Compaq Fortran
User Manual for
OpenVMS Alpha Systems

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3.3 Symbol Table and Traceback Information: Locating Run-Time Errors

Both the compiler and the OpenVMS Run-Time Library include facilities for detecting and reporting errors. You can use the OpenVMS Debugger and the traceback facility to help you locate errors that occur during program execution.

3.3.1 Effects of Error-Related Command Qualifiers

At each step in compiling, linking, and executing your program, you can specify command qualifiers that affect how errors are processed.

Table 3-2 summarizes the /DEBUG and /TRACEBACK qualifiers.

Table 3-2 /DEBUG and /TRACEBACK Qualifiers
Command Qualifier Effect
FORTRAN /DEBUG The Compaq Fortran compiler creates symbolic data needed by the debugger.


LINK /DEBUG Symbolic data created by the Compaq Fortran compiler is passed to the debugger.

Default: /NODEBUG

  /TRACEBACK Traceback information is passed to the debugger. Traceback will be produced.


RUN /DEBUG Invokes the debugger. The DBG> prompt will be displayed. Not needed if LINK/DEBUG was specified.
  /NODEBUG If /DEBUG was specified in the LINK command line, RUN/NODEBUG starts program execution without first invoking the debugger.

If an exception occurs and these qualifiers are not specified at any point in the compile-link-execute sequence, a traceback list is generated by default.

To perform symbolic debugging, you must use the /DEBUG qualifier with both the FORTRAN and LINK command lines; you do not need to specify it with the RUN command. If /DEBUG is omitted from either the FORTRAN or LINK command lines, you can still use it with the RUN command to invoke the debugger. However, any debugging you perform must then be done by specifying virtual addresses rather than symbolic names.

If you linked your program with the debugger, but wish to execute the program without debugger intervention, specify the following command:

$ RUN/NODEBUG program-name

If you specify LINK/NOTRACEBACK, you receive no traceback in the event of errors.

3.3.2 Sample Source Program and Traceback

Example 3-1 shows a sample source program and a traceback.

Example 3-1 Sample Compaq Fortran Program

       REAL ST 
       ST = 2.4E20   ! Constant inside range for REAL*4 formats 
       CALL SUB1(ST) 
       REAL RV,RES 
       RV = RV * RV  ! Generates overflow value 
       RES=LOG(RV)   ! Uses + Infinity value for IEEE floating-point data 

The program (TRACEBK.F90) shown in Example 3-1 is compiled, linked, and run to generate the traceback information shown after the RUN command:

%SYSTEM-F-HPARITH, high performance arithmetic trap, Imask=00000000, 
Fmask=00000001, summary=08, PC=0002007C, PS=0000001B
-SYSTEM-F-FLTOVF, arithmetic trap,floating overflow at PC=0002007C,PS=0000001B 
%TRACE-F-TRACEBACK, symbolic stack dump follows 
 Image Name   Module Name     Routine Name    Line Number  rel PC      abs PC 
 LINK_TRACEBA TRACE_TEST      SUB1                     10 0000007C    0002007C 
 LINK_TRACEBA TRACE_TEST      TRACE_TEST                6 00000030    00020030 
                                                        0 88EEFA50    88EEFA50

On the FORTRAN command line, the following qualifiers are used:

When an error condition is detected, you receive the appropriate message, followed by the traceback information. The Run-Time Library displays a message indicating the nature of the error and the address at which the error occurred (user PC). This is followed by the traceback information, which is presented in inverse order to the calls.

Values can be produced for relative and absolute PC, with no corresponding values for routine name and line. These PC values reflect procedure calls internal to the Run-Time Library.

Of particular interest are the values listed under "Routine Name" and "Line Number":

With this information, you can usually isolate the error in a short time.

If you specify either LINK/DEBUG or RUN/DEBUG, the debugger assumes control of execution and you do not receive a traceback list if an error occurs. To display traceback information, you can use the debugger command SHOW CALLS.

You should use the /NOOPTIMIZE qualifier on the FORTRAN command line whenever you use the debugger.

For More Information:

Chapter 4
Using the OpenVMS Debugger

This chapter is an introduction to using the OpenVMS Debugger with Compaq Fortran programs. This chapter provides the following information:

4.1 Overview

A debugger is a tool that helps you locate run-time errors quickly. It is used with a program that has already been compiled and linked successfully, but does not run correctly. For example, the output may be obviously wrong, or the program goes into an infinite loop or terminates prematurely. The debugger enables you to observe and manipulate the program's execution interactively so you can locate the point at which the program stopped working correctly.

