26
Fri, Apr
1 New Articles
  1. You are here:  
  2. Home
  3. Programming
  4. RPG
  5. TechTip: RPG IV Pointers--They're Easy!

TechTip: RPG IV Pointers--They're Easy!

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

I've heard comments about some RPG IV programmers being "pointer challenged." This bothers me, because this data type has been available since RPG IV's arrival in late 1994, almost 11 years. The pointer concept has been around since the invention of the computer! This tip explains the basics of RPG IV pointers and includes some examples.

The concept of a pointer (in any language) is that some variable contains an address of something else, rather than some kind of data item or procedure. If I refer to a standalone variable by its name, such as "Field5," and Field5 has been defined as a character variable with length 5, then every time I use Field5 I am referring to the storage (memory) address assigned by the compiler and loader. You could say that Field5 "points" to the five characters set up in storage. The address associated with Field5 is fixed. In contrast, a pointer variable contains an address that can change.

In RPG IV, a pointer variable is defined using the internal data type code of *. A quick check of the compile listing shows that the compiler sets aside 16 bytes for each pointer. Even though the i5 (or iSeries) uses a 64-bit address (8 bytes), 16 bytes are still used. Maybe IBM is planning ahead for 128-bit addressing--who knows? A pointer variable can contain either a procedure address or the address of a data item. If the pointer contains a procedure address, the keyword PROCPTR is used on the pointer definition. A pointer that is used to "point to" a data item is called a "basing pointer." Variables, data structures, and arrays may be used with basing pointers, and if so, the BASED keyword is specified and a pointer name is the value of the keyword.

Here's an important concept: When a data item is based, there is no storage allocated to it. It is a definition with a "yet to be determined" home in storage. I like to think of a based data item as a clear plastic ruler (or template) that's floating above the storage plane. It cannot be used until the pointer associated with it has a storage address. The value in the basing pointer determines where the template will be laid in the storage plane. Here is a simple example:

 

D Phrase S 25 Inz('Peter Piper picked a Plum')
 D Field5 S 5 Based(Ptr)
 D Ptr S *

 /free

 // You can't use Field5 yet.
// Now set the pointer.

 Ptr = %addr(Phrase);

// Ptr has a value, the memory address of
 // Phrase (left-most byte). Field5 can now
 // be used.

 // The current value of Field5 is 'Peter'
 // Now move the template (address in Ptr) to the right 6 bytes

 Ptr += 6;

 // The value of Field5 is now 'Piper'
 // Change Field5.

 Field5 = 'Smith';

 // The value of Phrase is now 'Peter Smith picked a Plum'

 /end-free

 

Based data structures and arrays can use dynamic storage (storage outside the program, but within the activation group). With dynamic storage, the amount of storage allocated to the program can vary as the needs of the program change. In the past, if we loaded an array with a variable number of elements, we would allocate the largest number of elements (Dim value) we thought might happen, even though the maximum number was seldom needed. Much main memory was wasted, and system performance could be adversely impacted. With a basing pointer and dynamic allocation of storage, we can allocate storage for an array as the needs arise. Here is an example of using an array that is based on a pointer and using storage from a dynamic "heap."

D Array            s           10     Dim(20000) Based(Ptr)
D Size             s            6  0
     
 /free
  // Allocate storage for the first 1000 elements
  Size = 1000 * %size(Array); // Size becomes 10,000
  Ptr  = %alloc(Size); // A pointer to the allocated storage is
                       // the return value.
  // The first 1000 elements of the array can now be loaded and
  // used. The storage allocated is not pre-initialized.

  // Additional storage can be allocated using the %realloc 
  // built-in function, using the same pointer, Ptr
 /end-free

Basing pointers are often used as parameters by APIs.

The other use of pointers is to contain the address of a procedure. The PROCPTR keyword is used on the pointer definition, and the built-in function %paddr (retrieve procedure address) can be used to load the pointer. In the following code segment, an ILE condition-handling procedure (ILEPROC) is specified on a pointer definition for use by the API prototype.

D CEEHDLR         PR
D  ptrCH                    *       Procptr
D  ptrStatus                *       Const
D  Feedback             12          Options(*Omit)

D ptrCH           s         *       Procptr(Inz(%paddr('ILEPROC'))

D Status         sds
D  Progname            *Proc

 /free
  ---
  ---
  // Register condition handler
  CallP     CEEHDLR(ptrCH:%addr(Status):*Omit);
  ---
  ---
 /end-free

Pointers provide great flexibility in programming, allowing us to write generic routines that pass pointers to data and procedures.

Jim Martin, the author of Free-Format RPG IV, is a corporate technical instructor at Jack Henry & Associates in Monett, Missouri. He is a veteran of RPG programming, beginning in 1967 with a position at IBM as a systems engineer and later becoming a staff programmer at the Rochester systems programming lab. For eight years, he was at Lakeview Technology as an AS/400 and RPG instructor and was a speaker at various local midrange user group meetings and conferences. He can be reached by email at This email address is being protected from spambots. You need JavaScript enabled to view it..

JIM MARTIN

Jim Martin holds a BS degree in mathematics and an MS in computer science. For 26 years, he was employed by IBM, where he wrote RPG applications for customers and worked in the programming laboratory as a programmer on portions of CPF and the OS/400 operating system. After leaving IBM, Jim took post-graduate work in computer science and performed RPG training. He is an IBM-certified RPG IV developer and author of multiple bestselling editions of Free-Format RPG IV, which, since the book's initial publication in 2005, have taught thousands of RPG IV programmers how to be successful with the free-format coding style.

MC Press books written by Jim Martin available now on the MC Press Bookstore.

Free-Format RPG IV: Third Edition Free-Format RPG IV: Third Edition
Improve productivity, readability, and program maintenance with the free-format style of programming in RPG IV.
List Price $59.95

Now On Sale

Free-Format RPG IV: Second Edition Free-Format RPG IV: Second Edition
>Make the transition from coding in fixed-format RPG to free format.
List Price $59.95

Now On Sale
Functions in Free-Format RPG IV Functions in Free-Format RPG IV
Here’s the ultimate guide to writing RPG IV programs with functions in the free-format style.
List Price $59.95

Now On Sale
BLOG COMMENTS POWERED BY DISQUS

LATEST COMMENTS

Support MC Press Online

$0.00 Raised:
$

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: