1 // Copyright 2011 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // Package build gathers information about Go packages.
9 // The Go path is a list of directory trees containing Go source code.
10 // It is consulted to resolve imports that cannot be found in the standard
11 // Go tree. The default path is the value of the GOPATH environment
12 // variable, interpreted as a path list appropriate to the operating system
13 // (on Unix, the variable is a colon-separated string;
14 // on Windows, a semicolon-separated string;
15 // on Plan 9, a list).
17 // Each directory listed in the Go path must have a prescribed structure:
19 // The src/ directory holds source code. The path below 'src' determines
20 // the import path or executable name.
22 // The pkg/ directory holds installed package objects.
23 // As in the Go tree, each target operating system and
24 // architecture pair has its own subdirectory of pkg
27 // If DIR is a directory listed in the Go path, a package with
28 // source in DIR/src/foo/bar can be imported as "foo/bar" and
29 // has its compiled form installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a"
30 // (or, for gccgo, "DIR/pkg/gccgo/foo/libbar.a").
32 // The bin/ directory holds compiled commands.
33 // Each command is named for its source directory, but only
34 // using the final element, not the entire path. That is, the
35 // command with source in DIR/src/foo/quux is installed into
36 // DIR/bin/quux, not DIR/bin/foo/quux. The foo/ is stripped
37 // so that you can add DIR/bin to your PATH to get at the
38 // installed commands.
40 // Here's an example directory layout:
42 // GOPATH=/home/user/gocode
47 // bar/ (go code in package bar)
49 // quux/ (go code in package main)
52 // quux (installed command)
56 // bar.a (installed package object)
60 // A build constraint, also known as a build tag, is a line comment that begins
64 // that lists the conditions under which a file should be included in the package.
65 // Constraints may appear in any kind of source file (not just Go), but
66 // they must appear near the top of the file, preceded
67 // only by blank lines and other line comments. These rules mean that in Go
68 // files a build constraint must appear before the package clause.
70 // To distinguish build constraints from package documentation, a series of
71 // build constraints must be followed by a blank line.
73 // A build constraint is evaluated as the OR of space-separated options;
74 // each option evaluates as the AND of its comma-separated terms;
75 // and each term is an alphanumeric word or, preceded by !, its negation.
76 // That is, the build constraint:
78 // // +build linux,386 darwin,!cgo
80 // corresponds to the boolean formula:
82 // (linux AND 386) OR (darwin AND (NOT cgo))
84 // A file may have multiple build constraints. The overall constraint is the AND
85 // of the individual constraints. That is, the build constraints:
87 // // +build linux darwin
90 // corresponds to the boolean formula:
92 // (linux OR darwin) AND 386
94 // During a particular build, the following words are satisfied:
96 // - the target operating system, as spelled by runtime.GOOS
97 // - the target architecture, as spelled by runtime.GOARCH
98 // - the compiler being used, either "gc" or "gccgo"
99 // - "cgo", if ctxt.CgoEnabled is true
100 // - "go1.1", from Go version 1.1 onward
101 // - "go1.2", from Go version 1.2 onward
102 // - any additional words listed in ctxt.BuildTags
104 // If a file's name, after stripping the extension and a possible _test suffix,
105 // matches any of the following patterns:
109 // (example: source_windows_amd64.go) or the literals:
112 // (example: windows.go) where GOOS and GOARCH represent any known operating
113 // system and architecture values respectively, then the file is considered to
114 // have an implicit build constraint requiring those terms.
116 // To keep a file from being considered for the build:
120 // (any other unsatisfied word will work as well, but ``ignore'' is conventional.)
122 // To build a file only when using cgo, and only on Linux and OS X:
124 // // +build linux,cgo darwin,cgo
126 // Such a file is usually paired with another file implementing the
127 // default functionality for other systems, which in this case would
128 // carry the constraint:
130 // // +build !linux,!darwin !cgo
132 // Naming a file dns_windows.go will cause it to be included only when
133 // building the package for Windows; similarly, math_386.s will be included
134 // only when building the package for 32-bit x86.