Packages named
main are ignored.
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.
Build the listed main package, plus all packages it imports,
into a C shared library. The only callable symbols will
be those functions exported using a cgo //export comment.
Requires exactly one main package to be listed.
Listed main packages are built into executables and listed
non-main packages are built into .a files (the default
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.
Build the listed main packages and everything they import into
executables. Packages not named main are ignored.
Build the listed main packages and everything they import into
position independent executables (PIE). Packages not named
main are ignored.
Build the listed main packages, plus all packages that they
import, into a Go plugin. Packages not named main are ignored.
var HelpCache = &base.Command{
UsageLine: "cache",
Short: "build and test caching",
Long: `
The go command caches build outputs for reuse in future builds.
The default location for cache data is a subdirectory named go-build
in the standard user cache directory for the current operating system.
Setting the GOCACHE environment variable overrides this default,
and running 'go env GOCACHE' prints the current cache directory.
You can set the variable to 'off' to disable the cache.
The go command periodically deletes cached data that has not been
used recently. Running 'go clean -cache' deletes all cached data.
The build cache correctly accounts for changes to Go source files,
compilers, compiler options, and so on: cleaning the cache explicitly
should not be necessary in typical use. However, the build cache
does not detect changes to C libraries imported with cgo.
If you have made changes to the C libraries on your system, you
will need to clean the cache explicitly or else use the -a build flag
(see 'go help build') to force rebuilding of packages that
depend on the updated C libraries.
The go command also caches successful package test results.
See 'go help test' for details. Running 'go clean -testcache' removes
all cached test results (but not cached build results).
The GODEBUG environment variable can enable printing of debugging
information about the state of the cache:
GODEBUG=gocacheverify=1 causes the go command to bypass the
use of any cache entries and instead rebuild everything and check
that the results match existing cache entries.
GODEBUG=gocachehash=1 causes the go command to print the inputs
for all of the content hashes it uses to construct cache lookup keys.
The output is voluminous but can be useful for debugging the cache.
GODEBUG=gocachetest=1 causes the go command to print details of its
decisions about whether to reuse a cached test result.