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<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>To request debugging information, specify the &lsquo;<samp><span class="samp">-g</span></samp>&rsquo; option when you run
the compiler.

   <p>Programs that are to be shipped to your customers are compiled with
optimizations, using the &lsquo;<samp><span class="samp">-O</span></samp>&rsquo; compiler option.  However, some
compilers are unable to handle the &lsquo;<samp><span class="samp">-g</span></samp>&rsquo; and &lsquo;<samp><span class="samp">-O</span></samp>&rsquo; options
together.  Using those compilers, you cannot generate optimized
executables containing debugging information.

   <p><span class="sc">gcc</span>, the <span class="sc">gnu</span> C/C<tt>++</tt> compiler, supports &lsquo;<samp><span class="samp">-g</span></samp>&rsquo; with or
without &lsquo;<samp><span class="samp">-O</span></samp>&rsquo;, making it possible to debug optimized code.  We
recommend that you <em>always</em> use &lsquo;<samp><span class="samp">-g</span></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>Older versions of the <span class="sc">gnu</span> C compiler permitted a variant option
&lsquo;<samp><span class="samp">-gg</span></samp>&rsquo;<!-- /@w --> for debugging information.  <span class="sc">gdb</span> no longer supports this
format; if your <span class="sc">gnu</span> C compiler has this option, do not use it.

   <p><span class="sc">gdb</span> 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><span class="option">-g</span></samp> flag alone, because this information is rather large. 
Version 3.1 and later of <span class="sc">gcc</span>, the <span class="sc">gnu</span> C compiler,
provides macro information if you specify the options
<samp><span class="option">-gdwarf-2</span></samp> and <samp><span class="option">-g3</span></samp>; the former option requests
debugging information in the Dwarf 2 format, and the latter requests
&ldquo;extra information&rdquo;.  In the future, we hope to find more compact
ways to represent macro information, so that it can be included with
<samp><span class="option">-g</span></samp> alone.

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