Programming Conventions


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''The following is opinion of [[User:Adamr|Adam Richardson]] and may or may not be agreed with by other RISC OS programmers.''
There's all sorts of things to know when you write a program, and not all of them are obvious or easily discovered - particularly if you don't know what you don't know! This guide is aimed at introducing you to some conventions and resources which you might or might not already know about. There are a few references dotted about: "PRM" refers to the ''Programmer's Reference Manual'' and "SG" refers to the ''[[Style Guide]]''.
There's all sorts of things to know when you write a program, and not all of them are obvious or easily discovered - particularly if you don't know what you don't know! This guide is aimed at introducing you to some conventions and resources which you might or might not already know about. There are a few references dotted about: "PRM" refers to the ''Programmer's Reference Manual'' and "SG" refers to the ''[[Style Guide]]''.

Revision as of 03:39, 8 January 2007

The following is opinion of Adam Richardson and may or may not be agreed with by other RISC OS programmers.

There's all sorts of things to know when you write a program, and not all of them are obvious or easily discovered - particularly if you don't know what you don't know! This guide is aimed at introducing you to some conventions and resources which you might or might not already know about. There are a few references dotted about: "PRM" refers to the Programmer's Reference Manual and "SG" refers to the Style Guide.

I will assume you are already familiar with "how" to do things, just not necessarily what the end result ought to be. Of course, this is all just a guide and you may decide that you have a better way of doing things. You would be wise to understand the ideas set out below before doing things your own way, though. ;-)




Since, well, ages ago when hard-discs became commonplace, it's been recommended that you store the choices for your application in a central place on the system. This makes it easy for a user to upgrade the program, allows it to run happily from a read only medium and on modern, multi-user OSes means each user can have different choices without you, as author, needing to worry about anything at all. You should not, therefore, store choices within your application directory.

So how can you deal with this? Briefly, you just write your choices to <Choices$Write>.AppName and read them from Choices:AppName. (This is a general rule: if you are reading from Example:Path, you would write to <Example$Write>.Path.) For a more detailed explanation, take a look here. You should register AppName with the allocation service (see below).

While it is generally good practice to write (and read) choices to the locations referenced by the variables outlined above, there is one caveat. It is entirely possible for a user to be running on a system where they have no write access to most of the hard drive. (Consider FSLock in Configure.) For example, this may be in a school or office situation where the system administrator wants to prevent users from altering too much on the machines. As a minimum, therefore, your programs should fail gracefully if they find they can't write to <Choices$Write>. Ideally, you would also provide some mechanism whereby the location of your choices can be chosen by the user.

Installing Things

You should be very cautious when fiddling about inside your users' boot sequences. Try to ensure that the users know what your program is going to do, keep a log of what happened and maybe provide a means to roll it back if it goes wrong.

To install a stand-alone program in the boot sequence, use the system variable Boot$ToBeTasks (unless your program is necessary for the completion of the boot sequence, in which case it should go in Boot$ToBeLoaded).

Scrap usage

Well, this is a short one. If you want to use the Scrap directory, just read and write to <Wimp$ScrapDir>.AppName. (Where AppName is the name of your program, and/or has been allocated to you specifically.)

Distribution of Modules

In the majority of cases you should distribute your module inside your application. However, if your module is likely to be used by other programs, you might want to consider distributing it for installation into !System. This has two benefits: it means that each program which uses it does not have to supply it separately and also makes keeping the module up to date (when newer versions are released) much easier for the user.

Modules which are destined for !System should be supplied in a dummy !System application which the user can then use with their computer's in-built "System Merge" utility. The structure of the dummy application will be:


where <minOS> is the minimum version of the OS which is required to run your module. So, for example, if your module is only suitable for an Iyonix, <minOS> will be 500.


Most end-users ought to be using the UniBoot distribution for their boot sequence, but this should not be assumed! If you require the use of the nested window manager, you should check for version 3.80 of WindowManager and if it's not there you might wish to point users in the right direction.


As a minimum, your application should have a !Sprites file in it which includes a mode 12 sprite called !AppName. Note that sprite names are limited to 12 characters so, if you give your program a long name, and the first 12 characters match another program, then the sprites will get confused.

!Sprites can also contain sm!AppName which should be half the size of the main sprite, for use when the user has "small icons" showing in their directory viewer. There's no point in just shrinking your sprite without touching it up, however as the filer will automatically scale the large version of your sprite if sm!AppName is not present. If present, and if the user's screen mode supports it, !Sprites22 will be used in preference to !Sprites. It should contain a mode 20 sprite. Likewise, you may include a !Sprites11 file which has a 180dpi sprite. From RISC OS 4 onwards, you can use 256 colour sprites. If ic_taskname is present, it will be used when your windows are iconised to the pinboard. (taskname is the string given in Wimp_Initialise.)

