C program compiled with cygwin in Windows works, segmentation fault under Linux. Is cygwin GCC 'bad'?

Question

For my Programming 102 class we are asked to deliver C code that compiles and runs under Linux. I don\'t have enough spare space on my hard drive to install Linux alongside Wind

Answer 1

I don't mean to sound rude, but it's probably your code that's bad, not the compiler. ;) Problems like this are actually more common than you'd think, because different OS's and compilers will have different ways of organizing your application's data in the stack and heap. The former can be particularly problematic, especially if you end up overwriting memory on the stack, or referencing freed memory which the system has decided to use for something else. So basically, you might get away with it sometimes, but other times your app will choke and die. Either way, if it segfaults, it's because you tried to reference memory which you were not allowed, so it's more of a "happy coincidence" that it didn't crash under another system/compiler.

But really, a segfault is a segfault, so you should instead debug your code looking for memory corruption instead of tweaking the compiler's configuration to figure out what's going wrong.

Edit: Ok, I see what you mean now... I thought you were coming at this problem with an "X sucks, but Y works just fine!" attitude, but it seems to be your TA who's got that. ;)

Anyways, here's some more hints for debugging problems like this:

• Look for pointer arithmetic, referencing/dereferencing for possible "doh!" errors. Any place where you are adding/subtracting one (aka, fencepost errors) are particularly suspect.
• Comment out calls to malloc/free around the problem area, and any associated areas where those pointers are used. If the code stops crashing, then you're headed in the right direction.
• Assuming you've at least identified the general area where your code is crashing, insert early return statements in there and find the point where your code doesn't crash. This can help to find an area somewhere between that point and where your code actually crashes. Remember, a segfault like this may not necessarily happen directly at the line of code where your bug is.
• Use the memory debugging tools available on your system.
  • On Unix, check out this guide for debugging memory on unix, and the valgrind profiler (@Sol, thx for reminding me about this one)
  • On Visual Studio/Windows, your good 'ol buddy CrtCheckMemory() comes in rather handy. Also, read up on the CRT memory debugging patterns, as they're one of the nicer features of working in VS. Often times, just leaving open a memory tab in VS is enough to diagnose bugs like this once you memorize the various patterns.
  • In Mac OSX, you can set a breakpoint on malloc_error_break (either from gdb or Xcode), which causes it the debugger to break whenever malloc detects memory corruption. I'm not sure whether that's available in other unix flavors, but a quick google search seems to indicate it's mac-only. Also, a rather "experimental" looking version of valgrind seems to exist for OSX.