Question

I have a C++ class Matrix22 with an array and a default constructor:

class Matrix22{
  /* something more */
  double mat[2][2];
  Matrix22(){
    for(int i=0; i<2; i++)
      for(int j=0; j<2; j++)
        mat[i][j] = i==j ? 1.0 : 0.0;
  }
};

I used it in my program and got a segmentation fault. As the rest was quite difficult and complicated I wrote a simple test routine, that just calls Matrix22(). No more seg fault.

I then ran gdb to debug the problem. If I call the constructor from the separate test routine, gcc reserves some memory for the member mat. I can navigate through the stack and see the return address some bytes after the array.

In the main program the compiler does not reserve enough space. The first element (mat[0][0]) gets written but any futher write just overwrites the next stack frame. I can also verify that as before the constructor the command btreturns a correct backtrace, where after the critical assignment the backtrace is corrupted.

So my question is: Why does in one case the compiler (or the linker?) reserve not enough space for the array, while in the other case that is not happening?

PS: Both “test cases” are compiled with the same compiler and flags and alsolinked against the same object files.

edit:

Here is the “simple” test case that works without seg fault:

void test_Matrix22()
{
  Framework::Math::Matrix22 matrix;
}

The code with creates a seg fault is in the class ModuleShaddower (intermixed header and implementation):

class ModuleShaddower{
    public:
        ModuleShaddower(PVModule& module, const EnvironmentalSetup& setup, const Position& position);
    private:
        Matrix22 rotMatrix90;
};

ModuleShaddower::ModuleShaddower(PVModule& module, const EnvironmentalSetup& setup, const Position& position)
  : module (module), position(position), setup(setup), logger(LoggerFactory::getLoggerInstance())
{
    double mat[][2] = {{0, -1},{1, 0}}; // This line will never be reached
    rotMatrix90 = Matrix22(mat);
}

As you see, it is quite from within the rest. I will maybe try to extract the problematic code but I think this won’t help much.

Answer 1

The problem was due to the fact that two object files in different locations had the same name. In the resulting static library, that was created from that object code, sometimes the wrong file gets replaced (both were called Shaddower.o). As I renamed one of the files all went well and no more errors.

I do not know the exact origin of this problem but it is solvable like that.