package build

import “go/build”

Package build gathers information about Go packages.

Go Path

The Go path is a list of directory trees containing Go source code. It is consulted to resolve imports that cannot be found in the standard Go tree. The default path is the value of the GOPATH environment variable, interpreted as a path list appropriate to the operating system (on Unix, the variable is a colon-separated string; on Windows, a semicolon-separated string; on Plan 9, a list).

Each directory listed in the Go path 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 Go path, 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” (or, for gccgo, “DIR/pkg/gccgo/foo/libbar.a”).

The bin/ directory holds compiled commands. Each command is named for its source directory, but only using 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/ is stripped so that you can add DIR/bin to your PATH to get at the installed commands.

Here’s an example directory layout:


/home/user/gocode/ 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) Build Constraints

A build constraint, also known as a build tag, is a line comment that begins

// +build that lists the conditions under which a file should be included in the package. Constraints may appear in any kind of source file (not just Go), but they must appear near the top of the file, preceded only by blank lines and other line comments. These rules mean that in Go files a build constraint must appear before the package clause.

To distinguish build constraints from package documentation, a series of build constraints must be followed by a blank line.

A build constraint is evaluated as the OR of space-separated options; each option evaluates as the AND of its comma-separated terms; and each term is an alphanumeric word or, preceded by !, its negation. That is, the build constraint:

// +build linux,386 darwin,!cgo corresponds to the boolean formula:

(linux AND 386) OR (darwin AND (NOT cgo)) A file may have multiple build constraints. The overall constraint is the AND of the individual constraints. That is, the build constraints:

// +build linux darwin // +build 386 corresponds to the boolean formula:

(linux OR darwin) AND 386 During a particular build, the following words are satisfied:

  • the target operating system, as spelled by runtime.GOOS

  • the target architecture, as spelled by runtime.GOARCH

  • the compiler being used, either “gc” or “gccgo”

  • “cgo”, if ctxt.CgoEnabled is true

  • “go1.1”, from Go version 1.1 onward

  • “go1.2”, from Go version 1.2 onward

  • “go1.3”, from Go version 1.3 onward

  • “go1.4”, from Go version 1.4 onward

  • “go1.5”, from Go version 1.5 onward

  • “go1.6”, from Go version 1.6 onward

  • “go1.7”, from Go version 1.7 onward

  • “go1.8”, from Go version 1.8 onward

  • any additional words listed in ctxt.BuildTags If a file’s name, after stripping the extension and a possible _test suffix, matches any of the following patterns:

*_GOOS *_GOARCH *_GOOS_GOARCH (example: source_windows_amd64.go) where GOOS and GOARCH represent any known operating system and architecture values respectively, then the file is considered to have an implicit build constraint requiring those terms (in addition to any explicit constraints in the file).

To keep a file from being considered for the build:

// +build ignore (any other unsatisfied word will work as well, but “ignore” is conventional.)

To build a file only when using cgo, and only on Linux and OS X:

// +build linux,cgo darwin,cgo Such a file is usually paired with another file implementing the default functionality for other systems, which in this case would carry the constraint:

// +build !linux,!darwin !cgo Naming a file dns_windows.go will cause it to be included only when building the package for Windows; similarly, math_386.s will be included only when building the package for 32-bit x86.

Using GOOS=android matches build tags and files as for GOOS=linux in addition to android tags and files.

Binary-Only Packages

It is possible to distribute packages in binary form without including the source code used for compiling the package. To do this, the package must be distributed with a source file not excluded by build constraints and containing a “//go:binary-only-package” comment. Like a build constraint, this comment must appear near the top of the file, preceded only by blank lines and other line comments and with a blank line following the comment, to separate it from the package documentation. Unlike build constraints, this comment is only recognized in non-test Go source files.

The minimal source code for a binary-only package is therefore:


package mypkg The source code may include additional Go code. That code is never compiled but will be processed by tools like godoc and might be useful as end-user documentation.

