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<title>GNU `make'</title>
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Node:<a name="Command%20Variables">Command Variables</a>,
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<h3 class="section">Variables for Specifying Commands</h3>

   <p>Makefiles should provide variables for overriding certain commands, options,
and so on.

   <p>In particular, you should run most utility programs via variables.
Thus, if you use Bison, have a variable named <code>BISON</code> whose default
value is set with <code>BISON = bison</code>, and refer to it with
<code>$(BISON)</code> whenever you need to use Bison.

   <p>File management utilities such as <code>ln</code>, <code>rm</code>, <code>mv</code>, and
so on, need not be referred to through variables in this way, since users
don't need to replace them with other programs.

   <p>Each program-name variable should come with an options variable that is
used to supply options to the program.  Append <code>FLAGS</code> to the
program-name variable name to get the options variable name--for
example, <code>BISONFLAGS</code>.  (The names <code>CFLAGS</code> for the C
compiler, <code>YFLAGS</code> for yacc, and <code>LFLAGS</code> for lex, are
exceptions to this rule, but we keep them because they are standard.)
Use <code>CPPFLAGS</code> in any compilation command that runs the
preprocessor, and use <code>LDFLAGS</code> in any compilation command that
does linking as well as in any direct use of <code>ld</code>.

   <p>If there are C compiler options that <em>must</em> be used for proper
compilation of certain files, do not include them in <code>CFLAGS</code>.
Users expect to be able to specify <code>CFLAGS</code> freely themselves.
Instead, arrange to pass the necessary options to the C compiler
independently of <code>CFLAGS</code>, by writing them explicitly in the
compilation commands or by defining an implicit rule, like this:

<pre class="smallexample">     CFLAGS = -g
     ALL_CFLAGS = -I. $(CFLAGS)
     .c.o:
             $(CC) -c $(CPPFLAGS) $(ALL_CFLAGS) $&lt;
     </pre>

   <p>Do include the <code>-g</code> option in <code>CFLAGS</code>, because that is not
<em>required</em> for proper compilation.  You can consider it a default
that is only recommended.  If the package is set up so that it is
compiled with GCC by default, then you might as well include <code>-O</code>
in the default value of <code>CFLAGS</code> as well.

   <p>Put <code>CFLAGS</code> last in the compilation command, after other variables
containing compiler options, so the user can use <code>CFLAGS</code> to
override the others.

   <p><code>CFLAGS</code> should be used in every invocation of the C compiler,
both those which do compilation and those which do linking.

   <p>Every Makefile should define the variable <code>INSTALL</code>, which is the
basic command for installing a file into the system.

   <p>Every Makefile should also define the variables <code>INSTALL_PROGRAM</code>
and <code>INSTALL_DATA</code>.  (The default for <code>INSTALL_PROGRAM</code> should
be <code>$(INSTALL)</code>; the default for <code>INSTALL_DATA</code> should be
<code>${INSTALL} -m 644</code>.)  Then it should use those variables as the
commands for actual installation, for executables and nonexecutables
respectively.  Use these variables as follows:

<pre class="example">     $(INSTALL_PROGRAM) foo $(bindir)/foo
     $(INSTALL_DATA) libfoo.a $(libdir)/libfoo.a
     </pre>

   <p>Optionally, you may prepend the value of <code>DESTDIR</code> to the target
filename.  Doing this allows the installer to create a snapshot of the
installation to be copied onto the real target filesystem later.  Do not
set the value of <code>DESTDIR</code> in your Makefile, and do not include it
in any installed files.  With support for <code>DESTDIR</code>, the above
examples become:

<pre class="example">     $(INSTALL_PROGRAM) foo $(DESTDIR)$(bindir)/foo
     $(INSTALL_DATA) libfoo.a $(DESTDIR)$(libdir)/libfoo.a
     </pre>

<p>Always use a file name, not a directory name, as the second argument of
the installation commands.  Use a separate command for each file to be
installed.

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