Debugging Fortran Programs

Debugging Fortran programs that have been built with g77 can be rather confusing as behinds the scenes g77 has converted the program into C and compiled that! The following points should help and to illustrate them consider the following (rather odd) function.

      SUBROUTINE MY_SUB(IFILL)
      COMMON /MY_COMMON/ MY_ARRAY(2,3)
 
      INTEGER  IFILL
      INTEGER  IROW, ICOL
 
      DO IROW = 1, 3
        DO ICOL = 1, 2
          MY_ARRAY(ICOL,IROW) = IFILL
        ENDDO
      ENDDO
 
      RETURN
      END

1. Use g77 version 3.0 or later; earlier versions did not provide debug information for COMMON blocks so you could not examine COMMON block variables!

2. All variable, function and subroutine names are converted to lower case.

   Example: To examine IROW:-

   print irow

3. All global names (i.e. function and subroutine) names have a trailing underscore added. Any name that already contains an underscore may have another trailing underscore added.

   Example: To set a breakpoint on MY_SUB:-

   break my_sub__

4. Arguments are passed by address so to examine an argument you must add an asterisk before to show that it is a pointer to be followed.

   Example: To examine IFILL:-

   print *ifill

5. In C, multi-dimensional arrays are ordered in the inverse way compared to FORTRAN. Using any reasonably up to date version of gcc/gdb should work fine and allow you to use standard Fortran array syntax to examine them.