Application sprites should be irregularly shaped, with transparent backgrounds and document sprites should have a solid outline, preferably with the snazzy little folded-corner bit! This helps the user distinguish easily and quickly between the two types of object.

Your icons should not attempt to be photographic quality pictures! They should be clear, iconic representations of whatever it is you are trying to communicate.

Application icons should go on the right of the icon bar and not have text under them, unless they are for devices. They should only be animated with good reason.

To avoid cluttering up the Wimp sprite pool, only your application sprite, iconised icon and any filetype sprites should be *IconSpriteed. If you wish to use further sprites in your program, load them into a bit of your own memory with OS_SpriteOp,&10A.

Window Design

To keep things consistent between applications you should try to stick to the standard sizes and spacings for icons and buttons in your windows.

  • Ensure that you check your window designs bearing in mind that users can (and do) customise their desktop font. At the very least, you should test your templates in 12-point Homerton. Ideally you should make use of the Wimp_TextOp SWI where your icons are likely to vary considerably in size.
  • You should clearly separate the user-changeable icons from the "action" buttons, perhaps with a horizontal ruler. In any case, your "Cancel" and "OK" buttons (in that order), should be at the bottom of the window, aligned to the right-hand side.
  • The default (e.g. responds to the Return key) button on a window should be 204x68 OS units big, while other action buttons are 188x52 OS units. You might use a small button only 140 units wide and if you need more width, adjust it in steps of 16 OS units.


It is good practise to design your program in such a way that third parties will be able to translate it if they choose. Through the use of the Territory module, RISC OS is able to deal with operating in different countries, timezones etc. and you can ensure your program deals with this appropriately by using ResFind. The full instructions for ResFind are included in its archive and are very thorough. Here's a brief description of the process to illustrate that it shouldn't involve too much work on your part :-)

Firstly, you must make sure that any user-facing messages are supplied in an editable form, away from your program's executable. (Of course, this is good practise anyhow.) You might wish to look at the MessageTrans SWIs if you're not already familiar with them.

In your !Run file (and maybe your !Boot file if you're setting Castle's help system variables), you run a little basic program called ResFind which assumes a certain directory structure within your application directory and intelligently sets up a couple of system variables depending on what it finds. From then on, you can simply refer to all your resources using the AppNameRes: path variable.

Application Structure

A common structure for your application would be:

This file is executed when an application's directory viewer is opened, or the application is Filer_Boot-ed.
This file is executed when the 'Help' item from the Filer menu is selected.
This file is executed when an application is run.
This is the conventional name given to the main executable for your application.
Either this or '!SpritesXX' is used for the application's filer sprite if no '!Boot' file is present.
This is a directory containing any resources your program might need.

Note that !Boot, !Help, !Run and !Sprites have special roles and should always be present. This structure certainly isn't set in stone however and for simple programs probably isn't that important. If you're using the ResFind system for internationalisation (described above) then you must have the Resources directory and it should contain a directory named UK. Further territories can be added like this:


Any territory-independent resources can go in !AppName.Resources


Castle have published a web page which makes various recommendations regarding implementing a help system in your programs. You can see the full document by clicking on this link, or you can read on for a summary...

  • The second entry down on an application's icon bar menu should be a Help entry which launches the program's help file. (E.g. with *Filer_Run <AppName$Dir>.!Help)
  • The application's main menu should include a Help entry as its last item. This may lead to a submenu of help sections.
  • If help is provided in some "marked up" form (e.g. HTML, StrongHelp) then a text alternative should be provided. This can be catered for by having the !Help file provide a small script which launches the appropriate file.
  • Authors should set up a number of "help" system variables, specifically:
AppName$Help: Path to help file
AppName$Version: Application's version number
AppName$Web: URL for application's home page on the Web
AppName$Title: Application's full title
AppName$Publisher: Your name!
AppName$Description: Concise description of the application

Note: it would be a good idea to make these variables dependent on territory too. You can do this by including them in an obey file which lives inside Resources.<territory>.

Setting System Variables

You should only set system variables which are currently unset in your !Boot file. For instance, your application should unilaterally set AppName$Dir in its !Run file but should be more careful about setting it in the !Boot file. For instance, you could use:

If "<AppName$Dir>" = "" Then Set AppName$Dir <Obey$Dir>

The same applies if you implement the "help" system variables.

