TweetFollow Us on Twitter

March 95 - Print Hints


Writing QuickDraw GX Drivers With Custom I/O and Buffering


[IMAGE 073-077_Print_Hints_html1.GIF]

One of the great features of QuickDraw GX is that it provides the printer driver developer with default implementations of commonly used routines. For example, just by specifying a few parameters in your driver's 'comm' (gxDeviceCommunicationsType) resource, your printer driver can connect to a printer either serially or through the Printer Access Protocol (PAP). You don't need to write a single line of communications code!

Another powerful feature of QuickDraw GX is that you can ignore the default implementations of printer driver routines and write your own routines instead. This feature enables you to tailor your printer driver so that it can accommodate unique situations. The ability to modify bits and pieces of the printing system is especially useful when it comes to writing printer drivers with custom communications code or buffering routines.

In general, to create custom communications code, youconfigure your driver's 'iobm' (gxUniversalIOPrefsType)resource, create a "not connected" 'comm' resource, and then override certain QuickDraw GX messages. For SCSI printers, however, you don't need to create the "not connected" resource, because a SCSI format of the 'comm' resource is already defined. We'll talk more about when you would want to use custom communications code, and how to write it, later in this column.

Also covered is how to write custom buffering routines. You may want to use custom buffering if, for example, you already have code that you want to use or you want to increase printer performance by taking advantage of a hardware buffer that you have available.

On this issue's CD, you'll find a sample printer driver called CustomWriter that illustrates how to implement "not connected" custom I/O and buffering. In addition, there's a sample LaserWriter IISC printer driver that shows how to create custom I/O code for a SCSI printer.

The default communications code in QuickDraw GX handles asynchronous communications for serial and PAP printers and QuickDraw GX shared printers. Even so, you may want to override this code in some cases, such as if your printer communicates using a protocol that QuickDraw GX doesn't support (like 200 Kbits/second serial), or if you have your own PAP code that you'd like to continue using.

QuickDraw GX also supports the special cases of "not connected" printers and SCSI printers. If you're writing a driver using either of these two types of connections, you'll need to write some custom I/O code. In fact, the "not connected" communications method is provided specifically for the developer writing a driver containing custom communications code. What does this type of communications method do? In the default implementation, nothing at all. In a minute, you'll see how to use this to your advantage.

The only SCSI support currently built into QuickDraw GX handles filling out the Chooser list with your devices' SCSI addresses and saving updated 'comm' resources for any desktop printers that are created. Otherwise, QuickDraw GX doesn't actually open connections or try to send commands, such as SCSIRead or SCSIWrite, to the printer. SCSI printers usually have unique command sets, and trying to provide a generic mechanism to support all of these devices is unrealistic. As a result, you must provide your own communications code if you're writing a SCSI driver.

Finally, if your device is connected through a hardware interface that QuickDraw GX doesn't provide default support for (such as a NuBusTM card), you'll need to provide all of the communications code for your driver.

The first step in writing a driver with custom communications code is to configure your driver's 'iobm' resource. This is a very easy (and very critical) exercise.

'iobm' stands for "Input/Output and Buffering preferences." So what does the "m" stand for? Great question. As it turns out, if you set up this resource incorrectly, it becomes an "I/O BooM" resource. (The system crashes.) The "m" is silent as long as the resource is set up correctly. *

The 'iobm' resource tells QuickDraw GX how your driver wants its communications and buffering environment set up. This resource has the following format:

type gxUniversalIOPrefsType {
    longint standardIO = 0x00000000,
            customIO = 0x00000001;
    longint;        // number of buffers to allocate,
                    // 0 = none
    longint;        // size of each buffer
    longint;        // number of IO requests that can
                    // be pending at any one time
    longint;        // open/close time-out in ticks
    longint;        // read/write time-out in ticks

The 'iobm' resource was described in thedevelop Issue 20 Print Hints column about QuickDraw GX buffering. Rather than reiterate that information here, we're going to briefly focus on the first three fields of the resource.

