Question

I am using a C++ library that provides an object that, for the sake of simplicity, is more or less like this:

class ExampleSO {
    public double* narray;
};

I have an instance of ExampleSO whose narray is about 200. Some other method ExampleSO::method() does a lot of arithmetic functions with this array and assigns it to different array elements:

ExampleSO::method() {
     // a lot of operations
     narray[50] = narray[1] * narray[2] / narray[40];
     // and so on

This code is generated by another program and it uses a bunch of defines to handle the array elements, so the code looks like this:

#define A narray[0]
#define X narray[1]
#define Y narray[2]
// ...
#define Z narray[40]
// ....
#define U narray[50]
// ... more, until narray[199]
ExampleSO::method() {
     // a lot of operations
     U = X * Y / Z;
     // and so on
}

My problem is that eventually some array elements are NaN and I am trying to debug the code to see why. I have already found out some of them, which are mainly caused by divisions by zero, others by exponentiation by very small numbers (small as in between 0 and +/-0.1).

With my little knowledge of gdb magic, I managed to see the array elements by display *(this->narray) @ 200, but this array is very big and therefore, unreadable.

So debugging this piece of code has turned out to be a bundersome task, because the #defines hide me the position of the element, the array is way too big and because so many elements become NaN that I get lost.

My question is: what ideas/suggestions do you have to help me debug this code? Perhaps a conditional breakpoint when the first array element becomes NaN would be useful? How could I do that with such structure?

Thanks!

Answer 1

1. Rewrite it. The structure you describe is horrible beyond description.
2. Write a python script to turn the #defines into gdb variable aliases, so that you can refer to them symbolically.
3. Use array syntax in gdb: p narray[12]
4. Add some debugging helper functions and call them from the debugger: p printMyFavoriteValues(narray)
5. Learn how to enable signalling NaNs. It is different on Windows versus Mac versus Linux.

#ifdef DARWIN
    _mm_setcsr( _MM_MASK_MASK &~
              (_MM_MASK_OVERFLOW|_MM_MASK_INVALID|_MM_MASK_DIV_ZERO) );
#else
    feenableexcept(FE_DIVBYZERO | FE_UNDERFLOW | FE_OVERFLOW | FE_INVALID);
#endif