You should make sure that you do not rely on anything which is set up in the !Boot file, as it is entirely possible that !Boot won't be seen before your program is run.


In order to avoid clashes between programs with similar names (for instance they might be resetting AppName$Dir every time they're run, or might write over each other's choices), you should ensure that your application name is allocated to you before it is released to the big wide world. This is not limited to your program's name however - there's a whole range of things which your should register, from SWI chunks to scrap locations.

For more detail, see this page. In particular, there is a link at the bottom of the page to a program which will make the whole process a a little easier.

Naming Conventions

Here's a list of a few naming conventions which you may care to follow:

  • Menu items and windows should have only their first word capitalised, unless they are proper nouns or acronyms. *Menu items which open new windows should end in "..." and if they do end in "..." they shouldn't have submenus. *SWI names should have each word capitalised with acronyms all in capitals.

General Hints

Now for a few general hints and tips to hopefully improve your programs...

Avoid Wimp_ReportError
Although Wimp_ReportError provides a quick and easy way of reporting errors and, indeed, asking questions of the user - it is not a great way of dealing with the problem. Just because your program has tied itself in knots or the user has entered something stupid, or a file is missing, or whatever, isn't really a good reason to bring the whole system to a grinding halt. Hopefully, with a little lateral thinking, you should be able to come up with a more friendly (and multitasking) scheme for giving messages to, and asking questions of, your users. (In order to address this problem in my !Run files, I wrote MultiError, which you might want to investigate.)
Don't Make Assumptions
Don't assume things about your user's systems. If there's a specific feature which your program required, test for it explicitly rather than guessing from the machine or OS version. (For example, do not assume that a user is running a 26bit machine just because the OS version is less than 5.) Make use of OS_ReadSysInfo and check for the existence of modules etc.
Backward Compatibility
It is probably reasonable, nowadays, to require at least RISC OS 4 for your programs. However, most things will still work fine on RISC OS 3.5 (especially if the user has installed UniBoot) so you might consider that as your lowest common denominator. It is not worth making any extra effort to support RISC OS 3.1.
Use the Global Clipboard
If you're writing a program which has content which could conceivably be useful in other programs (or visa versa), then make the effort to implement the global clipboard. Various programs make use of it already, including
For details, have a look here. If you'd like to go a step further and implement direct dragging of selections between applications, then look at the accompanying page on drag and drop.
Setting Choices
Lots of people don't want to spend lots of time designing and implementing elaborate configuration systems for their programs. But don't worry - there is a solution! Confix has been written specifically to address this problem. Have a look and see what you think...

How to Distribute Your Programs

Finally we come to how to distribute your programs once you've got them all polished off and ready to go! Ideally you'll have some webspace which you can use to publish your programs. If you refer to this in your documentation, it will be easy for users to find upgrades. If you're looking for free webspace, simply register with drobe and they'll give you some on their server.

To distribute your programs compress them into a zip file and upload it to your website. Make sure the filename ends in ".zip" to ensure that your website server delivers it correctly to end-users. This will also mean that the file is set to the correct type once it arrives on RISC OS (for those users with a working MimeMap installation). There are several free tools available to do the compression for you, mostly based on the open-source "InfoZip" routines. You could try try InfoZip, ZipEE, or even the command line InfoZip port. It's a good idea to test your archive in the different programs to make sure all is well and also in SparkPlug since many people will use that to unzip your archive and it's not based on "InfoZip" so is the most likely to cause you strife!

The End

Hopefully something in there has tickled your fancy. To finish off, here's a few links to some further resources which you probably already know about...

  • The PRMs can be bought pretty cheaply nowadays from your friendly local (OK, by mail order) RISC OS dealer. You should also find the StrongHelp copy of them useful though.
  • Have a look at the tofla site for an extensive collection of programming articles and resources.
  • Chris Bazley has a few items of interest here. (Including the global clipboard docs referred to above.)
  • Erik Groenhuis has some information on his website about implimenting OLE (Object Linking and Embedding) in your programs.
  • Here's some info on the nested wimp, also hosted by Erik.
  • The Google archive or FAQ of the comp.sys.acorn.programmer usenet group is well worth a trawl and if you can't find what you're after, you can always ask a question too. If you prefer a web forum, then the friendly (though low-traffic) Iconbar programming forum is well worth a visit.

Adamr 20:12, 7 Jan 2007 (GMT)

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