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layout: documentation
title: Functions
---
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<h1>Functions</h1>

#if (!$singlePage)
<div class="toc">
  <h1>Contents</h1>
  <ul>
    <li><a href="#package">package</a></li>
    <li><a href="#package_group">package_group</a></li>
    <li><a href="#exports_files">exports_files</a></li>
    <li><a href="#glob">glob</a></li>
    <li><a href="#select">select</a></li>
  </ul>
</div>
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<!-- =================================================================
                              package()
     =================================================================
-->

<h2 id="package">package</h2>

<pre>
package(default_deprecation, default_testonly, default_visibility, features)
</pre>
<p>This function declares metadata that applies to every subsequent rule in the
package. It is used at most once within a package (BUILD file).</p>

<p>The package() function should be called right after all the load() statements at the top of the
file, before any rule.</p>

<h3 id="package_args">Arguments</h3>

<table class="table table-condensed table-bordered table-params">
  <colgroup>
    <col class="col-param" />
    <col class="param-description" />
  </colgroup>
  <thead>
    <tr>
      <th>Attribute</th>
      <th>Description</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td id="package.default_visibility"><code>default_visibility</code></td>
      <td>
        <p><code>List of <a href="../build-ref.html#labels">labels</a>; optional</code></p>
        <p>The default visibility of the rules in this package.</p>
        <p>Every rule in this package has the visibility specified in this
        attribute, unless otherwise specified in the <code>visibility</code>
        attribute of the rule. For detailed information about the syntax of this
        attribute, see the documentation of the
        <a href="common-definitions.html#common.visibility">visibility</a>
        attribute. The package default visibility does not apply to
        <a href="#exports_files">exports_files</a>, which is
        public by default.</p>
      </td>
    </tr>
    <tr>
      <td id="package.default_deprecation"><code>default_deprecation</code></td>
      <td>
        <p><code>String; optional</code></p>
        <p>Sets the default <a href="common-definitions.html#common.deprecation">
        <code>deprecation</code></a> message for all rules in this package.</p>
      </td>
    </tr>
    <tr>
      <td id="package.default_testonly"><code>default_testonly</code></td>
      <td>
        <p><code>Boolean; optional; default is 0 except as noted</code></p>
        <p>Sets the default <a href="common-definitions.html#common.testonly">
        <code>testonly</code></a> property for all rules in this package.</p>
        <p>In packages under <code>javatests</code> the default value is 1.</p>
      </td>
    </tr>
    <tr>
      <td id="package.features"><code>features</code></td>
      <td>
        <p><code>List strings; optional</code></p>
        <p>Sets various flags that affect the semantics of this BUILD file.</p>
        <p>This feature is mainly used by the people working on the build system to
        tag packages that need some kind of special handling. Do not use this unless
        explicitly requested by someone working on the build system.</p>

      </td>
    </tr>

  </tbody>
</table>

<h3 id="package_example">Examples</h3>

The declaration below declares that the rules in this package are
visible only to members of package
group <code>//foo:target</code>. Individual visibility declarations
on a rule, if present, override this specification.

<pre class="code">
package(default_visibility = ["//foo:target"])
</pre>

<!-- =================================================================
                              package_group()
     =================================================================
-->

<h2 id="package_group">package_group</h2>

<pre>package_group(name, packages, includes)</pre>

<p>This function defines a set of packages and assigns a label to the
group. The label can be referenced in <code>visibility</code> attributes.</p>

<p>Package groups are used for visibility control.  You can grant access to a rule
to one or more package groups, every rule in the entire source tree, or only to rules declared
in the same package. For more detailed description of the visibility system, see
the <a href="common-definitions.html#common.visibility">visibility</a> attribute.