The OpenVMS Debugger is a symbolic debugger, which means that you can refer to program locations by the symbols (names) you used for those locations in your program---the names of variables, subroutines, labels, and so on. You do not need to use virtual addresses to refer to memory locations.

By issuing debugger commands at your terminal, you can perform the following operations:

Such techniques allow you to isolate an error in your code much more quickly than you could without the debugger.

Once you have found the error in the program, you can then edit the source code and compile, link, and run the corrected version.

4.2 Getting Started with the Debugger

This section explains how to use the debugger with Compaq Fortran programs. The section focuses on basic debugger functions, to get you started quickly. It also provides any debugger information that is specific to Compaq Fortran.

For More Information:

4.2.1 Compiling and Linking a Program to Prepare for Debugging

Before you can use the debugger, you must compile and link your program. The following example shows how to compile and link a Compaq Fortran program (consisting of a single compilation unit in the file INVENTORY.F90) prior to using the debugger.


The /DEBUG qualifier on the FORTRAN command line causes the compiler to write the debug symbol records associated with INVENTORY.F90 into the object module INVENTORY.OBJ. These records allow you to use the names of variables and other symbols declared in INVENTORY.F90 in debugger commands. (If your program has several compilation units, each of the program units that you want to debug must be compiled with the /DEBUG qualifier.)

Use the /NOOPTIMIZE qualifier when you compile a program in preparation for debugging. Otherwise, the object code is optimized (to reduce the size of the program and make it run faster), so that the symbolic evaluation of some program locations may be inconsistent with what you might expect from viewing the source code. For example, a variable in an optimized program may not be available. (After debugging the program, recompile it without the /NOOPTIMIZE qualifier.) For a description of the various optimizations performed by the compiler, see Chapter 5.

The /DEBUG qualifier on the LINK command line causes the linker to include all symbol information that is contained in INVENTORY.OBJ in the executable image. This qualifier also causes the OpenVMS image activator to start the debugger at run time. (If your program has several object modules, you may need to specify the other modules on the LINK command line.)

For More Information:

On the effects of specifying the /DEBUG qualifier on the FORTRAN, LINK, and RUN command lines, see Section 2.3.12 and Section 3.3.

4.2.2 Establishing the Debugging Configuration and Interface

Before invoking the debugger, check that the debugging configuration is appropriate for the kind of program you want to debug.

The configuration depends on the current value of the logical name DBG$PROCESS. Before invoking the debugger, issue the DCL command SHOW LOGICAL DBG$PROCESS to determine the current definition of DBG$PROCESS.

The default configuration is appropriate for almost all programs. To request the default debugging configuration. the logical name DBG$PROCESS is undefined or has the value DEFAULT. For example, the following command shows when DBG$PROCESS is undefined:

%SHOW-S-NOTRAN, no translation for logical name DBG$PROCESS

To define DBG$PROCESS to have a value of DEFAULT, type:


To remove (deassign) a logical name definition, use the DEASSIGN command.

If the DECwindowstm Motif product is installed and running on your workstation, by default the OpenVMS Debugger uses the DECwindows Motif interface. To use the character cell interface on a DECwindows system, define the logical:


To define this logical name for multiple users, use other logical name tables.

To enable use of the DECwindows interface, deassign the logical:


The DECwindows interface provides a main window in which portions are updated as the program executes, including the source code, typed commands, and debugger messages. This interface provides pull-down menus and uses the kept debugger (equivalent of DEBUG/KEEP).

The examples in this chapter show the command line (character cell) interface to the OpenVMS debugger.

The character cell interface to the OpenVMS debugger provides the following debugging interfaces:

Screen mode is activated by pressed PF3 on the keypad (or type the command SET MODE SCREEN). Screen mode allows the debugger character cell interface to simultaneously display separate groups of data similar to the DECwindows interface. For example, your screen might show the source code (SRC), debugger output (OUT), and debugger command input (PROMPT) displays.

While in screen mode, use the SHOW DISPLAY command to view the predefined displays and the DISPLAY command to define a new display. To view the keypad definitions in screen mode, press PF2 on the keypad.

To leave screen mode and resume line-oriented mode, press PF1 PF3 (or type the command SET SCREEN NOSCREEN).

For More Information:

On the DECwindows Motif debugger interface (including a source browser) and screen mode, see the OpenVMS Debugger Manual.

