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<h3 class="section">12.1 Have You Found a Bug?</h3>

<p><a name="index-bug-criteria-3257"></a>
If you are not sure whether you have found a bug, here are some guidelines:

     
<a name="index-fatal-signal-3258"></a>
<a name="index-core-dump-3259"></a>
<ul><li>If the compiler gets a fatal signal, for any input whatever, that is a
compiler bug.  Reliable compilers never crash.

     <p><a name="index-invalid-assembly-code-3260"></a><a name="index-assembly-code_002c-invalid-3261"></a><li>If the compiler produces invalid assembly code, for any input whatever
(except an <code>asm</code> statement), that is a compiler bug, unless the
compiler reports errors (not just warnings) which would ordinarily
prevent the assembler from being run.

     <p><a name="index-undefined-behavior-3262"></a><a name="index-undefined-function-value-3263"></a><a name="index-increment-operators-3264"></a><li>If the compiler produces valid assembly code that does not correctly
execute the input source code, that is a compiler bug.

     <p>However, you must double-check to make sure, because you may have a
program whose behavior is undefined, which happened by chance to give
the desired results with another C or C++ compiler.

     <p>For example, in many nonoptimizing compilers, you can write &lsquo;<samp><span class="samp">x;</span></samp>&rsquo;
at the end of a function instead of &lsquo;<samp><span class="samp">return x;</span></samp>&rsquo;, with the same
results.  But the value of the function is undefined if <code>return</code>
is omitted; it is not a bug when GCC produces different results.

     <p>Problems often result from expressions with two increment operators,
as in <code>f (*p++, *p++)</code>.  Your previous compiler might have
interpreted that expression the way you intended; GCC might
interpret it another way.  Neither compiler is wrong.  The bug is
in your code.

     <p>After you have localized the error to a single source line, it should
be easy to check for these things.  If your program is correct and
well defined, you have found a compiler bug.

     <li>If the compiler produces an error message for valid input, that is a
compiler bug.

     <p><a name="index-invalid-input-3265"></a><li>If the compiler does not produce an error message for invalid input,
that is a compiler bug.  However, you should note that your idea of
&ldquo;invalid input&rdquo; might be someone else's idea of &ldquo;an extension&rdquo; or
&ldquo;support for traditional practice&rdquo;.

     <li>If you are an experienced user of one of the languages GCC supports, your
suggestions for improvement of GCC are welcome in any case. 
</ul>

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