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<h3 class="section">13.1 Have You Found a Bug?</h3>
<p><a name="index-bug-criteria-4302"></a>
If you are not sure whether you have found a bug, here are some guidelines:
<a name="index-fatal-signal-4303"></a>
<a name="index-core-dump-4304"></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-4305"></a><a name="index-assembly-code_002c-invalid-4306"></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-4307"></a><a name="index-undefined-function-value-4308"></a><a name="index-increment-operators-4309"></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 ‘<samp><span class="samp">x;</span></samp>’
at the end of a function instead of ‘<samp><span class="samp">return x;</span></samp>’, 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-4310"></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
“invalid input” might be someone else's idea of “an extension” or
“support for traditional practice”.
<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.
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