<h3 id="package_group_args">Arguments</h3>

<table class="table table-condensed table-bordered table-params">
  <colgroup>
    <col class="col-param" />
    <col class="param-description" />
  </colgroup>
  <thead>
    <tr>
      <th>Attribute</th>
      <th>Description</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td id="package_group.name"><code>name</code></td>
      <td>
        <p><code><a href="../build-ref.html#name">Name</a>; required</code></p>
        <p>A unique name for this target.</p>
      </td>
    </tr>
    <tr>
      <td id="package_group.packages"><code>packages</code></td>
      <td>
        <p><code>List of <a href="../build-ref.html#packages">Package</a>; optional</code></p>
        <p>A complete enumeration of packages in this group.</p>

        <p>Packages should be referred to using their full names,
        starting with a double slash. For
        example, <code>//foo/bar/main</code> is a valid element
        of this list.</p>

        <p>You can also specify wildcards: the specification
        <code>//foo/...</code> specifies every package under
        <code>//foo</code>, including <code>//foo</code> itself.</p>

        <p>Package specifications can be prefixed with <code>-</code> to
        indicate negation: the specification <code>-//foo/bar/...</code>
        excludes all packages under <code>//foo/bar</code> that would
        otherwise have been matched by the package patterns in the
        current <code>package_group</code>. When used together with
        <code>includes</code>, the set of packages for each package group
        is computed and then the results are unioned: negative patterns
        in one package group do not affect the result of included package
        groups.

        <p>If this attribute is missing, the package group itself will contain
        no packages (but it can still include other package groups).</p>
      </td>
    </tr>
    <tr>
      <td id="package_group.includes"><code>includes</code></td>
      <td>
        <p><code>List of <a href="../build-ref.html#labels">labels</a>; optional</code></p>
        <p>Other package groups that are included in this one.</p>

        <p>The labels in this attribute must refer to other package
        groups. Packages in referenced package groups are taken to be part
        of this package group. This is transitive, that is, if package
        group <code>a</code> contains package group <code>b</code>,
        and <code>b</code> contains package group <code>c</code>, every
        package in <code>c</code> will also be a member of <code>a</code>.</p>
      </td>
    </tr>
  </tbody>
</table>

<h3 id="package_group_example">Examples</h3>

<p>The following <code>package_group</code> declaration specifies a
package group called "tropical" that contains tropical fruits.</p>

<pre class="code">
package_group(
    name = "tropical",
    packages = [
        "//fruits/mango",
        "//fruits/orange",
        "//fruits/papaya/...",
    ],
)
</pre>

<p>The following declarations specify the package groups of a fictional
application:</p>

<pre class="code">
package_group(
    name = "fooapp",
    includes = [
        ":controller",
        ":model",
        ":view",
    ],
)

package_group(
    name = "model",
    packages = ["//fooapp/database"],
)

package_group(
    name = "view",
    packages = [
        "//fooapp/swingui",
        "//fooapp/webui",
    ],
)

package_group(
    name = "controller",
    packages = ["//fooapp/algorithm"],
)
</pre>

<!-- =================================================================
                        exports_files([label, ...])
     =================================================================
  -->

<h2 id="exports_files">exports_files</h2>

<pre>exports_files([<i>label</i>, ...], visibility, licenses)</pre>

<p>
  <code>exports_files()</code> specifies a list of files belonging to
  this package that are exported to other packages.
</p>

<p>
  The BUILD file for a package may only refer directly to source files belonging
  to another package if they are explicitly exported with an
  <pre>exports_files()</pre> statement.
  As a legacy behaviour, also files mentioned as input to a rule are exported
  with the default visibility until the flag
  <a href="https://github.com/bazelbuild/bazel/issues/10225"><code>--incompatible_no_implicit_file_export</code></a>
  is flipped. However, this behavior should not be relied upon and actively
  migrated away from.
</p>

<h3 id="exports_files_args">Arguments</h3>

<p>
  The argument is a list of names of files within the current package. A
  visibility declaration can also be specified; in this case, the files will be
  visible to the targets specified. If no visibility is specified, the files
  will be visible to every package, even if a package default visibility was
  specified in the <code><a href="functions.html#package">package</a></code>
  function. The <a href="common-definitions.html#common.licenses">licenses</a>
  can also be specified.
</p>

<h3 id="exports_files_example">Example</h3>

<p>
  The following example exports <code>golden.txt</code>, a
  text file from the <code>test_data</code> package, so that other
  packages may use it, for example, in the <code>data</code> attribute
  of tests.
</p>

<pre class="code">
# from //test_data/BUILD

exports_files(["golden.txt"])
</pre>

<!-- =================================================================
                               glob()
     =================================================================
  -->

