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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 ‘<samp><span class="samp">-g</span></samp>’ option when you run |
| the compiler. |
| |
| <p>Programs that are to be shipped to your customers are compiled with |
| optimizations, using the ‘<samp><span class="samp">-O</span></samp>’ compiler option. However, some |
| compilers are unable to handle the ‘<samp><span class="samp">-g</span></samp>’ and ‘<samp><span class="samp">-O</span></samp>’ 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 ‘<samp><span class="samp">-g</span></samp>’ with or |
| without ‘<samp><span class="samp">-O</span></samp>’, making it possible to debug optimized code. We |
| recommend that you <em>always</em> use ‘<samp><span class="samp">-g</span></samp>’ 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 |
| ‘<samp><span class="samp">-gg</span></samp>’<!-- /@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 |
| “extra information”. 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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