On Wed, Jul 9, 2008 at 12:28 PM, Lionel B <lionelb.nospam@xxxxxxxxx> wrote: > On Wed, 09 Jul 2008 03:31:35 +0100, James Gregory wrote: > >> 1. Compile with MSVC 7.1, no optimization: my program doesn't freeze 2. >> Compile with MSVC 7.1, with optimization: my program doesn't freeze 3. >> Compile with gcc 4.2.3 (Ubuntu 64 bit), no optimization: my program >> doesn't freeze >> 4. Compile with gcc 4.2.3 (Ubuntu 64 bit), with optimization: my program >> freezes in a particular function 5. Compile with gcc 4.2.3 (Ubuntu 64 >> bit), with optimization, and include <iostream> at the top of the file >> where the function which freezes is defined: my program doesn't freeze > > You give *way* too little information to even begin to guess what the > problem might be... but this smells like a classic case of Undefined > Behaviour - sometimes it appears to work, sometimes it doesn't. I.e. you > have a bug in your program. Sure, I agree it is 99.999999% certain I am doing something wrong hence the undefined behaviour. But doesn't it seem somewhat out of the ordinary that the behaviour can be affected by which standard library header files I include? How can this make a difference to the code that gcc produces? It seems unlikely that it's a total coincidence that whenever I run the version with iostream included it doesn't freeze and whenever I run the version without it does (I just tried recompiling the two versions again and the pattern holds). > >> Debugging this is difficult. > > It frequently is ;-) but what other choice do you have? There are tools > to help (debuggers such as gdb, memory leak checkers such as > valgrind, ...) I have used gdb to see which function it freezes in, but as noted I cannot compile with full debugging symbols and watch variables/step through the code as this removes the bug. valgrind doesn't report any problems at all (except a few warnings about functions called in external libraries at start up). The reason I originally wanted to include iostream was an effort to output the value of variables at various points in the function which freezes, but as noted this also removes the bug. > > First I would try stripping your program down to a minimal core that > exhibits the buggy behaviour. Often in doing so you will isolate and > identify the bug. If not seek help here or in a C (or C++, you don't say > which language your program is in) forum. You can post minimal code > demonstrating the problem. But if even including a certain header file can affect the behaviour, how can I strip down the program except through random trial and error? I guess what I'm trying to say is not "What is wrong with my program?", when clearly I haven't given enough information for anyone to help, but rather "Does anyone know what sort of thing might be affected in the code gcc produces if I include iostream is included?". (And it is C++ not C, hence <iostream>) James
