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<a name="A-simple-makefile"></a>
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<p>
Next: <a href="Linking-with-external-libraries.html#Linking-with-external-libraries" accesskey="n" rel="next">Linking with external libraries</a>, Previous: <a href="Recompiling-and-relinking.html#Recompiling-and-relinking" accesskey="p" rel="prev">Recompiling and relinking</a>, Up: <a href="Compiling-a-C-program.html#Compiling-a-C-program" accesskey="u" rel="up">Compiling a C program</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="A-simple-makefile-1"></a>
<h3 class="section">2.6 A simple makefile</h3>
<a name="index-makefile_002c-example-of"></a>
<a name="index-GNU-Make"></a>

<p>For those unfamiliar with <code>make</code>, this section provides a simple
demonstration of its use.  Make is a program in its own right and can be
found on all Unix systems.  To learn more about the GNU version of
<code>make</code> you will need to consult the <cite>GNU Make</cite> manual by
Richard M. Stallman and Roland McGrath (see <a href="Further-reading.html#Further-reading">Further reading</a>).
</p>
<a name="index-target_002c-in-makefile"></a>
<a name="index-dependency_002c-in-makefile"></a>
<a name="index-command_002c-in-makefile"></a>
<p>Make reads a description of a project from a <em>makefile</em> (by default,
called <samp>Makefile</samp> in the current directory).  A makefile specifies
a set of compilation rules in terms of <em>targets</em> (such as
executables) and their <em>dependencies</em> (such as object files and
source files) in the following format:
</p>
<div class="example">
<pre class="example"><i>target</i>: <i>dependencies</i>
        <i>command</i>
</pre></div>
<a name="index-tab_002c-in-makefiles"></a>
<a name="index-separator_002c-in-makefiles"></a>
<p>For each target, make checks the modification time of the corresponding
dependency files to determine whether the target needs to
be rebuilt using the corresponding command.  Note that the
<code><i>command</i></code> lines in a makefile must be indented with a single
<tt class="key">TAB</tt> character, not spaces.
</p>
<a name="index-implicit-rules_002c-in-makefile"></a>
<a name="index-rules_002c-in-makefile"></a>
<a name="index-CFLAGS_002c-make-variable"></a>
<a name="index-CC_002c-make-variable"></a>
<a name="index-CXX_002c-make-variable"></a>
<a name="index-CXXFLAGS_002c-make-variable"></a>
<a name="index-CPPFLAGS_002c-make-variable"></a>
<a name="index-variables_002c-in-make"></a>
<p>GNU Make contains many default rules, referred to as <em>implicit</em>
rules, to simplify the construction of makefiles.  For example, these
specify that <samp>.o</samp> files can be obtained from <samp>.c</samp> files by
compilation, and that an executable can be made by linking together
<samp>.o</samp> files.  Implicit rules are defined in terms of <em>make
variables</em>, such as <code>CC</code> (the C compiler) and <code>CFLAGS</code> (the
compilation options for C programs), which can be set using
<code><i>VARIABLE</i>=<i>VALUE</i></code> lines in the makefile.  For C++ the
equivalent variables are <code>CXX</code> and <code>CXXFLAGS</code>, while the make
variable <code>CPPFLAGS</code> sets the preprocessor options.  The implicit and
user-defined rules are automatically chained together as necessary by
GNU Make.
</p>
<p>A simple <samp>Makefile</samp> for the project above can be written as
follows:
</p>
<div class="example">
<pre class="verbatim">CC=gcc 
CFLAGS=-Wall
main: main.o hello_fn.o

clean:
	rm -f main main.o hello_fn.o
</pre></div>
<p>The file can be read like this: using the C compiler <code>gcc</code>,
with compilation option <samp>-Wall</samp>, build the target executable
<code>main</code> from the object files <samp>main.o</samp> and <samp>hello_fn.o</samp>
(these, in turn, will be built via implicit rules from <samp>main.c</samp>
and <samp>hello_fn.c</samp>).  The target <code>clean</code> has no dependencies
and simply removes all the compiled files.<a name="DOCF5" href="#FOOT5"><sup>4</sup></a> The option <samp>-f</samp> (force) on the <code>rm</code> command
suppresses any error messages if the files do not exist.
</p>
<p>To use the makefile, type <code>make</code>.  When called with no arguments,
the first target in the makefile is built, producing the executable
<samp>main</samp>:
</p>
<div class="example">
<pre class="example">$ make
gcc -Wall   -c -o main.o main.c
gcc -Wall   -c -o hello_fn.o hello_fn.c
gcc   main.o hello_fn.o   -o main
$ ./main
Hello, world!
</pre></div>
<p>To rebuild the executable after modifying a source file, simply type
<code>make</code> again. By checking the timestamps of the target and dependency files, 
make identifies the files which have changed and regenerates the corresponding
intermediate files needed to update the targets:
</p>
<div class="example">
<pre class="example">$ emacs main.c  <span class="roman">(edit the file)</span>
$ make
gcc -Wall   -c -o main.o main.c
gcc    main.o hello_fn.o   -o main
$ ./main
Hello, everyone!
</pre></div>
<p>Finally, to remove the generated files, type <code>make clean</code>:
</p>
<div class="example">
<pre class="example">$ make clean
rm -f main main.o hello_fn.o
</pre></div>
<p>A more sophisticated makefile would usually contain additional targets
for installation (<code>make install</code>) and testing (<code>make check</code>).
</p>
<p>The examples in the rest of this book are small enough not to need
makefiles, but the use of make is recommended for any larger programs.
</p>
<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT5" href="#DOCF5">(4)</a></h3>
<p>This assumes that
there is no file called <samp>clean</samp> in the current directory&mdash;see
the discussion of &ldquo;phony targets&rdquo; in the GNU Make manual for
details.</p>
</div>
<hr>
<div class="header">
<p>
Next: <a href="Linking-with-external-libraries.html#Linking-with-external-libraries" accesskey="n" rel="next">Linking with external libraries</a>, Previous: <a href="Recompiling-and-relinking.html#Recompiling-and-relinking" accesskey="p" rel="prev">Recompiling and relinking</a>, Up: <a href="Compiling-a-C-program.html#Compiling-a-C-program" accesskey="u" rel="up">Compiling a C program</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
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gccintro 1.0-3 / usr / share / doc / gccintro / gccintro / A-simple-makefile.html
