22
Mon, Apr
1 New Articles

Subprocedure Basics

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

In my never-ending attempt to move RPG IV programmers toward using subprocedures, I found that a basic introduction to subprocedure architecture was missing. There are several specific examples out there of using APIs or RPG-only routines, but there really isn't a good subprocedure orientation.

Subprocedures are the RPG IV implementation of ILE procedures. Most high-level languages—such as C, COBOL, Visual Basic, and Java—have some sort of subroutine feature. Even RPG III includes "subroutines" (as does RPG IV). But RPG IV subprocedures are more like the features of those other languages than subroutines.

Local Variables

Subprocedures are isolated routines that are similar to standalone programs. They allow you to declare variables and write code that is independent of the source member in which it is contained. For example, if you have a traditional RPG program with mainline calculations, you probably have several work fields declared. Often those work field names are vague at best; names such as WRK1, WRK#, or @WORK are some heart-breaking examples.

But with a subprocedure, all declared fields are unique. For example, when program A is created, its fields are separate and distinct from those of program B. Likewise, a subprocedure's fields are separate and distinct from the rest of the source member. This means that you can declare a work field with a distinctive name within a subprocedure. So rather than declaring an ambiguous field named WRK#, you could declare a field name, such as AcctBalance (or ACTBAL if you prefer shorter names).

Fields declared in subprocedures are called "local variables." The content exists while the subprocedure is running and is destroyed when the subprocedure ends. So when the subprocedure is called a second time, the fields are reinitialized. This is similar to calling a program, setting on LR in that program, returning to its caller, and then calling the same program again. But with a subprocedure, you don't set LR on or off.

Passing Parameters

Subprocedures accept parameters just like programs accept parameters. But unlike a program-to-program CALL operation in RPG, when a subprocedure is called, you should use prototyping. When a call to the subprocedure is performed, parameters are passed to the subprocedure in a way that is syntactically similar to the way CL program-to-program calls are specified. That is, you don't use PLISTs (parameter lists); you simply put the data or the variable containing the data on the call statement itself. A simple example might be something like the following:

 callp  GetBalance( custNo : actBal );

This call to the subprocedure named GetBalance passes two parameters. From the name, we can infer that the account balance for the given customer number is being retrieved. Note that this example is in free-format RPG IV, but the hybrid syntax (using the fixed-format columns) has the same syntax except for the terminating semicolon, as follows:

     C                   callp     GetBalance( custNo : actBal)

Unlike program parameters, subprocedure parameters can include extended attributes that allow you to control whether or not they can be modified, whether the value passed can be a literal or a field, and several other options.

Prototyping: What Is It?

I am often asked what the prototype is used for. Sometimes, I think prototyping was added to RPG IV just to make the compiler more anal-retentive. The fact is that prototypes, in the context of subprocedures, are simply a declaration of the parameter list used by a subprocedure. Normally, a prototype is a direct copy of the subprocedure's parameter list.

The compiler uses the prototype source code to validate that the parameter values specified when the procedure is called match what the subprocedure is expecting. Of course, you can cheat and change the parameter definitions for a prototype (a prototype is nothing more than source code after all), hence lying to the compiler and causing a runtime error.

Prototypes are usually stored in a source member and then /COPYed into the source member where the subprocedure is defined, as well as any other source member that calls the subprocedure. This means the source code of the prototype can be /COPYed into many source members; it should, therefore, never be hard-coded in the source member where the subprocedure is also defined.

Return Values

When you see a subprocedure in an expression, it is often used as follows:

   eval  balance = GetBalance( custNo );

The field named BALANCE is assigned the return value from the GetBalance() subprocedure. The return value is sent back to the calling RPG expression by the subprocedure using the RETURN operation code. Whatever value is specified in Factor 2 of the RETURN operation code is sent back to the caller and inserted into the expression in place of the call to the subprocedure. This gives you the ability to avoid modifying parameters when a simple return value is generated by the subprocedure.

A return value is not required for subprocedures. In the C language, a return value is often used as an "OK" code for the successful execution of the subprocedure; that is, if no return value is desired, one is implemented anyway to provide the user of the subprocedure a way to determine whether or not the subprocedure ran successfully.

To define the return value, specify its attributes, just as you would with a parameter, but code it on the first line of the procedure interface (i.e., parameter list), as follows:

http://www.mcpressonline.com/articles/images/2002/Subprocedure%20Basics10180600.jpg

A return value is entirely optional (as are parameters). To define a subprocedure without a return value, simply omit the return value definition, as follows:

http://www.mcpressonline.com/articles/images/2002/Subprocedure%20Basics10180601.jpg

Subprocedures can be an effective way to write code that can be used and reused over and over again. It also allows others to take advantage of the code you have written. Lastly, it gives us the ability to write libraries of routines that we can make publicly available for use by any RPG or CL application.

Bob Cozzi is a programmer/consultant, writer/author, and software developer of the RPG xTools, a popular add-on subprocedure library for RPG IV. His book The Modern RPG Language has been the most widely used RPG programming book for nearly two decades. He, along with others, speaks at and runs the highly-popular RPG World conference for RPG programmers.

BOB COZZI

Bob Cozzi is a programmer/consultant, writer/author, and software developer. His popular RPG xTools add-on subprocedure library for RPG IV is fast becoming a standard with RPG developers. His book The Modern RPG Language has been the most widely used RPG programming book for more than a decade. He, along with others, speaks at and produces the highly popular RPG World conference for RPG programmers.


MC Press books written by Robert Cozzi available now on the MC Press Bookstore.

RPG TnT RPG TnT
Get this jam-packed resource of quick, easy-to-implement RPG tips!
List Price $65.00

Now On Sale

The Modern RPG IV Language The Modern RPG IV Language
Cozzi on everything RPG! What more could you want?
List Price $99.95

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: