User's Guide

A Sample dbx Session for an XL Fortran Program

You can debug XL Fortran programs with any dbx-compatible symbolic debugger. For background information on dbx, see the AIX General Concepts and Procedures book. For information on dbx subcommands, see the AIX Commands Reference.

The following example represents a typical XL Fortran problem that you may be able to resolve through dbx. Although it demonstrates only a small subset of dbx features and uses memory-allocation techniques made obsolete by Fortran 90/Fortran 95 allocatable arrays, it can serve as an introduction if you have not used this debugger before.

Problem with Dynamic Memory Allocation

The following program tries to allocate an array at run time by using the AIX system subroutine malloc. When you use the following command to compile the program and then run the program, the program produces a core dump:

   xlf95 -qddim testprog.f -o testprog

At this point, you may be wondering whether the C malloc routine is working correctly or whether this is the right way to allocate an array in a main program when the dimensions are not known until run time.

          program main
 
	  pointer(p, array(nvar,nrec))
	  real*8 array
 
	  nvar = 2
	  nrec = 3
	  p = malloc(nvar*nrec*8)
 
	  call test_sub(array, nvar, nrec)
 
	  end
 
	  subroutine test_sub(array, nvar, nrec)
 
	  dimension array(nvar, nrec)
 
	  array(1,1) = 1.
	  array(2,1) = 2.
	  array(1,2) = 3.
	  array(2,2) = 4.
	  array(1,3) = 5.
	  array(2,3) = 6.
 
	  write(*, 100) array(1,1), array(2,1), array(1,2),
     1          array(2,2), array(1,3), array(2,3)
  100     format(//t2,f4.1/t2,f4.1/t2,f4.1/t2,f4.1/
     1          t2,f4.1/t2,f4.1)
 
	  return
	  end

You might go through the debugging process as follows:

1. Compile the program with the -g option, to allow debugging under dbx:

   
   

    -> xlf95  -qddim -g testprog.f -o testprog
   ** main   === End of Compilation 1 ===
   ** test_sub   === End of Compilation 2 ===
   1501-510  Compilation successful for file testprog.f.
   

2. Run the program to verify the problem and create a core dump:

   
   

    -> testprog
   Segmentation fault(coredump)
    ->
   

3. Find out where in the program the core dump occurs:

   
   

    -> dbx testprog core
   dbx version 3.1 for AIX.
   Type 'help' for help.
   reading symbolic information ...
   [using memory image in core]
    
   segmentation violation in test_sub at line 21 in file "testprog.f"
      21             array(1,1) = 1.
   (dbx)
   

4. Use the where command to get a traceback of the calls that led to that point in the program:

   
   

   (dbx) where
   test_sub(array = (...), nvar = warning: Unable to access address 0x200aee94 
    from core
   -1, nrec = warning: Unable to access address 0x200aee98 from core
   -1), line 21 in "testprog.f"
   main(), line 12 in "testprog.f"
   (dbx)
   

   main calls test_sub at line 12. The warning indicates that a problem occurs while evaluating the arguments for this call.

5. Look at the value of the first argument of the array:

   
   

   (dbx) print array(1,1)
   reference through nil pointer
   (dbx)
   

   This suggests that array does not have a value assigned. To verify that possibility, try to look at the address of an element in the array:

   
   

   (dbx) p &array(1,1)
   (nil)
   (dbx)
   

   It seems that XL Fortran has not allocated the space for the array. To verify that it has not, print the value of the pointer that points to the array:

   
   

   (dbx) print p
   warning: Unable to access address 0x200aee90 from core
   0xffffffff
   

6. To find out what happens to p during execution, restart the program and trace the use of p:

   
   

   (dbx) stop in main
   [1] stop in main
   (dbx) run
   [1] stopped in main at line 7 in file "testprog.f"
       7             nvar = 2
   (dbx) trace p
   [3] trace p
   (dbx) cont
   initially (at line 8 in "testprog.f"):  p = nil
    
   segmentation violation in test_sub at line 21 in file "testprog.f"
      21             array(1,1) = 1.
   (dbx) p p
   nil
   (dbx)
   

   Because p is never set to a valid value, something must be wrong with the line that allocates space for the array:

   
   

        9            p = malloc(nvar*nrec*8)
   

7. The next step is to research why the call to malloc does not work. Because malloc is a C function, you should read Chapter 10, Interlanguage Calls for background knowledge and specific guidelines.

   When you read that section, you find that calls to C functions require arguments to be passed by value, rather than by reference. To fix the problem in this sample program, replace the line:

   p = malloc(nvar*nrec*8)
   

   with the line:

   p = malloc(%val(nvar*nrec*8))
   

8. Compiling and running the fixed program (solution.f) again produces the correct result:

   
   

    -> xlf95  -qddim -g solution.f -o solution
   ** main   === End of Compilation 1 ===
   ** test_sub   === End of Compilation 2 ===
   1501-510  Compilation successful for file solution.f.
    -> solution
    
    
     1.0
     2.0
     3.0
     4.0
     5.0
     6.0
   

9. It might be informative to trace the execution of the corrected program:

   
   

    -> dbx solution
   dbx version 3.1 for AIX.
   Type 'help' for help.
   Core file program (testprog) does not match current program (core ignored)
   reading symbolic information ...
   (dbx) trace p
   [1] trace p
   (dbx) run
   initially (at line 7 in "solution.f"):  p = nil
   after line 9 in "solution.f":   p = 0x200af100
    
    
     1.0
     2.0
     3.0
     4.0
     5.0
     6.0
    
   execution completed
   (dbx)
   

To check whether the values of p and array are appropriate, turn off the trace:

(dbx) status
[1] trace p
(dbx) delete all
(dbx) status
(dbx)

Then set new break points and run through the program again. Notice that the address of array(1,1) is the same as the contents of p(0x200af100), as expected:

(dbx) stop at 9
[11] stop at "solution.f":9
(dbx) run
[11] stopped in main at line 9 in file "solution.f"
    9             p = malloc(%val(nvar*nrec*8))
(dbx) p p
nil
(dbx) next
stopped in main at line 12 in file "solution.f"
   12             call test_sub(array, nvar, nrec)
(dbx) p p
0x200af100     <-------------
(dbx)
(dbx) step      /* Notice we use step to step into subroutine test_sub. */
stopped in test_sub at line 21 in file "solution.f"
   21             array(1,1) = 1.
(dbx) p &array(1,1)
0x200af100     <---------------
(dbx) next
stopped in test_sub at line 22 in file "solution.f"
   22             array(2,1) = 2.
(dbx) p array(1,1)
1.0
(dbx)