The first item in the 'iobm' resource (standardIO or customIO) tells QuickDraw GX whether you want to use its built-in communications code. You must specify customIO if you want to use your own custom I/O code. When customIO is specified, QuickDraw GX won't go through the overhead of initialization and data allocation for the internal communications routines; as a result, youmust override certain messages, as described in a following section. When you specify customIO, the last three longint fields of this resource are ignored.

The two fields in the 'iobm' resource that follow the I/O type field indicate the number and size of the buffers your driver would like QuickDraw GX to create. Note that you can use QuickDraw GX's built-in buffering even if you're writing your own communications code. If, however, you're creating and disposing of your own buffers, you should set the "number of buffers" field to 0, so that QuickDraw GX won't waste time and memory allocating buffers that are never used. For code that communicates synchronously, multiple buffers don't improve performance, so you should set this field to 1.

Later in this column we'll take a closer look at what's required to create and manage your own I/O buffers.

Unless you're writing custom I/O routines to support a SCSI printer, you'll want to create a "not connected" 'comm' resource for your driver. Below is the declaration of a 'comm' resource for the "not connected" case.

For the full description of a 'comm' resource, see Inside Macintosh: QuickDraw GX Printing Extensions and Drivers .*

type gxDeviceCommunicationsType {
    unsigned longint = 'nops';

There's not a whole lot to it, is there? When you specify customIO in your 'iobm' resource, QuickDraw GX never does anything with your desktop printer's 'comm' resources other than examine the first longint. So, all sorts of possibilities become apparent. As long as that first longint is 'nops', you can extend the definition of this resource to suit your needs. Whether to change the definition of the resource in the PrintingResTypes.r interface file or not is up to you. Instead, you could just resize the resource when you update it at desktop printer creation time, as we'll discuss momentarily.

When you supply your own I/O routines, there are several messages that you need to override. Some of these messages will always need to betotally overridden, meaning that your overrides for these messages should never forward the messages. Other messages should bepartially overridden, in which case the message is forwarded at some point in your override code.

Now we'll look at the messages you need to override. (Table 1 summarizes these messages and the ones to override for custom buffering.) If you want more information on writingmessage overrides, see Sam Weiss's article, "DevelopingQuickDraw GX Printing Extensions," in develop Issue 15, andInside Macintosh: QuickDraw GX Printing Extensions and Drivers .

Table 1. Overriding QuickDraw GX messages

When to OverrideCustom I/OCustom Buffering
Always override (partially)GXOpenConnectionGXOpenConnection
Always override (totally)GXDumpBufferGXBufferData
Usually override (partially)GXDefaultDesktopPrinter
Sometimes override (totally)GXFreeBuffer

Always override (partially)

  • GXOpenConnection
  • GXCloseConnection
  • GXCleanupOpenConnection
  • GXWriteData

When you override these messages, you should first forward the message, then execute your added code. Your overrides for the first three messages should contain code to open and close a connection to your device.

GXCloseConnection is sent to close a connection if no errors occur during the device communications phase of printing; GXCleanupOpenConnection is sent if an error does occur during this time. The goal for both of these overrides is to "undo" any data allocation or initialization that occurred in the GXOpenConnection override. Often, your GXCleanupOpenConnection message override can simply execute the same code as your GXCloseConnection override.

The GXWriteData override should forward the message(with a nil pointer and a length of 0) to flush any data that's buffered, and then send the data to the printer.

Always override (totally)

  • GXDumpBuffer

Your override for this message should execute code that sends the indicated data to your printer. When this message is sent, a connection to your device will already have been established through the successful execution of your GXOpenConnection override. The GXDumpBuffer message is used to send data to the printer whenever an I/O buffer becomes full.

Usually override (partially)

  • GXDefaultDesktopPrinter
  • GXChooserMessage

When QuickDraw GX creates a desktop printer, it stores in it a 'comm' resource that specifies how to communicate with the printer. By default, this 'comm' resource is just a filled-in copy of one of your driver's 'comm' resources. Depending on the setting in the Chooser's "Connect via:" menu, the 'comm' resource is updated with information about the printer, such as the selected serial port, SCSI address, and network address for an AppleTalk printer. If your driver uses a "not connected" 'comm' resource (as described earlier), it will be copied verbatim, without updated information about the selected printer. As a result, you might need to step in and fill out the resource yourself.

