Debugging Fortran

The following tips and general concepts are provided to help you while debugging Fortran programs. For information on debugging Fortran OpenMP code with dbx, see Interacting With Events.

Current Procedure and File

During a debug session, dbx defines a procedure and a source file as current. Requests to set breakpoints and to print or set variables are interpreted relative to the current function and file. Thus, stop at 5 sets different breakpoints, depending on which file is current.

Uppercase Letters

If your program has uppercase letters in any identifiers, dbx recognizes them. You need not provide case-sensitive or case-insensitive commands, as in some earlier versions.

Fortran 95 and dbx must be in the same case-sensitive or case-insensitive mode:

• Compile and debug in case-insensitive mode without the -U option. The default value of the dbx input_case_sensitive environment variable is then false.

  If the source has a variable named LAST, then in dbx, both the print LAST or print last commands work. Fortran 95 and dbx consider LAST and last to be the same, as requested.

• Compile and debug in case-sensitive mode using -U. The default value of the dbx input_case_sensitive environment variable is then true.

  If the source has a variable named LAST and one named last, then in dbx, print last works, but print LAST does not work. Fortran 95 and dbx distinguish between LAST and last, as requested.

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  Note - File or directory names are always case-sensitive in dbx, even if you have set the dbx input_case_sensitive environment variable to false.

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Sample dbx Session

The following examples use a sample program called my_program.

Main program for debugging, a1.f:

    PARAMETER ( n=2 )
    REAL twobytwo(2,2) / 4 *-1 /
    CALL mkidentity( twobytwo, n )
    PRINT *, determinant( twobytwo )
    END

Subroutine for debugging, a2.f:

    SUBROUTINE mkidentity ( array, m )
    REAL array(m,m)
    DO 90 i = 1, m
        DO 20 j = 1, m
            IF ( i .EQ. j ) THEN
            array(i,j) = 1.
            ELSE
            array(i,j) = 0.
            END IF
20        CONTINUE
90    CONTINUE
    RETURN
    END

Function for debugging, a3.f:

    REAL FUNCTION determinant ( a )
    REAL a(2,2)
    determinant = a(1,1) * a(2,2) - a(1,2) / a(2,1)
    RETURN
    END

Running the Sample dbx Session

1. Compile and link with the- g option.

   You can do this in one or two steps.

   Compile and link in one step, with -g:

    demo% f95 -o my_program -g a1.f a2.f a3.f

   Or, compile and link in separate steps:

    demo% f95 -c -g a1.f a2.f a3.f
    demo% f95 -o my_program a1.o a2.o a3.o

2. Start dbx on the executable named my_program.

    demo% dbx my_program
    Reading symbolic information…

3. Set a simple breakpoint by typing stop in subnam, where subnam names a subroutine, function, or block data subprogram.

   To stop at the first executable statement in a main program.

    (dbx) stop in MAIN
    (2) stop in MAIN

   Although MAIN must be all uppercase, subnam can be uppercase or lowercase.

4. Type the run command, which runs the program in the executable files named when you started dbx.

    (dbx) run
    Running: my_program
    stopped in MAIN at line 3 in file "a1.f"
        3         call mkidentity( twobytwo, n )

   When the breakpoint is reached, dbx displays a message showing where it stopped, in this case, at line 3 of the a1.f file.

5. To print a value, type the print command.

   Print value of n:

    (dbx) print n
    n = 2

   Print the matrix twobytwo; the format might vary:

    (dbx) print twobytwo
    twobytwo =
       (1,1)       -1.0
       (2,1)       -1.0
       (1,2)       -1.0
       (2,2)       -1.0

   Print the matrix array:

   (dbx) print array
   dbx: "array" is not defined in the current scope
   (dbx)

   The print fails because array is not defined here, only in mkidentity.

6. To advance execution to the next line, type the next command.

   Advance execution to the next line:

   (dbx) next
   stopped in MAIN at line 4 in file "a1.f"
       4             print *, determinant( twobytwo )
   (dbx) print twobytwo
   twobytwo =
       (1,1)       1.0
       (2,1)       0.0
       (1,2)       0.0
       (2,2)       1.0
   (dbx) quit
   demo%

   The next command executes the current source line and stops at the next line. It counts subprogram calls as single statements.

   Compare the next command with the step command. The step command executes the next source line or the next step into a subprogram. If the next executable source statement is a subroutine or function call, then:

   • The step command sets a breakpoint at the first source statement of the subprogram.

   • The nextcommand sets the breakpoint at the first source statement after the call, but still in the calling program.

7. To quit dbx, type the quit command.

   (dbx)quit        
   demo%