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Have You Found a Bug?

If you are not sure whether you have found a bug, the following documentation discusses some guidelines.

- If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash.
- If the compiler produces invalid assembly code, for any input what-ever (except an asm statement), that is a compiler bug, unless the compiler reports errors (not just warnings) which would ordinarily prevent the assembler from being run.
- If the compiler produces valid assembly code that does not correctly execute the input source code, that is a compiler bug.
However, you must double-check to make sure, because you may have run into an incompatibility between GNU C and traditional C (see Incompatibilities of GNU CC). These incompatibilities might be considered bugs, but they are inescapable consequences of valuable features. Or you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C or C++ compiler.
For example, in many nonoptimizing compilers, you can write ‘x;’ at the end of a function instead of ‘return x;’, with the same results.
But the value of the function is undefined if return is omitted; it is not a bug when GNU CC produces different results.
Problems often result from expressions with two increment operators, as in ‘f(*p++, *p++)’. Your previous compiler might have interpreted that expression the way you intended; GNU CC might interpret it another way. Neither compiler is wrong. The bug is in your code.
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.
- If the compiler produces an error message for valid input, that is a compiler bug.
- 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 my idea of “an extension” or “support for traditional practice.”
- If you are an experienced user of C or C++ compilers, your suggestions for improvement of GNU CC or GNU C++ are welcome in any case.