To update the 'comm' resource, you need to override the GXDefaultDesktopPrinter message as shown in Listing 1. Here we forward the message so that QuickDraw GX completes creation of the desktop printer; then we retrieve the 'comm' resource from the desktop printer, update it, and replace the old version with the updated version.

When you update the 'comm' resource, you need to know which printer the user selected, as well as its addressing information and so forth. You can find this information by overriding GXChooserMessage, which is sent by the GXHandleChooserMessage API call. In this override, possibly with some help from your Chooser PACK's LDEF, you can determine the relevant information about the selected printer.

For example, you can store this information in a column of cells that's appended to the printer list. Or you can store it in the list record's userHandle or, by using PC-relative addressing, in a data storage area following your jump table. When you retrieve this data in your GXChooserMessage override, simply store it using one of QuickDraw GX's global data functions for printing. Finally, retrieve the information from your GXDefaultDesktopPrinter override and store it in the desktop printer's 'comm' resource.

It's important to note that you can't use any of the functions GetMessageHandlerInstanceContext, SetMessageHandlerInstanceContext, GXGetJobRefCon,GXSetJobRefCon, and NewMessageGlobals for the example in Listing 1, because the GXChooserMessage and GXDefaultDesktopPrinter messages are sent to two different message handler instances. Therefore, you should use GetMessageHandlerClassContext, SetMessageHandlerClassContext, or some other method that works across message handler instances.

Sometimes override (totally)

  • GXFreeBuffer

If your communications code runs asynchronously, you must override GXFreeBuffer so that QuickDraw GX can tell when operations on a buffer have completed. The GXFreeBuffer message is sent to make sure that all the data in the buffer has been processed before the buffer is used again. When GXFreeBuffer returns, the indicated buffer is ready to accept more data. An override for this message should loop (calling GXJobIdle) until I/O on the specified buffer is complete, and then return.

Listing 1. Updating a 'comm' resource when a desktop printer is created

OSErr MyDefaultDesktopPrinter (Str31 dtpName) {
    OSErr   anyErrors;
    Handle  theCommResource;
    // Forward the message so that the desktop printer is created.
    anyErrors = Forward_GXDefaultDesktopPrinter(dtpName);
    nrequire(anyErrors, Abort);
    // Load the data for the 'comm' resource that was stored in the
    // desktop printer.
    anyErrors = GXFetchDTPData(dtpName, gxDeviceCommunicationsType,
       gxDeviceCommunicationsID, &theCommResource);
    require_action(theCommResource != nil, Abort,
       anyErrors = resNotFound;);
    // Update the 'comm' data with info about the selected printer,
    // and store the updated copy 
    // back in the desktop printer.
    anyErrors = GXWriteDTPData(dtpName, gxDeviceCommunicationsType,
       gxDeviceCommunicationsID, theCommResource);
    // Finally, dispose of the handle we received from
    // GXFetchDTPData. It's a detached resource
    // handle, so DON'T USE RELEASERESOURCE!!

    return anyErrors;

At this point, we've discussed everything that's needed to handle your own custom I/O code. Now we'll take a quick look at what's required if you want to create and maintain your own buffers, instead of using those that the default implementation provides.

First things first. Go back to your 'iobm' resource, and set the number of buffers to 0. This tells QuickDraw GX not to waste time and memory allocating buffers that you aren't going to use.

When you implement your own buffering scheme, you can use any sort of internal representation for your buffers that you want to. However, since some of the buffering messages take a pointer to a gxPrintingBuffer, you'll need to use that format for passing your buffers between certain messages. But as far as the actual buffer structures go, you can use a handle, a linked list, or any other configuration that's convenient or necessary to use.

To support custom buffering code, you'll need to override the following messages.