<h2 id="glob">glob</h2>

<pre>glob(include, exclude=[], exclude_directories=1, allow_empty=True)</pre>

<p>
Glob is a helper function that finds all files that match certain path patterns,
and returns a list of their paths. Glob only searches files in its own package,
and looks only for source files (not generated files nor other targets).
</p>

<p>
A source file's Label is included in the result if the file's package-relative
path matches any of the <code>include</code> patterns and none of the
<code>exclude</code> patterns.
</p>

<p>
The <code>include</code> and <code>exclude</code> lists contain path patterns
that are relative to the current package. Every pattern may consist of one or
more path segments. As usual with Unix paths, these segments are separated by
<code>/</code>. Segments may contain the <code>*</code> wildcard: this matches
any substring in the path segment (even the empty substring), excluding the
directory separator <code>/</code>. This wildcard can be used multiple times
within one path segment. Additionally, the <code>**</code> wildcard can match
zero or more complete path segments, but it must be declared as a standalone
path segment.
</p>

Examples:
<ul>
<li><code>foo/bar.txt</code> matches exactly the <code>foo/bar.txt</code> file
in this package</li>
<li><code>foo/*.txt</code> matches every file in the <code>foo/</code> directory
if the file ends with
<code>.txt</code> (unless <code>foo/</code> is a subpackage)</li>
<li><code>foo/a*.htm*</code> matches every file in the <code>foo/</code>
directory that starts with <code>a</code>, then has an arbitrary string (could
be empty), then has <code>.htm</code>, and ends with another arbitrary string;
such as <code>foo/axx.htm</code> and <code>foo/a.html</code> or
<code>foo/axxx.html</code></li>
<li><code>**/a.txt</code> matches every <code>a.txt</code> file in every
subdirectory of this package</li>
<li><code>**/bar/**/*.txt</code> matches every <code>.txt</code> file in every
subdirectory of this package, if at least one directory on the resulting path is
called <code>bar</code>, such as <code>xxx/bar/yyy/zzz/a.txt</code> or
<code>bar/a.txt</code> (remember that <code>**</code> also matches zero
segments) or <code>bar/zzz/a.txt</code></li>
<li><code>**</code> matches every file in every subdirectory of this
package</li>
<li><code>foo**/a.txt</code> is an invalid pattern, because <code>**</code> must
stand on its own as a segment</li>
</ul>

<p>
If the <code>exclude_directories</code> argument is enabled (set to 1), files of
type directory will be omitted from the results (default 1).
</p>
<p>
If the <code>allow_empty</code> argument is set to <code>False</code>, the
<code>glob</code> function will error-out if the result would otherwise be the
empty list.
</p>
<p>
There are several important limitations and caveats:
</p>

<ol>
  <li>
    <p>
    Since <code>glob()</code> runs during BUILD file evaluation,
    <code>glob()</code> matches files only in your source tree, never
    generated files.  If you are building a target that requires both
    source and generated files, you must append an explicit list of generated
    files to the glob. See the <a href="#glob_example">example</a>
    below with <code>:mylib</code> and <code>:gen_java_srcs</code>.
    </p>
  </li>

  <li>
    <p>
      If a rule has the same name as a matched source file, the rule will
      "shadow" the file.
    </p>
    <p>
      To understand this, remember that <code>glob()</code> returns a list of
      paths, so using <code>glob()</code> in other rules' attribute (e.g.
      <code>srcs = glob(["*.cc"])</code>) has the same effect as listing the
      matched paths explicitly.  If for example <code>glob()</code> yields
      <code>["Foo.java", "bar/Baz.java"]</code> but there's also a rule in the
      package called "Foo.java" (which is allowed, though Bazel warns about it),
      then the consumer of the <code>glob()</code> will use the "Foo.java" rule
      (its outputs) instead of the "Foo.java" file. See
      <a href="https://github.com/bazelbuild/bazel/issues/10395#issuecomment-583714657">GitHub
      issue #10395</a> for more details.
    </p>
  </li>

  <li>
    Globs may match files in subdirectories.  And subdirectory names
    may be wildcarded.  However...
  </li>

  <li>
    <p>
    Labels are not allowed to cross the package boundary and glob does
    not match files in subpackages.
    </p>

