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<h1 class="chapter"> 12. Features of GNU <code>make</code> </h1>

<p>Here is a summary of the features of GNU <code>make</code>, for comparison
with and credit to other versions of <code>make</code>.  We consider the
features of <code>make</code> in 4.2 BSD systems as a baseline.  If you are
concerned with writing portable makefiles, you should not use the
features of <code>make</code> listed here, nor the ones in <a href="make_13.html#SEC135">Incompatibilities and Missing Features</a>.
</p>
<p>Many features come from the version of <code>make</code> in System V.
</p>
<ul>
<li>
The <code>VPATH</code> variable and its special meaning.
See section <a href="make_4.html#SEC38">Searching Directories for Prerequisites</a>.
This feature exists in System V <code>make</code>, but is undocumented.
It is documented in 4.3 BSD <code>make</code> (which says it mimics System V's
<code>VPATH</code> feature).

</li><li>
Included makefiles.  See section <a href="make_3.html#SEC16">Including Other Makefiles</a>.
Allowing multiple files to be included with a single directive is a GNU
extension.

</li><li>
Variables are read from and communicated via the environment.
See section <a href="make_6.html#SEC85">Variables from the Environment</a>.

</li><li>
Options passed through the variable <code>MAKEFLAGS</code> to recursive
invocations of <code>make</code>.
See section <a href="make_5.html#SEC70">Communicating Options to a Sub-<code>make</code></a>.

</li><li>
The automatic variable <code>$%</code> is set to the member name
in an archive reference.  See section <a href="make_10.html#SEC121">Automatic Variables</a>.

</li><li>
The automatic variables <code>$@</code>, <code>$*</code>, <code>$&lt;</code>, <code>$%</code>,
and <code>$?</code> have corresponding forms like <code>$(@F)</code> and
<code>$(@D)</code>.  We have generalized this to <code>$^</code> as an obvious
extension.  See section <a href="make_10.html#SEC121">Automatic Variables</a>.

</li><li>
Substitution variable references.
See section <a href="make_6.html#SEC75">Basics of Variable References</a>.

</li><li>
The command-line options &lsquo;<samp>-b</samp>&rsquo; and &lsquo;<samp>-m</samp>&rsquo;, accepted and
ignored.  In System V <code>make</code>, these options actually do something.

</li><li>
Execution of recursive commands to run <code>make</code> via the variable
<code>MAKE</code> even if &lsquo;<samp>-n</samp>&rsquo;, &lsquo;<samp>-q</samp>&rsquo; or &lsquo;<samp>-t</samp>&rsquo; is specified.
See section <a href="make_5.html#SEC67">Recursive Use of <code>make</code></a>.

</li><li>
Support for suffix &lsquo;<samp>.a</samp>&rsquo; in suffix rules.  See section <a href="make_11.html#SEC133">Suffix Rules for Archive Files</a>.  This feature is obsolete in GNU <code>make</code>, because the
general feature of rule chaining (see section <a href="make_10.html#SEC117">Chains of Implicit Rules</a>) allows one pattern rule for installing members in an
archive (see section <a href="make_11.html#SEC130">Implicit Rule for Archive Member Targets</a>) to be sufficient.

</li><li>
The arrangement of lines and backslash-newline combinations in
commands is retained when the commands are printed, so they appear as
they do in the makefile, except for the stripping of initial
whitespace.
</li></ul>

<p>The following features were inspired by various other versions of
<code>make</code>.  In some cases it is unclear exactly which versions inspired
which others.
</p>
<ul>
<li>
Pattern rules using &lsquo;<samp>%</samp>&rsquo;.
This has been implemented in several versions of <code>make</code>.
We're not sure who invented it first, but it's been spread around a bit.
See section <a href="make_10.html#SEC118">Defining and Redefining Pattern Rules</a>.

</li><li>
Rule chaining and implicit intermediate files.
This was implemented by Stu Feldman in his version of <code>make</code>
for AT&amp;T Eighth Edition Research Unix, and later by Andrew Hume of
AT&amp;T Bell Labs in his <code>mk</code> program (where he terms it
&ldquo;transitive closure&rdquo;).  We do not really know if
we got this from either of them or thought it up ourselves at the
same time.  See section <a href="make_10.html#SEC117">Chains of Implicit Rules</a>.

</li><li>
The automatic variable <code>$^</code> containing a list of all prerequisites
of the current target.  We did not invent this, but we have no idea who
did.  See section <a href="make_10.html#SEC121">Automatic Variables</a>.  The automatic variable
<code>$+</code> is a simple extension of <code>$^</code>.

</li><li>
The &ldquo;what if&rdquo; flag (&lsquo;<samp>-W</samp>&rsquo; in GNU <code>make</code>) was (as far as we know)
invented by Andrew Hume in <code>mk</code>.
See section <a href="make_9.html#SEC108">Instead of Executing the Commands</a>.

</li><li>
The concept of doing several things at once (parallelism) exists in
many incarnations of <code>make</code> and similar programs, though not in the
System V or BSD implementations.  See section <a href="make_5.html#SEC61">Command Execution</a>.

</li><li>
Modified variable references using pattern substitution come from
SunOS 4.  See section <a href="make_6.html#SEC75">Basics of Variable References</a>.
This functionality was provided in GNU <code>make</code> by the
<code>patsubst</code> function before the alternate syntax was implemented
for compatibility with SunOS 4.  It is not altogether clear who
inspired whom, since GNU <code>make</code> had <code>patsubst</code> before SunOS
4 was released.

</li><li>
The special significance of &lsquo;<samp>+</samp>&rsquo; characters preceding command lines
(see section <a href="make_9.html#SEC108">Instead of Executing the Commands</a>) is
mandated by
<cite>IEEE Standard 1003.2-1992</cite> (POSIX.2).

