Hi Johannes, I'm just chiming in to make a point to the list in general here. Nothing personal ;-)
Johannes Meixner writes: > Hello, > > On Sep 24 22:22 Olaf Meeuwissen wrote (excerpt): >> One, in sanei_ir.c, deserves some attention as it produces potentially >> undefined behaviour. It's really the same issue as reported in 311857, >> which I recently reopened. >> >> ../../../sanei/sanei_ir.c:481:11: warning: multiple unsequenced >> modifications to 'outi' [-Wunsequenced] >> *outi++ = *outi++ >> is; >> >> The two increments may occur in any given order, IIUC[3]. >> >> [3] http://c-faq.com/expr/seqpoints.html > > In general I would even demand that nowadays such kind > of fancy coding stlye should no longer be done at all. Not totally clear on what the FAQ is saying, I decided to put code like the above to the test. Consider this code: #include <stdio.h> #include <stdlib.h> int main (int argc, char *argv[]) { int a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; int i; int *p = a; for (i = 0; i < sizeof (a) / sizeof (*a) / 2; ++i) { *p++ = *p++ >> 3; } fprintf (stdout, "a = { %i", a[0]); for (i = 1; i < sizeof (a) / sizeof (*a); ++i) { fprintf (stdout, ", %i", a[i]); } fprintf (stdout, " }\n"); return EXIT_SUCCESS; } Those who think they know what the output will be, by all means, write it down now before reading any further. I compiled the above code on Debian stable (amd64) with the default versions of clang and gcc. The only compile-time flag I used was -ansi. BTW, optimization level does not matter. I ran both programs and this is what I got: clang: a = { 0, 0, 2, 0, 4, 0, 6, 0, 8, 1 } gcc : a = { 0, 1, 0, 3, 0, 5, 0, 7, 1, 9 } If any of you think either compiler is buggy (or even both?), feel free to submit a bug report with the respective compiler projects. In the mean time, I think we should any code that is flagged as -Wunsequenced (by clang) or -Wsequence-point (by gcc). # If any of you think everyone uses gcc, think again. Debian, for one, # has been very busy compiling *all* of its packages with clang. I "reverse engineered" the compilers' sequencing order. It turns out that gcc uses the equivalent of int q = *p >> 3; *p = q; p++; p++; where as clang decided to use the equivalent of int q = *p >> 3; p++; *p = q; p++; When forcing the sequence order as above, both compilers yield the same results. So, if we know which of these compilers the code was written against, we can fix our code. If not, and that includes the "yet-another-compiler" scenario, we cannot and need the original author of the code to step in and fix this *bug*. > With nowadays compiler optimizations it does not matter > if one writes > > y = x++ * 2; > > or > > y = x * 2; > x++; > > The former is selfish and oversophisticated coding. > In this case it is a mix-up of two separated things. > > The latter is altruistic and explicit coding that > makes it obvious for others what actually is meant. I'm not that much of a source code "extremist", but I do subscribe to the (self-invented?) AWARE principle: All Warnings Are Really Errors As such, warnings should be fixed. I am well aware (no pun intended!) of the various versions of various compilers on various platforms all having their own ideas of what should and should not be flagged as a warning when. That notwithstanding, a project can select a canonical setup up, use -Werror there and not release unless the compile passes on that setup. # Now here's an idea for 1.0.26! Hope this helps, -- Olaf Meeuwissen, LPIC-2 FSF Associate Member since 2004-01-27 Support Free Software Support the Free Software Foundation https://my.fsf.org/donate https://my.fsf.org/join -- sane-devel mailing list: sane-devel@lists.alioth.debian.org http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/sane-devel Unsubscribe: Send mail with subject "unsubscribe your_password" to sane-devel-requ...@lists.alioth.debian.org
