Right. There are two things here: 1. Resolving the immediate problem (and understanding exactly why it fails might be a good first step).
2. The quality of the code. You are right; it is poor code. I try to write pretty good code. I take no pride in avoiding the use of unnecessary parentheses. I confess, I (a.) failed to consider that testWord = valueToTest >> 32 would not reliably operate as intended; and (b.) I was completely satisfied when the function passed basic unit tests and though no more of it. Lesson learned, hopefully. Not certain exactly what the lesson is, but I will be more careful in the future. I have learned to be cautious about integer type conversions, but obviously not cautious enough. I guess the lesson is just like for sequences of logical operators: use parentheses to force what you expect. Charles -----Original Message----- From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf Of Harry Wahl Sent: Sunday, July 21, 2013 11:21 AM To: IBM-MAIN@LISTSERV.UA.EDU Subject: Re: Looking for help with an obscure C integer problem Charles, Hi, here is my opinion (and this definitely falls under the category of "obscure C"): You are not considering the implications of "sequence points" in your C/C++. "Sequence points" should not be confused with "operator precedence." Operator precedence is determinate, sequence points are not. I believe IBM XLC is at the C++11 level of C/C++. The C/C++ level is relevant here because there are sometimes subtle, and sometimes not so subtle, differences in how sequence points apply between various levels of C++. While C++11 (the most recent level of C/C++) seems to a have only tiny, mostly irrelevant and evolutionary changes from prior levels of C/C++; there are significant differences in how "sequence points" are defined and effect execution. Still, C++11 and the level of the C/C++ compiler that is compiling your program is only tangential to the situation you describe. Your code will execute with undefined behavior regardless of what compiler you use. But, knowing the level of the C/C++ compiler may be important if you wish to reconcile why it behaves one way sometimes and other ways other times (e.g. on a different z/OS). To me, your failure to consider the subtle impact of sequence points renders your code ambiguous and subject to undefined behavior. This manifests itself, for example, by executing differently when optimized. It is at the compiler's and optimizer's discretion to decide the order of execution for code that the C++ standard does not specifically define. This includes overlapping execution. I think the C/C++ compiler and optimizer are working exactly as specified by applicable ISO/IEC standards. "The fault, dear Brutus, it not in our stars,But, in ourselves, that we are underlings" Cassius in Shakespeare's Julius Caesar ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