Index Variables func ArchChar(goarch string) (string, error) func IsLocalImport(path string) bool type Context func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Package, error) func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) func (ctxt *Context) MatchFile(dir, name string) (match bool, err error) func (ctxt *Context) SrcDirs() []string type ImportMode type MultiplePackageError func (e *MultiplePackageError) Error() string type NoGoError func (e *NoGoError) Error() string type Package func Import(path, srcDir string, mode ImportMode) (*Package, error) func ImportDir(dir string, mode ImportMode) (*Package, error) func (p *Package) IsCommand() bool Package Files

build.go doc.go read.go syslist.go

Variables ❖ var ToolDir = filepath.Join(runtime.GOROOT(), “pkg/tool/”+runtime.GOOS+”_”+runtime.GOARCH) ToolDir is the directory containing build tools.

func ArchChar ❖ func ArchChar(goarch string) (string, error) ArchChar returns “?” and an error. In earlier versions of Go, the returned string was used to derive the compiler and linker tool names, the default object file suffix, and the default linker output name. As of Go 1.5, those strings no longer vary by architecture; they are compile, link, .o, and a.out, respectively.

func IsLocalImport ❖ func IsLocalImport(path string) bool IsLocalImport reports whether the import path is a local import path, like “.”, “..”, “./foo”, or “../foo”.

type Context ❖ 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

// The build and release tags specify build constraints
// that should be considered satisfied when processing +build lines.
// Clients creating a new context may customize BuildTags, which
// defaults to empty, but it is usually an error to customize ReleaseTags,
// which defaults to the list of Go releases the current release is compatible with.
// In addition to the BuildTags and ReleaseTags, build constraints
// consider the values of GOARCH and GOOS as satisfied tags.
BuildTags   []string
ReleaseTags []string

// The install suffix specifies a suffix to use in the name of the installation
// directory. By default it is empty, but custom builds that need to keep
// their outputs separate can set InstallSuffix to do so. For example, when
// using the race detector, the go command uses InstallSuffix = "race", so
// that on a Linux/386 system, packages are written to a directory named
// "linux_386_race" instead of the usual "linux_386".
InstallSuffix string

// JoinPath joins the sequence of path fragments into a single path.
// If JoinPath is nil, Import uses filepath.Join.
JoinPath func(elem ...string) string

// SplitPathList splits the path list into a slice of individual paths.
// If SplitPathList is nil, Import uses filepath.SplitList.
SplitPathList func(list string) []string

// IsAbsPath reports whether path is an absolute path.
// If IsAbsPath is nil, Import uses filepath.IsAbs.
IsAbsPath func(path string) bool

// IsDir reports whether the path names a directory.
// If IsDir is nil, Import calls os.Stat and uses the result's IsDir method.
IsDir func(path string) bool

// HasSubdir reports whether dir is lexically a subdirectory of
// root, perhaps multiple levels below. It does not try to check
// whether dir exists.
// If so, HasSubdir sets rel to a slash-separated path that
// can be joined to root to produce a path equivalent to dir.
// If HasSubdir is nil, Import uses an implementation built on
// filepath.EvalSymlinks.
HasSubdir func(root, dir string) (rel string, ok bool)

// ReadDir returns a slice of os.FileInfo, sorted by Name,
// describing the content of the named directory.
// If ReadDir is nil, Import uses ioutil.ReadDir.
ReadDir func(dir string) ([]os.FileInfo, error)

// OpenFile opens a file (not a directory) for reading.
// If OpenFile is nil, Import uses os.Open.
OpenFile func(path string) (io.ReadCloser, error)

} A Context specifies the supporting context for a build.

❖ var Default Context = defaultContext() Default is the default Context for builds. It uses the GOARCH, GOOS, GOROOT, and GOPATH environment variables if set, or else the compiled code’s GOARCH, GOOS, and GOROOT.

func (*Context) Import

❖ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Package, error) Import returns details about the Go package named by the import path, interpreting local import paths relative to the srcDir directory. If the path is a local import path naming a package that can be imported using a standard import path, the returned package will set p.ImportPath to that path.

In the directory containing the package, .go, .c, .h, and .s files are considered part of the package except for:

  • .go files in package documentation

  • files starting with _ or . (likely editor temporary files)

  • files with build constraints not satisfied by the context If an error occurs, Import returns a non-nil error and a non-nil *Package containing partial information.

func (*Context) ImportDir

❖ func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) ImportDir is like Import but processes the Go package found in the named directory.

func (*Context) MatchFile

❖ func (ctxt *Context) MatchFile(dir, name string) (match bool, err error) MatchFile reports whether the file with the given name in the given directory matches the context and would be included in a Package created by ImportDir of that directory.

MatchFile considers the name of the file and may use ctxt.OpenFile to read some or all of the file’s content.

func (*Context) SrcDirs

❖ func (ctxt *Context) SrcDirs() []string SrcDirs returns a list of package source root directories. It draws from the current Go root and Go path but omits directories that do not exist.

type ImportMode ❖ type ImportMode uint An ImportMode controls the behavior of the Import method.

