[Dune] Debugging tricks

[Sven Marnach]

Hi all,

though a bit off-topic, I'd like to share two solutions of problems I
stumbled on again and again while debugging, so there is a chance that
you sometimes have the same problems.

1. Floating point exceptions

   By default, the FPU doesn't throw floating exceptions on errors.
   When, for example, dividing by zero, the FPU just returns "nan" and
   the program continues computing.  That's usually not the kind of
   behaviour a programmer of numerical software wants.

   glibc users can enable floating point excetions by putting

     #include <fenv.h>
     //...
     int main()
     {     
         feenableexcept (FE_INVALID|FE_DIVBYZERO|FE_OVERFLOW);
         //...
     }

   in the source code.  Now, your program receives SIGFPE each time a
   floating point exception of one of the specified types occurs.  By
   default, this will make the program abort and print an error
   message.  You could also install a signal handler using signal(2).

   Note that the FPU runs asynchronously, so your program receives the
   signal not exactly at the instruction that caused the exception (so
   the backtrace in gdb might be corrupted).

   You can also put the call to feenableexcept into the initialization
   code of a library and link your program with that library, see

     http://gcc.gnu.org/onlinedocs/gcc-3.3.6/g77/Floating_002dpoint-Exception-Handling.html

2. Printing STL vectors in GDB

   ...is quite inconvenient.  It will get a whole lot easier if you
   put the following lines in your ~/.gdbinit:

     define pvec
         p *&$arg0[0]@$arg0.size()
     end

     document pvec
     pvec <vector>: print an STL vector in readable format
     end

   You can customise the display format by

      set print array-indexes
      set print array

Greetings from Heidelberg,
    Sven