    <p>
    For example, the glob expression <code>**/*.cc</code> in package
    <code>x</code> does not include <code>x/y/z.cc</code> if
    <code>x/y</code> exists as a package (either as
    <code>x/y/BUILD</code>, or somewhere else on the package-path). This
    means that the result of the glob expression actually depends on the
    existence of BUILD files &mdash; that is, the same glob expression would
    include <code>x/y/z.cc</code> if there was no package called
    <code>x/y</code> or it was marked as deleted using the
    <a href="../user-manual.html#flag--deleted_packages">--deleted_packages</a>
    flag.
    </p>

  </li>

  <li>
    The restriction above applies to all glob expressions,
    no matter which wildcards they use.
  </li>
  <li>
    A hidden file with filename starting with <code>.</code> is completely matched by
    both the <code>**</code> and the <code>*</code> wildcards. If you want to match a hidden file
    with a compound pattern, your pattern needs to begin with a <code>.</code>. For example,
    <code>*</code> and <code>.*.txt</code> will match <code>.foo.txt</code>, but <code>*.txt</code>
    will not.

    Hidden directories are also matched in the same manner. Hidden directories
    may include files that are not required as inputs, and can increase the
    number of unnecessarily globbed files and memory consumption. To exclude
    hidden directories, add them to the "exclude" list argument.
  </li>

  <li>
    The "**" wildcard has one corner case: the pattern
    <code>"**"</code> doesn't match the package's directory path. That is to
    say, <code>glob(["**"], exclude_directories = 0)</code> matches all files
    and directories transitively strictly under the current package's directory
    (but of course not going into directories of subpackages - see the previous
    note about that).
  </li>
</ol>

<p>
In general, you should <b>try to provide an appropriate extension (e.g. *.html)
instead of using a bare '*'</b> for a glob pattern. The more explicit name
is both self documenting and ensures that you don't accidentally match backup
files, or emacs/vi/... auto-save files.
</p>

<p>
When writing build rules you can enumerate the elements of the glob. This
enables generating individual rules for every input, for example. See the
<a href="#expanded_glob_example">expanded glob example</a> section below.
</p>

<h3 id="glob_example">Glob Examples</h3>

<p> Create a Java library built from all java files in this directory,
and all files generated by the <code>:gen_java_srcs</code> rule.</p>
<pre class="code">
java_library(
    name = "mylib",
    srcs = glob(["*.java"]) + [":gen_java_srcs"],
    deps = "...",
)

genrule(
    name = "gen_java_srcs",
    outs = [
        "Foo.java",
        "Bar.java",
    ],
    ...
)
</pre>

<p>Include all txt files in directory testdata except experimental.txt.
Note that files in subdirectories of testdata will not be included. If
you want those files to be included, use a recursive glob (**).</p>
<pre class="code">
sh_test(
    name = "mytest",
    srcs = ["mytest.sh"],
    data = glob(
        ["testdata/*.txt"],
        exclude = ["testdata/experimental.txt"],
    ),
)
</pre>

<h3 id="recursive_glob_example">Recursive Glob Examples</h3>

<p>Make the test depend on all txt files in the testdata directory and any
   of its subdirectories (and their subdirectories, and so on).
   Subdirectories containing a BUILD file are ignored. (See limitations
   and caveats above.)</p>
<pre class="code">
sh_test(
    name = "mytest",
    srcs = ["mytest.sh"],
    data = glob(["testdata/**/*.txt"]),
)
</pre>

<p>Create a library built from all java files in this directory and all
subdirectories except those whose path includes a directory named testing.
<b>This pattern should be avoided if possible, as it can reduce build
incrementality and therefore increase build times.</b>
</p>
<pre class="code">
java_library(
    name = "mylib",
    srcs = glob(
        ["**/*.java"],
        exclude = ["**/testing/**"],
    ),
)
</pre>

<h3 id="expanded_glob_example">Expanded Glob Examples</h3>

<p>
Create an individual genrule for *_test.cc in the current directory
that counts the number of lines in the file.
</p>