❖ const ( // If FindOnly is set, Import stops after locating the directory // that should contain the sources for a package. It does not // read any files in the directory. FindOnly ImportMode = 1 << iota

// If AllowBinary is set, Import can be satisfied by a compiled
// package object without corresponding sources.
// Deprecated:
// The supported way to create a compiled-only package is to
// write source code containing a //go:binary-only-package comment at
// the top of the file. Such a package will be recognized
// regardless of this flag setting (because it has source code)
// and will have BinaryOnly set to true in the returned Package.

// If ImportComment is set, parse import comments on package statements.
// Import returns an error if it finds a comment it cannot understand
// or finds conflicting comments in multiple source files.
// See for more information.

// By default, Import searches vendor directories
// that apply in the given source directory before searching
// the GOROOT and GOPATH roots.
// If an Import finds and returns a package using a vendor
// directory, the resulting ImportPath is the complete path
// to the package, including the path elements leading up
// to and including "vendor".
// For example, if Import("y", "x/subdir", 0) finds
// "x/vendor/y", the returned package's ImportPath is "x/vendor/y",
// not plain "y".
// See for more information.
// Setting IgnoreVendor ignores vendor directories.
// In contrast to the package's ImportPath,
// the returned package's Imports, TestImports, and XTestImports
// are always the exact import paths from the source files:
// Import makes no attempt to resolve or check those paths.

) type MultiplePackageError ❖ type MultiplePackageError struct { Dir string // directory containing files Packages []string // package names found Files []string // corresponding files: Files[i] declares package Packages[i] } MultiplePackageError describes a directory containing multiple buildable Go source files for multiple packages.

func (*MultiplePackageError) Error

❖ func (e *MultiplePackageError) Error() string type NoGoError ❖ type NoGoError struct { Dir string } NoGoError is the error used by Import to describe a directory containing no buildable Go source files. (It may still contain test files, files hidden by build tags, and so on.)

func (*NoGoError) Error

❖ func (e *NoGoError) Error() string type Package ❖ type Package struct { Dir string // directory containing package sources Name string // package name ImportComment string // path in import comment on package statement Doc string // documentation synopsis ImportPath string // import path of package (“” if unknown) Root string // root of Go tree where this package lives SrcRoot string // package source root directory (“” if unknown) PkgRoot string // package install root directory (“” if unknown) PkgTargetRoot string // architecture dependent install root directory (“” if unknown) BinDir string // command install directory (“” if unknown) Goroot bool // package found in Go root PkgObj string // installed .a file AllTags []string // tags that can influence file selection in this directory ConflictDir string // this directory shadows Dir in $GOPATH BinaryOnly bool // cannot be rebuilt from source (has //go:binary-only-package comment)

// Source files
GoFiles        []string // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
CgoFiles       []string // .go source files that import "C"
IgnoredGoFiles []string // .go source files ignored for this build
InvalidGoFiles []string // .go source files with detected problems (parse error, wrong package name, and so on)
CFiles         []string // .c source files
CXXFiles       []string // .cc, .cpp and .cxx source files
MFiles         []string // .m (Objective-C) 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 system object files to add to archive

// Cgo directives
CgoCFLAGS    []string // Cgo CFLAGS directives
CgoCPPFLAGS  []string // Cgo CPPFLAGS directives
CgoCXXFLAGS  []string // Cgo CXXFLAGS directives
CgoFFLAGS    []string // Cgo FFLAGS directives
CgoLDFLAGS   []string // Cgo LDFLAGS directives
CgoPkgConfig []string // Cgo pkg-config directives

// Dependency information
Imports   []string                    // import paths from GoFiles, CgoFiles
ImportPos map[string][]token.Position // line information for Imports

// Test information
TestGoFiles    []string                    // _test.go files in package
TestImports    []string                    // import paths from TestGoFiles
TestImportPos  map[string][]token.Position // line information for TestImports
XTestGoFiles   []string                    // _test.go files outside package
XTestImports   []string                    // import paths from XTestGoFiles
XTestImportPos map[string][]token.Position // line information for XTestImports

} A Package describes the Go package found in a directory.

func Import

❖ func Import(path, srcDir string, mode ImportMode) (*Package, error) Import is shorthand for Default.Import.

func ImportDir

❖ func ImportDir(dir string, mode ImportMode) (*Package, error) ImportDir is shorthand for Default.ImportDir.

func (*Package) IsCommand

❖ func (p *Package) IsCommand() bool IsCommand reports whether the package is considered a command to be installed (not just a library). Packages named “main” are treated as commands.