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<a name="Compilation"></a>
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<a name="Compiling-for-Debugging"></a>
<h3 class="section">4.1 Compiling for Debugging</h3>

<p>In order to debug a program effectively, you need to generate
debugging information when you compile it.  This debugging information
is stored in the object file; it describes the data type of each
variable or function and the correspondence between source line numbers
and addresses in the executable code.
</p>
<p>To request debugging information, specify the &lsquo;<samp>-g</samp>&rsquo; option when you run
the compiler.
</p>
<p>Programs that are to be shipped to your customers are compiled with
optimizations, using the &lsquo;<samp>-O</samp>&rsquo; compiler option.  However, some
compilers are unable to handle the &lsquo;<samp>-g</samp>&rsquo; and &lsquo;<samp>-O</samp>&rsquo; options
together.  Using those compilers, you cannot generate optimized
executables containing debugging information.
</p>
<p><small>GCC</small>, the <small>GNU</small> C/C<tt>++</tt> compiler, supports &lsquo;<samp>-g</samp>&rsquo; with or
without &lsquo;<samp>-O</samp>&rsquo;, making it possible to debug optimized code.  We
recommend that you <em>always</em> use &lsquo;<samp>-g</samp>&rsquo; whenever you compile a
program.  You may think your program is correct, but there is no sense
in pushing your luck.  For more information, see <a href="Optimized-Code.html#Optimized-Code">Optimized Code</a>.
</p>
<p>Older versions of the <small>GNU</small> C compiler permitted a variant option
&lsquo;<samp><span class="nolinebreak">-gg</span></samp>&rsquo;<!-- /@w --> for debugging information.  <small>GDB</small> no longer supports this
format; if your <small>GNU</small> C compiler has this option, do not use it.
</p>
<p><small>GDB</small> knows about preprocessor macros and can show you their
expansion (see <a href="Macros.html#Macros">Macros</a>).  Most compilers do not include information
about preprocessor macros in the debugging information if you specify
the <samp>-g</samp> flag alone.  Version 3.1 and later of <small>GCC</small>,
the <small>GNU</small> C compiler, provides macro information if you are using
the DWARF debugging format, and specify the option <samp>-g3</samp>.
</p>
<p>See <a href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging-Options">Options for Debugging Your Program or GCC</a> in <cite>Using the <small>GNU</small> Compiler Collection (GCC)</cite>, for more
information on <small>GCC</small> options affecting debug information.
</p>
<p>You will have the best debugging experience if you use the latest
version of the DWARF debugging format that your compiler supports.
DWARF is currently the most expressive and best supported debugging
format in <small>GDB</small>.
</p>
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