22
Sun, Dec
3 New Articles
  1. You are here:  
  2. Home
  3. Programming
  4. Programming - Other
  5. General
  6. TechTip: Discover the Debug Built-ins of the IBM i System Debugger

TechTip: Discover the Debug Built-ins of the IBM i System Debugger

General
Typography
  • Smaller Small Medium Big Bigger
  • Default Helvetica Segoe Georgia Times

Learn a way to make debugging a bit easier.

Written by Junlei Li

I used to debug programs under Linux with gdb. The versatile supports for expressions provided by gdb allow programmers to evaluate expressions containing any kind of constants, variables, High-Level Language (HLL)-specific operators, type casts, and even function calls. The IBM i System Debugger supports neither calls to user procedures nor calls to language-specific library routines or built-in functions (BIFs)—for example, RPG BIFs, COBOL intrinsic functions, or CL BIFs. Well, the IBM i System Debugger supports a number of debug built-ins that can be used in expressions within a debug command.

 

The debug BIFs supported by the System Debugger include %SUBSTR, %ADDR, %INDEX, %VAR, and %LOCALVARS. Note that not every debug BIF can be used when debugging a program written in a specific HLL. Debug BIFs available for debugging different IBM i HLLs are listed in the following table.

 

Debug BIFs

HLL

%SUBSTR

%ADDR

%INDEX

%VAR

%LOCALVARS

RPG

Y

Y

Y

Y

Y

COBOL

Y

Y

N

Y

Y

CL

Y

Y

N

Y

N

C/C++

N

N

N

N

Y

Java

N

N

N

N

Y

 

Documents about the Debug BIFs can be found in language-specific manuals, including these:

 

%SUBSTR

The %SUBSTR debug BIF allows you to substring a string variable. The syntax for the %SUBSTR BIF is as follows:

 

%SUBSTR(identifier   start-element number-of-elements)

 

The first parameter, identifier, must be a string identifier; the second parameter, start-element, is the starting position; and the third parameter, number-of-elements, is the number of single-byte or double-byte characters. In addition, the second and third parameters must be positive integer literals. Parameters are delimited by one or more spaces.

 

%SUBSTR can be used to do the following:

  • Evaluate a substring of a string identifier.
  • Assign a portion of a string identifier.
  • Use a portion of a string identifier in the WHEN expression of a conditional break debug command.
  • Use a portion of a string identifier in a WATCH expression of the WATCH debug command.

 

Debug the following RPG program:

 

     d ptr            s               *   inz(%addr(a))

     d a               s             8a   inz('aaaaa')

     d b               s             8a   inz('bbbbb')

     d

     /free

           *inlr = *on; // Line 6

     /end-free

 

In the debug session, when the program is stopped at line 6, enter the following debug commands:

 

> EVAL %substr(a 1 2)

   %SUBSTR(A 1 2) = 'aa'

> EVAL %substr(b 1 2)

   %SUBSTR(B 1 2) = 'bb'

> EVAL %substr(a 1 2)=%substr(b 1 2)

   %SUBSTR(A 1 2)=%SUBSTR(B 1 2) = 'bb'

> EVAL a

A = 'bbaaa   '

> BREAK 10 when %substr(a 1 2)='cc'

> WATCH %substr(a 1 2)

 

Now, the Display Watch display might look like this:

 

                         Display Watch

 

Watch Number . . . :   1

Address   . . . . . :   E720569891071378

Length . . . . . . :   2

Number of Hits . . :   0

 

Scope when watch was set:

   Program/Library/Type:   A171         LSBIN       *PGM

 

   Module . :   A171

   Procedure:   A171

   Variable :   %SUBSTR(A 1 2)

 

F12=Cancel

 

%ADDR

The %ADDR debug BIF can be used to retrieve the address of a variable for display or assign the address of a variable to a pointer variable. See the following RPG example:

 

     d info_t         ds                 qualified

     d     a                           10a

     d     b                          10i 0

     d i1             ds                 likeds(info_t)

     d i2             ds                 likeds(info_t)

     d ptr             s               *

     d i               ds                 likeds(info_t)

     d                                    based(ptr)

 

     /free

           i1.a = 'A';

           i1.b = 1;

           i2.a = 'B';

           i2.b = 2;

 

           *inlr = *on; // Line 16

     /end-free

 

In the debug session, when the program is stopped at a breakpoint at line 16, issue the following command. The output might be as shown:

 

> EVAL i1

   I1.A = 'A         '