Always override (partially)

  • GXOpenConnection
  • GXCloseConnection
  • GXCleanupOpenConnection
The partial overrides for these messages should forward the messages and then allocate or dispose of your internal buffer structures. If you're using custom I/O, you already provide overrides of these messages. In that case, simply add this new code to the existing overrides.

Always override (totally)

  • GXBufferData
  • GXWriteData

Provide an override of GXBufferData that stores the passed data in your next available buffer. If a buffer becomes full, call Send_GXDumpBuffer. Before you attempt to add data to this buffer again, call Send_GXFreeBuffer to make sure that all of the buffer's data has been sent to the printer.

Your override for the GXWriteData message should flush all data from your buffers and then immediately send the passed data to the printer. To do this, call Send_GXDumpBuffer on all buffers, followed bySend_GXFreeBuffer on all buffers. If you're performingcustom I/O, just add this code to your existing override.

You may wonder why you don't need to override the GXDumpBuffer message when you perform custom buffering. Unlike the messages listed above, GXDumpBuffer takes a pointer to a gxPrintingBuffer. Whenever your code calls Send_GXDumpBuffer, you must pass data in a gxPrintingBuffer structure, regardless of the internal buffer representation that you're using. Since the buffered data is passed in the format that GXDumpBuffer already expects, there's no need to override the message.

That's all there is to it. So, the next time someone asks, "Can you write QuickDraw GX printer drivers for my 200 Kbits/second serial typesetter, my SCSI copier/printer, and my NuBus-interfaced cutter plotter?" tell them, "Yoooooooou betcha!"

DAVE HERSEY (AppleLink HERSEY) left Apple's Developer Technical Support (DTS) group about six months ago to join the Print Shop software development group. He now fixes the QuickDraw GX bugs that he reported while in DTS, and works on QuickDraw GX 2.0 -- the "knock your socks off" release. *

Thanks to Tom Dowdy, David Hayward, and Nick Thompson for reviewing this column. *


Community Search:
MacTech Search:

Software Updates via MacUpdate

Planet Diver guide - How to survive long...
Planet Diver is an endless arcade game about diving through planets while dodging lava, killing bats, and collecting Starstuff. Here are some tips to help you go the distance. [Read more] | Read more »
KORG iDS-10 (Music)
KORG iDS-10 1.0.0 Device: iOS iPhone Category: Music Price: $9.99, Version: 1.0.0 (iTunes) Description: ** Debut Discount: 50% OFF! Sale Price US$9.99 (Regular price US$19.99). Other all Korg apps are also 50% OFF until Dec 28! **... | Read more »
World of Tanks Generals guide - Tips and...
World of Tanks Generals is a brand new card game by the developer behind the World of Tanks shooter franchise. It plays like a cross between chess and your typical card game. You have to keep in consideration where you place your tanks on the board... | Read more »
TruckSimulation 16 guide: How to succeed...
Remember those strangely enjoyable truck missions in Grand Theft Auto V whereit was a disturbing amount of fun to deliver cargo? TruckSimulation 16 is reminiscent of that, and has you play the role of a truck driver who has to deliver various... | Read more »
The best GIF making apps
Animated GIFs have exploded in popularity recently which is likely thanks to a combination of Tumblr, our shorter attention spans, and the simple fact they’re a lot of fun. [Read more] | Read more »
The best remote desktop apps for iOS
We've been sifting through the App Store to find the best ways to do computer tasks on a tablet. That gave us a thought - what if we could just do computer tasks from our tablets? Here's a list of the best remote desktop apps to help you use your... | Read more »
Warhammer 40,000: Freeblade guide - How...
Warhammer 40,000: Freebladejust launched in the App Store and it lets you live your childhood dream of blowing up and slashing a bunch of enemies as a massive, hulking Space Marine. It's not easy being a Space Marine though - and particularly if... | Read more »
Gopogo guide - How to bounce like the be...
Nitrome just launched a new game and, as to be expected, it's a lot of addictive fun. It's called Gopogo, and it challenges you to hoparound a bunch of platforms, avoiding enemies and picking up shiny stuff. It's not easy though - just like the... | Read more »
Sago Mini Superhero (Education)
Sago Mini Superhero 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: KAPOW! Jack the rabbit bursts into the sky as the Sago Mini Superhero! Fly with Jack as he lifts impossible weights,... | Read more »
Star Wars: Galaxy of Heroes guide - How...
Star Wars: Galaxy of Heroes is all about collecting heroes, powering them up, and using them together to defeat your foes. It's pretty straightforward stuff for the most part, but increasing your characters' stats can be a bit confusing because it... | Read more »