4.2.3 Invoking the Debugger

After you compile and link your program and establish the appropriate debugging configuration, you can then invoke the debugger. To do so, enter the DCL command RUN, specifying the executable image of your program as the parameter. For example, enter the following command to debug the program INVENTORY:

         OpenVMS Alpha DEBUG Version x.x-xxx 
%DEBUG-I-INITIAL, language is FORTRAN, module set to INVENTORY
%DEBUG-I-EXITSTATUS, is '%SYSTEM-S-NORMAL, normal successful completion'

The diagnostic message that is displayed at the debugger startup indicates that this debugging session is initialized for a Compaq Fortran program and that the name of the main program unit is INVENTORY. In the initial "%DEBUG-I-INITIAL" message, the OpenVMS Debugger term "module" is equivalent to a Compaq Fortran "procedure".

When some qualifiers are used to compile (/WARNINGS=ALIGNMENT or most /CHECK keywords), the debugger does not start up in the main program. When this happens, type GO once to get to the beginning of the main program.

The DBG> prompt indicates that you can now type debugger commands. At this point, if you type the GO command, program execution begins and continues until it is forced to pause or stop (for example, if the program prompts you for input or an error occurs).

You can specify the DEBUG command /KEEP qualifier to use the kept debugger. The kept debugger allows you to run one (or more) programs with the RUN command, rerun the last program run with a RERUN command, and connect and disconnect to a running process. For example:

         OpenVMS Alpha DEBUG Version x.x-xxx
%DEBUG-I-INITIAL, language is FORTRAN, module set to SQUARES
%DEBUG-I-EXITSTATUS, is '%SYSTEM-S-NORMAL, normal successful completion'
 stepped to SQUARES\%LINE 4 
%DEBUG-I-EXITSTATUS, is '%SYSTEM-S-NORMAL, normal successful completion'

For more information on using the debugger and invoking the debugger for other display modes, see the OpenVMS Debugger Manual.

4.2.4 Debugger Commands Used Often

You can use the following debugger commands when debugging any program:

For More Information:

4.2.5 Debugger Breakpoints, Tracepoints, and Watchpoints

The OpenVMS Debugger supports breakpoints, tracepoints, and watchpoints to help you find out what happens at critical points in your program.

Set a breakpoint if you want the debugger to stop program execution at a certain point in your program (routine, line number, and so on). After you set a breakpoint and begin program execution, execution stops at the breakpoint, allowing you to look at the contents of program variables to see if they contain the correct values.

Use the following commands to control breakpoints:

Set a tracepoint to request that the debugger display messages when certain parts of your program execute. You can also specify an action for the tracepoint, such as displaying the value of a variable. Unlike breakpoints, execution continues past the tracepoint. For example, a tracepoint lets you see how many times a routine gets called.

Use the following commands to control tracepoints:

Set a watchpoint to request that the debugger stop execution when the values of certain variables (or memory locations) change. A breakpoint stops execution when a certain part of program is reached. In contrast, a watchpoint stops execution when a certain value changes.

The following commands are usually used to control watchpoints:

Before you set a breakpoint, tracepoint, or watchpoint, you can define the scope to be used (by using the SET SCOPE or SET MODULE command, for instance) or you can add a pathname prefix before a symbol name.

4.2.6 Ending a Debugging Session

To end a debugging session and return to the DCL level, type EXIT or press Ctrl/Z:


The following message, displayed during a debugging session, indicates that your program has completed normally:

%DEBUG-I-EXITSTATUS, is '%SYSTEM-S-NORMAL, normal successful completion' 

To continue debugging after seeing this message, type EXIT and start a new debugging session with the DCL RUN command.

If you specified the DEBUG command with the /KEEP qualifier when you invoked the debugger, you can run the same program again from within the debugging session (RERUN) and perform other functions.

4.2.7 Notes on Debugger Support for Compaq Fortran

In general, the debugger supports the data types and operators of Compaq Fortran and the other debugger-supported languages. However, the following are language-specific limitations and differences:

For More Information:

4.3 Sample Debugging Session

Example 4-1 shows a program called SQUARES that requires debugging. The program was compiled and linked without diagnostic messages from either the compiler or the linker. Compiler-assigned line numbers have been added in the example so that you can identify the source lines referenced in the explanatory text.

