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 Windows, and so I use cygwin to compile my programs.

The most recent program I had to give in compiles and runs fine under cygwin. It compiles fine under Linux, but half-way through execution produces a segmentation fault. I explained this to the grad student who gives us class and he said that cygwin's version of GCC allows for sloppier code to be compiled and executed.

The few references I have found via google haven't been conclusive. One thread I found said that the cause for the seg fault under Linux is a memory leak. Why would this not affect the cygwin version?

I would use the University's computers, but I can't use Subversion on them which would significantly hinder my efforts. (I'm new to coding and often need to be able to be able to revert to X revisions ago).

Is cygwin's version of GCC really more 'lax' with the code it compiles? If so, are there any obvious issues to look out for when coding? Are there any alternatives for being able to write code that will run under Linux?

Edit

Thanks for the replies. I wasn't explicit enough in my original post: that there is a bug in my code was pretty much a given for me (I am quite new to programming, and really green when it comes to C, after all). My TA implied cygwin's GCC is a less reliable compiler -allowing for much sloppier code to run- than the one found under GNU/Linux. I found this strange and so had a search on the internet, but couldn't really find any references to that fact.

More than blaming the compiler vs. my code, I was wondering what the reason could be for the program to run under Windows and crash under Linux. The replies re: different memory managers and heap/stack layout under Windows/Linux were illustrating in that regard.

Would the conclusion that cygwin's GCC is just as 'good' as GNU/Linux', and it's the underlying operating systems/sheer luck that my buggy program runs under one and not the other be pretty much correct?

Regarding posting the source code, it's a homework assignment so I'd prefer to find the issue myself if at all possible :)

Edit 2

I've accepted jalf's answer as it talks about what makes the program run under Windows and not under Linux, which was what I really wanted to know. Thanks to everyone else who contributed, they were all very interesting and informative replies.

When I've found the issue and fixed it I'll upload a zip file with all the source code of this non-working version, in case anyone is curious to see what the hell I did :)

Edit 3

For those interested in seeing the code, I found the problem, and it was indeed due to pointers. I was trying to return a pointer from a function. The pointer I was trying to return was being declared inside the function and so was being destroyed once the function executed. Problematic code is commented out on lines 22-24.

Feel free to ridicule my code.

/**
*  Returns array of valid searches based on current coordinate
*/
void determine_searches(int row, int col, int last_row, int last_col, int *active_search){
    // define coordinate categories and related valid search directions
    int Library0[] = {2, 3, 4, -1};
    int Library1[] = {4, 5, 6, -1};
    int Library2[] = {2, 3, 4, 5, 6, -1};
    int Library3[] = {0, 1, 2, 3, 4, 5, 6, 7, -1};
    int Library4[] = {0, 1, 2, -1};
    int Library5[] = {0, 6, 7, -1};
    int Library6[] = {0, 1, 2, 6, 7, -1};
    int Library7[] = {0, 1, 2, 3, 4, -1};
    int Library8[] = {0, 4, 5, 6, 7, -1};

    int * Library[] = { 
        Library0, Library1, Library2,
        Library3, Library4, Library5,
        Library6, Library7, Library8,
    };

    // declare (and assign memory to) the array of valid search directions that will be returned
    //int *active_search;
    //active_search = (int *) malloc(SEARCH_DIRECTIONS * sizeof(int));


    // determine which is the correct array of search directions based on the current coordinate
    // top left corner
        int i = 0;
    if(row == 0 && col == 0){
        while(Library[0][i] != -1){
            active_search[i] = Library[0][i];
            i++;
        }
    }
    // top right corner
    else if(row == 0 && col == last_col){
        while(Library[1][i] != -1){
            active_search[i] = Library[1][i];
            i++;
        }
    }
    // non-edge columns of first row
    else if(row == 0 && (col != 0 || col != last_col)){
        while(Library[2][i] != -1){
            active_search[i] = Library[2][i];
            i++;
        }
    }
    // non-edge coordinates (no edge columns nor rows)
    else if(row != 0 && row != last_row && col != 0 && col != last_col){
        while(Library[3][i] != -1){
            active_search[i] = Library[3][i];
            i++;
        }
    }
    // bottom left corner
    else if(row == last_row && col == 0){
        while(Library[4][i] != -1){
            active_search[i] = Library[4][i];
            i++;
        }
    }
    // bottom right corner
    else if(row == last_row && col == last_col){
        while(Library[5][i] != -1){
            active_search[i] = Library[5][i];
            i++;
        }
    }
    // non-edge columns of last row
    else if(row == last_row && (col != 0 || col != last_col)){
        while(Library[6][i] != -1){
            active_search[i] = Library[6][i];
            i++;
        }
    }
    // non-edge rows of first column
    else if((row != 0 || row != last_row) && col == 0){
        while(Library[7][i] != -1){
            active_search[i] = Library[7][i];
            i++;
        }
    }
    // non-edge rows of last column
    else if((row != 0 || row != last_row) && col == last_col){
        while(Library[8][i] != -1){
            active_search[i] = Library[8][i];
            i++;
        }
    }
    active_search[i] = -1;
}

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.

Answer 2

Like others have said, you might want to post some of your code here, even if that's not the real point of your question. It might still be a good learning experience to have everyone here poke through your code and see if they can find what caused the segfault.

But yeah, the problem is that there are so many platform-dependent, as well as basically random, factors influencing a C program. Virtual memory means that sometimes, accessing unallocated memory will seem to work, because you hit an unused part of a page that's been allocated at some earlier point. Other times, it'll segfault because you hit a page that hasn't been allocated to your process at all. And that is really impossible to predict. It depends on where your memory was allocated, was it at the edge of a page, or in the middle? That's up to the OS and the memory manager, and which pages have been allocated so far, and...... You get the idea. Different compilers, different versions of the same compilers, different OS'es, different software, drivers or hardware installed on the system, anything can change whether or not you get a segfault when you access unallocated memory.

As for the TA's claim that cygwin is more "lax", that's rubbish, for one simple reason. Neither compiler caught the bug! If the "native" GCC compiler had truly been less lax, it would have given you an error at compile-time. Segfaults are not generated by the compiler. There's not much the compiler can do to ensure you get a segfault instead of a program that seemingly works.