Price Scanner via

New MacBook Air 13-Inch and 15-Inch Coming At...
The Taipei, Taiwan based Chinese language Economic Daily News’s Xie Yili reports that major redesign of the MacBook Air, which currently dates back to October, 2010, is expected to be unveiled next... Read more
World’s First USB-C Adapter For MacBook Suppo...
Innergie, a brand of Delta Electronics, has announced its official release of the world’s first USB-C adapter supporting four DC output voltages, the PowerGear USB-C 45. This true Type C adapter... Read more
13-inch and 11-inch MacBook Airs on sale for...
B&H Photo has 13″ and 11″ MacBook Airs on sale for up to $120 off MSRP as part of their Holiday sale including free shipping plus NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air: $819 $90 off... Read more
13-inch MacBook Pros on sale for up to $150 o...
Take up to $150 off MSRP on the price of a new 13″ MacBook Pro at B&H Photo today as part of their Holiday sale. Shipping is free, and B&H charges NY tax only. These prices are currently the... Read more
13-inch 128GB MacBook Air now on sale for $79...
Best Buy has just lowered their price on the 2015 13″ 1.6GHz/128GB MacBook Air to $799.99 on their online store for Cyber Monday. Choose free shipping or free local store pickup (if available). Sale... Read more
Best Buy lowers 13-inch MacBook Pro prices, n...
Best Buy has lowered prices on select 13″ MacBook Pros this afternoon. Now save up to $200 off MSRP for Cyber Monday on the following models. Choose free shipping or free local store pickup (if... Read more
Cyber Monday: Apple MacBooks on sale for up t...
Apple resellers have MacBook Pros, MacBook Airs, and MacBooks on sale for up to $250 off MSRP for Cyber Monday 2015. The following is a roundup of the lowest prices available for new models from any... Read more
Cyber Monday: Apple Watch on sale for up to $...
B&H Photo has the Apple Watch on sale for Cyber Monday for $50-$100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - Apple Watch Sport: $50 off - Apple Watch: $50-$100 off B... Read more
Cyber Monday: 15% off Apple products, and sto...
Use code CYBER15 on Cyber Monday only to take 15% on Apple products at Target, and store-wide. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-... Read more
iPad Air 2 And iPad mini Among Top Five Black...
Adobe has released its 2015 online shopping data for Black Friday and Thanksgiving Day. The five best selling electronic products on Black Friday were Samsung 4K TVs, Apple iPad Air 2, Microsoft Xbox... Read more

Jobs Board

*Apple* New Products Tester Needed - Apple (...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, continues Read more
Software Engineer, *Apple* Watch - Apple (U...
# Software Engineer, Apple Watch Job Number: 33362459 Santa Clara Valley, Califo ia, United States Posted: Jul. 28, 2015 Weekly Hours: 40.00 **Job Summary** Join the Read more
SW Engineer - *Apple* Music - Apple (United...
# SW Engineer - Apple Music Job Number: 40899104 San Francisco, Califo ia, United States Posted: Aug. 18, 2015 Weekly Hours: 40.00 **Job Summary** Join the Android Read more
Sr Software Engineer *Apple* Pay - Apple (U...
# Sr Software Engineer Apple Pay Job Number: 44003019 Santa Clara Valley, Califo ia, United States Posted: Nov. 13, 2015 Weekly Hours: 40.00 **Job Summary** Apple Read more
*Apple* Site Security Manager - Apple (Unite...
# Apple Site Security Manager Job Number: 42975010 Culver City, Califo ia, United States Posted: Oct. 2, 2015 Weekly Hours: 40.00 **Job Summary** The Apple Site Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.