</li><li>
The &lsquo;<samp>+=</samp>&rsquo; syntax to append to the value of a variable comes from SunOS
4 <code>make</code>.  See section <a href="make_6.html#SEC82">Appending More Text to Variables</a>.

</li><li>
The syntax &lsquo;<samp><var>archive</var>(<var>mem1</var> <var>mem2</var>&hellip;)</samp>&rsquo; to list
multiple members in a single archive file comes from SunOS 4 <code>make</code>.
See section <a href="make_11.html#SEC129">Archive Members as Targets</a>.

</li><li>
The <code>-include</code> directive to include makefiles with no error for a
nonexistent file comes from SunOS 4 <code>make</code>.  (But note that SunOS 4
<code>make</code> does not allow multiple makefiles to be specified in one
<code>-include</code> directive.)  The same feature appears with the name
<code>sinclude</code> in SGI <code>make</code> and perhaps others.
</li></ul>

<p>The remaining features are inventions new in GNU <code>make</code>:
</p>
<ul>
<li>
Use the &lsquo;<samp>-v</samp>&rsquo; or &lsquo;<samp>--version</samp>&rsquo; option to print version and
copyright information.

</li><li>
Use the &lsquo;<samp>-h</samp>&rsquo; or &lsquo;<samp>--help</samp>&rsquo; option to summarize the options to
<code>make</code>.

</li><li>
Simply-expanded variables.  See section <a href="make_6.html#SEC76">The Two Flavors of Variables</a>.

</li><li>
Pass command-line variable assignments automatically through the
variable <code>MAKE</code> to recursive <code>make</code> invocations.
See section <a href="make_5.html#SEC67">Recursive Use of <code>make</code></a>.

</li><li>
Use the &lsquo;<samp>-C</samp>&rsquo; or &lsquo;<samp>--directory</samp>&rsquo; command option to change
directory.  See section <a href="make_9.html#SEC112">Summary of Options</a>.

</li><li>
Make verbatim variable definitions with <code>define</code>.
See section <a href="make_6.html#SEC84">Defining Variables Verbatim</a>.

</li><li>
Declare phony targets with the special target <code>.PHONY</code>.

<p>Andrew Hume of AT&amp;T Bell Labs implemented a similar feature with a
different syntax in his <code>mk</code> program.  This seems to be a case of
parallel discovery.  See section <a href="make_4.html#SEC45">Phony Targets</a>.
</p>
</li><li>
Manipulate text by calling functions.
See section <a href="make_8.html#SEC92">Functions for Transforming Text</a>.

</li><li>
Use the &lsquo;<samp>-o</samp>&rsquo; or &lsquo;<samp>--old-file</samp>&rsquo;
option to pretend a file's modification-time is old.
See section <a href="make_9.html#SEC109">Avoiding Recompilation of Some Files</a>.

</li><li>
Conditional execution.

<p>This feature has been implemented numerous times in various versions
of <code>make</code>; it seems a natural extension derived from the features
of the C preprocessor and similar macro languages and is not a
revolutionary concept.  See section <a href="make_7.html#SEC88">Conditional Parts of Makefiles</a>.
</p>
</li><li>
Specify a search path for included makefiles.
See section <a href="make_3.html#SEC16">Including Other Makefiles</a>.

</li><li>
Specify extra makefiles to read with an environment variable.
See section <a href="make_3.html#SEC17">The Variable <code>MAKEFILES</code></a>.

</li><li>
Strip leading sequences of &lsquo;<samp>./</samp>&rsquo; from file names, so that
&lsquo;<tt>./<var>file</var></tt>&rsquo; and &lsquo;<tt><var>file</var></tt>&rsquo; are considered to be the
same file.

</li><li>
Use a special search method for library prerequisites written in the
form &lsquo;<samp>-l<var>name</var></samp>&rsquo;.
See section <a href="make_4.html#SEC44">Directory Search for Link Libraries</a>.

</li><li>
Allow suffixes for suffix rules
(see section <a href="make_10.html#SEC126">Old-Fashioned Suffix Rules</a>) to contain any
characters.  In other versions of <code>make</code>, they must begin with
&lsquo;<samp>.</samp>&rsquo; and not contain any &lsquo;<samp>/</samp>&rsquo; characters.

</li><li>
Keep track of the current level of <code>make</code> recursion using the
variable <code>MAKELEVEL</code>.  See section <a href="make_5.html#SEC67">Recursive Use of <code>make</code></a>.

</li><li>
Provide any goals given on the command line in the variable
<code>MAKECMDGOALS</code>.  See section <a href="make_9.html#SEC107">Arguments to Specify the Goals</a>.

</li><li>
Specify static pattern rules.  See section <a href="make_4.html#SEC51">Static Pattern Rules</a>.

</li><li>
Provide selective <code>vpath</code> search.
See section <a href="make_4.html#SEC38">Searching Directories for Prerequisites</a>.

</li><li>
Provide computed variable references.
See section <a href="make_6.html#SEC75">Basics of Variable References</a>.

</li><li>
Update makefiles.  See section <a href="make_3.html#SEC20">How Makefiles Are Remade</a>.
System V <code>make</code> has a very, very limited form of this
functionality in that it will check out SCCS files for makefiles.

</li><li>
Various new built-in implicit rules.
See section <a href="make_10.html#SEC115">Catalogue of Implicit Rules</a>.

</li><li>
The built-in variable &lsquo;<samp>MAKE_VERSION</samp>&rsquo; gives the version number of
<code>make</code>.
<a name="IDX630"></a>
</li></ul>

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