Example 4-1 Sample Program SQUARES

     1        PROGRAM SQUARES 
     2        INTEGER (KIND=4) :: INARR(20), OUTARR(20) 
     3   ! Read the input array from the data file. 
     4        OPEN(UNIT=8, FILE='datafile.dat', STATUS='OLD') 
     5        READ(8,*,END=5) N, (INARR(I), I=1,N) 
     6     5  CLOSE (UNIT=8) 
     7   ! Square all nonzero elements and store in OUTARR. 
     8        K = 0 
     9        DO I = 1, N 
     10         IF (INARR(I) .NE. 0) THEN 
     11            OUTARR(K) = INARR(I)**2 
     12         ENDIF 
     13       END DO 
     15  ! Print the squared output values.  Then stop. 
     16       PRINT 20, K 
     17   20  FORMAT (' Number of nonzero elements is',I4) 
     18       DO I = 1, K 
     19         PRINT 30, I, OUTARR(I) 
     20   30    FORMAT(' Element',I4,' has value',I6) 
     21       END DO 

The program SQUARES performs the following functions:

  1. Reads a sequence of integer numbers from a data file and saves these numbers in the array INARR (lines 4 and 5). The file DATAFILE.DAT contains one record with the integer values 4, 3, 2, 5, and 2. The first number (4) indicates the number of data items (array elements) that follow.
  2. Enters a loop in which it copies the square of each nonzero integer into another array OUTARR (lines 9 through 13).
  3. Prints the number of nonzero elements in the original sequence and the square of each such element (lines 16 through 21).

When you run SQUARES, it produces the following output, regardless of the number of nonzero elements in the data file:

Number of nonzero elements is   0

The error occurs because variable K, which keeps track of the current index into OUTARR, is not incremented in the loop on lines 9 through 13. The statement K = K + 1 should be inserted just before line 11.

Example 4-2 shows how to start the debugging session and use the debugger to find the error in the program in Example 4-1. Comments keyed to the callouts follow the example.

Example 4-2 Sample Debugging Session Using Program SQUARES

$ LINK/DEBUG SQUARES                    (2)
$ SHOW LOGICAL DBG$PROCESS              (3)
%SHOW-S-NOTRAN, no translation for logical name DBG$PROCESS
$ RUN SQUARES                           (4)
         OpenVMS Alpha DEBUG Version x.x-xxx 
%DEBUG-I-INITIAL, language is FORTRAN, module set to SQUARES
DBG> STEP 5                             (5)
stepped to SQUARES\%LINE 9
     9:         DO 10 I = 1, N
DBG> EXAMINE N,K                        (6)
SQUARES\N:       4
SQUARES\K:       0
DBG> STEP 2                             (7)
stepped to SQUARES\%LINE 11
    11:                 OUTARR(K) = INARR(I)**2
DBG> EXAMINE I,K                        (8)
SQUARES\I:       1
SQUARES\K:       0
DBG> DEPOSIT K = 1                      (9)
DBG> GO                                 (11)
Number of nonzero elements is   4
Element   1 has value     9
Element   2 has value     4
Element   3 has value    25
Element   4 has value     4
%DEBUG-I-EXITSTATUS, is 'SYSTEM-S-NORMAL, normal successful completion'
DBG> EXIT                               (12)
$ EDIT SQUARES.FOR                      (13)
10:       IF(INARR(I) .NE. 0) THEN
11:           K = K + 1
12:           OUTARR(K) = INARR(I)**2
13:       ENDIF
$ RUN SQUARES                           (15)
         OpenVMS Alpha DEBUG Version x.x-xxx
%DEBUG-I-INITIAL, language is FORTRAN, module set to SQUARES
breakpoint at SQUARES\%LINE 12
   do (EXAMINE I,K)
DBG> TYPE 7:14 
module SQUARES
     7: C ! Square all nonzero elements and store in OUTARR.
     8:         K = 0
     9:         DO I = 1, N
    10:           IF (INARR(I) .NE. 0) THEN
    11:              K = K + 1
    12:              OUTARR(K) = INARR(I)**2
    13:           ENDIF
    14:         END DO
DBG> GO                                 (17)
break at SQUARES\%LINE 12 
    12:              OUTARR(K) = INARR(I)**2
SQUARES\I:        1
SQUARES\K:        1
break at SQUARES\%LINE 12 
    12:              OUTARR(K) = INARR(I)**2
SQUARES\I:        2
SQUARES\K:        2
break at SQUARES\%LINE 12 
    12:              OUTARR(K) = INARR(I)**2
SQUARES\I:        3
SQUARES\K:        3
break at SQUARES\%LINE 12 
    12:              OUTARR(K) = INARR(I)**2
SQUARES\I:        4
SQUARES\K:        4
Number of nonzero elements is   4 
Element   1 has value     9 
Element   2 has value     4 
Element   3 has value    25 
Element   4 has value     4 
%DEBUG-I-EXITSTATUS, is '%SYSTEM-S-NORMAL, normal successful completion'
DBG> EXIT                               (18)

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