   I1.B = 1

> EVAL i2

   I2.A = 'B         '

   I2.B = 2

> EVAL ptr = %addr(i1)

   PTR = %ADDR(I1) = SPP:E720569891077310

> EVAL i

   I.A = 'A         '

I.B   = 1

%INDEX

The %INDEX debug BIF can be used to change the index of a table or multiple-occurrence data structure. See the following RPG example:

 

     d x               ds                 occurs(5)

     d     c                           10a

     d     n                           10i   0

 

     c       1             occur     x

     c                   eval     c = 'one'

     c                   eval     n = 1

     c       2             occur     x

     c                   eval     c = 'two'

     c                  eval     n = 2

     c

     c                   seton                                       lr

 

In the debug session, when the program is stopped at the SETON LR line, enter the following debug commands:

 

> EVAL c

   C = 'two       '

> EVAL n

N =   2

> EVAL x=%index(1)

   X=%INDEX(1) = 1

> EVAL c

   C = 'one       '

> EVAL n

   N = 1

%VAR

The %VAR debug BIF identifies the specified parameter as a variable. The syntax is as follows:

 

   %VAR(identifier)

 

It is used to tell the debugger that identifier is not a debug keyword. If you were trying to debug the following RPG program, you would need the %VAR BIF quite a lot.

 

     d eval           s             1a

     d break           s             1a

     d watch           s             1a

     d pgm             s             1a

     d step           s             1a

     d clear           s             1a

     d qual           s             1a

     /free

           *inlr = *on; // Line 9

     /end-free

 

In the debug session, when the program is stopped at line 9, enter the following debug commands:

 

> EVAL eval

   Syntax error occurred.

> EVAL break

   Syntax error occurred.

> EVAL watch

   Syntax error occurred.

> EVAL pgm

   Syntax error occurred.

> EVAL step

   Syntax error occurred.

> EVAL clear

   Syntax error occurred.

> EVAL qual

   Syntax error occurred.

> EVAL %var(eval)

   EVAL = ' '

%LOCALVARS

An EVAL %LOCALVARS debug command will result in the evaluation of every variable originating in the current scope. See the following Java example.

 

/// @file ahya.java

public class ahya {

   public static final String _stat = "When   and where";

   public static void main(String[] args) {

       String a = "abc";

       int b = 456;

       byte[] c = new byte[] {1, 2, 3};

 

       System.out.println("AH YA?! ....   ..."); // Line: 9

   }

}

 

In the debug session, when the program is stopped at a breakpoint at line 9, issue the EVAL %localvars command. The output might be as follows:

 

> EVAL %localvars

   args = ARR:DF727F1B0E004FE0

   a = abc

   b = 456

   c = ARR:DF727F1B0E005000

Go Forth and Debug

I hope these debug BIFs are as helpful to you as they have been to me.

BLOG COMMENTS POWERED BY DISQUS

LATEST COMMENTS

Support MC Press Online

$

Book Reviews

Resource Center

  • SB Profound WC 5536 Have you been wondering about Node.js? Our free Node.js Webinar Series takes you from total beginner to creating a fully-functional IBM i Node.js business application. You can find Part 1 here. In Part 2 of our free Node.js Webinar Series, Brian May teaches you the different tooling options available for writing code, debugging, and using Git for version control. Brian will briefly discuss the different tools available, and demonstrate his preferred setup for Node development on IBM i or any platform. Attend this webinar to learn:

  • SB Profound WP 5539More than ever, there is a demand for IT to deliver innovation. Your IBM i has been an essential part of your business operations for years. However, your organization may struggle to maintain the current system and implement new projects. The thousands of customers we've worked with and surveyed state that expectations regarding the digital footprint and vision of the company are not aligned with the current IT environment.

  • SB HelpSystems ROBOT Generic IBM announced the E1080 servers using the latest Power10 processor in September 2021. The most powerful processor from IBM to date, Power10 is designed to handle the demands of doing business in today’s high-tech atmosphere, including running cloud applications, supporting big data, and managing AI workloads. But what does Power10 mean for your data center? In this recorded webinar, IBMers Dan Sundt and Dylan Boday join IBM Power Champion Tom Huntington for a discussion on why Power10 technology is the right strategic investment if you run IBM i, AIX, or Linux. In this action-packed hour, Tom will share trends from the IBM i and AIX user communities while Dan and Dylan dive into the tech specs for key hardware, including:

  • Magic MarkTRY the one package that solves all your document design and printing challenges on all your platforms. Produce bar code labels, electronic forms, ad hoc reports, and RFID tags – without programming! MarkMagic is the only document design and print solution that combines report writing, WYSIWYG label and forms design, and conditional printing in one integrated product. Make sure your data survives when catastrophe hits. Request your trial now!  Request Now.