<pre class="code">
# Conveniently, the build language supports list comprehensions.
[genrule(
    name = "count_lines_" + f[:-3],  # strip ".cc"
    srcs = [f],
    outs = ["%s-linecount.txt" % f[:-3]],
    cmd = "wc -l $&lt; &gt;$@",
 ) for f in glob(["*_test.cc"])]
</pre>

<p>
If the BUILD file above is in package //foo and the package contains three
matching files, a_test.cc, b_test.cc and c_test.cc then running
<code>bazel query '//foo:all'</code> will list all rules that were generated:

<pre>
$ bazel query '//foo:all' | sort
//foo:count_lines_a_test
//foo:count_lines_b_test
//foo:count_lines_c_test
</pre>

<!-- =================================================================
                              select()
     =================================================================
-->

<h2 id="select">select</h2>

<pre>
select(
    {conditionA: valuesA, conditionB: valuesB, ...},
    no_match_error = "custom message"
)
</pre>

<p><code>select()</code> is the helper function that makes a rule attribute
  <a href="common-definitions.html#configurable-attributes">configurable</a>.
  It can replace the right-hand side of

  <i>almost</i>
  any attribute assignment so its value depends on command-line Bazel flags.
  This can be used, for example, to define platform-specific dependencies or to
  embed different resources depending on whether a rule is built in "developer"
  vs. "release" mode.
</p>

<p>Basic usage is as follows:</p>

<pre class="code">
sh_binary(
    name = "myrule",
    srcs = select({
        ":conditionA": ["myrule_a.sh"],
        ":conditionB": ["myrule_b.sh"],
        "//conditions:default": ["myrule_default.sh"]
    })
)
</pre>

<p>This makes the <code>srcs</code> attribute of
  a <code>sh_binary</code> configurable by replacing its normal label
  list assignment with a <code>select</code> call that maps
  configuration conditions to matching values. Each condition is a label
  reference to
  a <code><a href="general.html#config_setting">config_setting</a></code> instance,
  which "matches" if the rule's configuration matches an expected set of
  values. The value of <code>myrule#srcs</code> then becomes whichever
  label list matches the current invocation.
</p>

<p>Further notes:</p>

<ul>
  <li>Exactly one condition is selected on any invocation.
  </li>
  <li>If multiple conditions match and one is a specialization of the others,
    the specialization takes precedence. Condition B is considered a
    specialization of condition A if B has all the same flags as A plus some
    additional flags. However, the number of constraint values that A and B have
    are not considered in this comparison -- one condition cannot match a
    platform <i>more than</i> another condition does.
  </li>
  <li>If multiple conditions match and one is not a specialization of all the
    others, Bazel fails with an error.
  </li>
  <li>The special pseudo-label <code>//conditions:default</code> is
    considered to match if no other condition matches. If this condition
    is left out, some other rule must match to avoid an error.
  </li>
  <li><code>select</code> can be embedded <i>inside</i> a larger
    attribute assignment. So <code>srcs = ["common.sh"]
    + select({ ":conditionA": ["myrule_a.sh"], ...})</code> and <code>
    srcs = select({ ":conditionA": ["a.sh"]}) + select({ ":conditionB":
    ["b.sh"]})</code> are valid expressions.
  </li>
  <li><code>select</code> works with most, but not all, attributes. Incompatible
  attributes are marked <code>nonconfigurable</code> in their documentation.

  </li>
</ul>

By default, Bazel produces the following error when no conditions match:
<pre class="code">
Configurable attribute "foo" doesn't match this configuration (would a default
condition help?).
Conditions checked:
 //pkg:conditionA.
 //pkg:conditionB.
</pre>

<code>no_match_error</code> can be used to signal more precise errors.

<h3 id="select_example">Examples</h3>

<pre class="code">
config_setting(
    name = "windows",
    values = {
        "crosstool_top": "//crosstools/windows",
    },
)

cc_binary(
    name = "multiplatform_app",
    ...
    linkopts = select({
        ":windows": [
            "-Wl,windows_support1.lib",
            "-Wl,windows_support2.lib",
        ],
        "//conditions:default": [],
    ...
)
</pre>

<p>In the above example, <code>multiplatform_app</code> links with additional
  options when invoked with <code>bazel build //pkg:multiplatform_app
  --crosstool_top=//crosstools/windows </code>.

<p>

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