  • SB HelpSystems ROBOT GenericForms of ransomware has been around for over 30 years, and with more and more organizations suffering attacks each year, it continues to endure. What has made ransomware such a durable threat and what is the best way to combat it? In order to prevent ransomware, organizations must first understand how it works.

  • SB HelpSystems ROBOT GenericIT security is a top priority for businesses around the world, but most IBM i pros don’t know where to begin—and most cybersecurity experts don’t know IBM i. In this session, Robin Tatam explores the business impact of lax IBM i security, the top vulnerabilities putting IBM i at risk, and the steps you can take to protect your organization. If you’re looking to avoid unexpected downtime or corrupted data, you don’t want to miss this session.

  • SB HelpSystems ROBOT GenericCan you trust all of your users all of the time? A typical end user receives 16 malicious emails each month, but only 17 percent of these phishing campaigns are reported to IT. Once an attack is underway, most organizations won’t discover the breach until six months later. A staggering amount of damage can occur in that time. Despite these risks, 93 percent of organizations are leaving their IBM i systems vulnerable to cybercrime. In this on-demand webinar, IBM i security experts Robin Tatam and Sandi Moore will reveal:

  • FORTRA Disaster protection is vital to every business. Yet, it often consists of patched together procedures that are prone to error. From automatic backups to data encryption to media management, Robot automates the routine (yet often complex) tasks of iSeries backup and recovery, saving you time and money and making the process safer and more reliable. Automate your backups with the Robot Backup and Recovery Solution. Key features include:

  • FORTRAManaging messages on your IBM i can be more than a full-time job if you have to do it manually. Messages need a response and resources must be monitored—often over multiple systems and across platforms. How can you be sure you won’t miss important system events? Automate your message center with the Robot Message Management Solution. Key features include:

  • FORTRAThe thought of printing, distributing, and storing iSeries reports manually may reduce you to tears. Paper and labor costs associated with report generation can spiral out of control. Mountains of paper threaten to swamp your files. Robot automates report bursting, distribution, bundling, and archiving, and offers secure, selective online report viewing. Manage your reports with the Robot Report Management Solution. Key features include:

  • FORTRAFor over 30 years, Robot has been a leader in systems management for IBM i. With batch job creation and scheduling at its core, the Robot Job Scheduling Solution reduces the opportunity for human error and helps you maintain service levels, automating even the biggest, most complex runbooks. Manage your job schedule with the Robot Job Scheduling Solution. Key features include:

  • LANSA Business users want new applications now. Market and regulatory pressures require faster application updates and delivery into production. Your IBM i developers may be approaching retirement, and you see no sure way to fill their positions with experienced developers. In addition, you may be caught between maintaining your existing applications and the uncertainty of moving to something new.

  • LANSAWhen it comes to creating your business applications, there are hundreds of coding platforms and programming languages to choose from. These options range from very complex traditional programming languages to Low-Code platforms where sometimes no traditional coding experience is needed. Download our whitepaper, The Power of Writing Code in a Low-Code Solution, and:

  • LANSASupply Chain is becoming increasingly complex and unpredictable. From raw materials for manufacturing to food supply chains, the journey from source to production to delivery to consumers is marred with inefficiencies, manual processes, shortages, recalls, counterfeits, and scandals. In this webinar, we discuss how:

  • The MC Resource Centers bring you the widest selection of white papers, trial software, and on-demand webcasts for you to choose from. >> Review the list of White Papers, Trial Software or On-Demand Webcast at the MC Press Resource Center. >> Add the items to yru Cart and complet he checkout process and submit

  • Profound Logic Have you been wondering about Node.js? Our free Node.js Webinar Series takes you from total beginner to creating a fully-functional IBM i Node.js business application.

  • SB Profound WC 5536Join us for this hour-long webcast that will explore:

  • Fortra IT managers hoping to find new IBM i talent are discovering that the pool of experienced RPG programmers and operators or administrators with intimate knowledge of the operating system and the applications that run on it is small. This begs the question: How will you manage the platform that supports such a big part of your business? This guide offers strategies and software suggestions to help you plan IT staffing and resources and smooth the transition after your AS/400 talent retires. Read on to learn: