TweetFollow Us on Twitter

March 95 - Print Hints

PRINT HINTS

Writing QuickDraw GX Drivers With Custom I/O and Buffering

DAVE HERSEY

[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.

CUSTOM I/O -- WHO NEEDS IT?
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.

HOW TO GET STARTED
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.

WHEN "NOT CONNECTED" MEANS "CONNECTED"
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.

CUSTOM I/O -- THE MESSAGES
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
GXCloseConnectionGXCloseConnection
GXCleanupOpenConnectionGXCleanupOpenConnection
GXWriteData
Always override (totally)GXDumpBufferGXBufferData
GXWriteData
Usually override (partially)GXDefaultDesktopPrinter
GXChooserMessage
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.
    MyUpdateCommResource(theCommResource);
    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!!
    DisposeHandle(theCommResource);

Abort:
    return anyErrors;
}

USING YOUR OWN BUFFERING SCHEME
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.

DRIVE SAFELY
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

DiskCatalogMaker 7.2.7 - Catalog your di...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast Finder-like intuitive look and feel Super-fast search algorithm Can compress catalog data for... Read more
Iridient Developer 3.2.1 - Powerful imag...
Iridient Developer (was RAW Developer) is a powerful image-conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more
BusyContacts 1.2.7 - Fast, efficient con...
BusyContacts is a contact manager for OS X that makes creating, finding, and managing contacts faster and more efficient. It brings to contact management the same power, flexibility, and sharing... Read more
MegaSeg 6.0.5 - Professional DJ and radi...
MegaSeg is a complete solution for pro audio/video DJ mixing, radio automation, and music scheduling with rock-solid performance and an easy-to-use design. Mix with visual waveforms and Magic... Read more
BusyContacts 1.2.7 - Fast, efficient con...
BusyContacts is a contact manager for OS X that makes creating, finding, and managing contacts faster and more efficient. It brings to contact management the same power, flexibility, and sharing... Read more
MegaSeg 6.0.5 - Professional DJ and radi...
MegaSeg is a complete solution for pro audio/video DJ mixing, radio automation, and music scheduling with rock-solid performance and an easy-to-use design. Mix with visual waveforms and Magic... Read more
Iridient Developer 3.2.1 - Powerful imag...
Iridient Developer (was RAW Developer) is a powerful image-conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more
iFFmpeg 6.6.1 - Convert multimedia files...
iFFmpeg is a comprehensive media tool to convert movie, audio and media files between formats. The FFmpeg command line instructions can be very hard to master/understand, so iFFmpeg does all the hard... Read more
Tor Browser Bundle 7.0.11 - Anonymize We...
The Tor Browser Bundle is an easy-to-use portable package of Tor, Vidalia, Torbutton, and a Firefox fork preconfigured to work together out of the box. It contains a modified copy of Firefox that... Read more
iShowU Instant 1.2.4 - Full-featured scr...
iShowU Instant gives you real-time screen recording like you've never seen before! It is the fastest, most feature-filled real-time screen capture tool from shinywhitebox yet. All of the features you... Read more

Latest Forum Discussions

See All

Reigns: Her Majesty guide - how to use e...
Ruling a kingdom isn't easy--doubly so for a queen whose every decision is questioned by the other factions seeking a slice of power. Reigns: Her Majesty builds on the original game's swipey tactics, adding items that you can use to move the story... | Read more »
The best new games we played this week -...
Friday has crept up on us once again, so it's time to honor the best new games we've played over the past few days. This past week was a pretty exciting one, with the debut of lots of beautiful new indies and some familiar faces returning to the... | Read more »
Portal Knights guide- beginner tips and...
Portal Knights is finally making the jump to iOS and Android, and it's already climbing the ranks to become the next big MMO experience on mobile. This sprawling sandbox game will let you pursue any adventure you wish, whether you want to sling... | Read more »
Reigns: Her Majesty guide - how to swipe...
Reigns: Her Majesty is storming the App Store this week, bringing more tinder-esque kingdom building to eager players everywhere. If you've played the original Reigns, you'll know that leading a kingdom is never easy. It's a careful balancing act... | Read more »
Getting Over It (Games)
Getting Over It 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: A game I madeFor a certain kind of person To hurt them. • Climb up an enormous mountain with nothing but a hammer and a pot.•... | Read more »
Reigns: Her Majesty (Games)
Reigns: Her Majesty 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Pocket Legends Adventures guide - how to...
Pocket Legends Adventures is a fun action adventure RPG that takes control when you want it to, but also opens itself for player input, too, if you're looking to tkae a more active role in combat. Regardless of play style, the game can be quite... | Read more »
Portal Knights (Games)
Portal Knights 1.2.4 Device: iOS Universal Category: Games Price: $4.99, Version: 1.2.4 (iTunes) Description: Craft your adventure. Forge your hero. Become the ultimate Portal Knight! | Read more »
Ashworld (Games)
Ashworld 1.3 Device: iOS Universal Category: Games Price: $3.99, Version: 1.3 (iTunes) Description: | Read more »
The best deals on the App Store this wee...
Another week means another roundup of quality bargain games. This week's list features a few games we haven't featured before in our weekly roundups. If you're on the prowl for new games, now's an optimal time to try something completely new for... | Read more »

Price Scanner via MacPrices.net

Beats by Dr. Dre – BeatsX Earphones on sale f...
Best Buy has BeatsX Earphones on sale for $109, $40 off, on their online store. Sale price for online orders only. Choose free store pickup, if available, or choose free shipping. Read more
10″ 64GB WiFi Apple iPad Pros on sale for $59...
MacMall has 10.5″ 64GB Apple iPad Pros on sale for $599 including free shipping. That’s $50 off MSRP and among the lowest prices available for these iPads from any Apple reseller. Read more
15″ 2.2GHz MacBook Pros on sale for $200-$300...
B&H Photo has the 15″ 2.2GHz MacBook Pro available for $200 off MSRP including free shipping plus NY & NJ sales tax only: – 15″ 2.2GHz MacBook Pro (MJLQ2LL/A): $1799 $200 off MSRP Apple has... Read more
Holiday sale: 15″ MacBook Pros for $200-$420...
MacMall has 15″ MacBook Pros on sale for $220-$300 off MSRP, each including free shipping: – 15″ 2.8GHz MacBook Pro Space Gray (MPTR2LL/A): $2179, $220 off MSRP – 15″ 2.8GHz MacBook Pro Silver (... Read more
Holiday sale: 13″ MacBook Airs for up to $150...
B&H Photo has 13″ MacBook Airs on sale for $100 off MSRP as part of their Holiday sale. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13″ 1.8GHz/128GB MacBook... Read more
The best Holiday sale prices on 13″ MacBook P...
B&H Photo has 13″ MacBook Pros on sale this weekend, with models available for $100-$150 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13-inch 2.3GHz... Read more
Holiday sale: 27″ Apple iMacs for $150-$200 o...
MacMall has 27″ iMacs on sale for $150-$200 off MSRP as part of their Holiday sale, each including free shipping: – 27″ 3.8GHz iMac (MNED2LL/A): $2099 $200 off MSRP – 27″ 3.5GHz iMac (MNEA2LL/A): $... Read more
Save on clearance 2016 15″ MacBook Pros, up t...
B&H Photo has clearance 2016 15″ MacBook Pros available for up to $700 off original MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: – 15″ 2.7GHz Touch Bar MacBook Pro... Read more
Apple offers Certified Refurbished 9.7″ iPads...
Apple has Certified Refurbished 9.7″ WiFi iPads available for $50-$70 off the cost of new models. An Apple one-year warranty is included with each iPad, and shipping is free: – 9″ 32GB WiFi iPad: $... Read more
Save $200-$240 on a 12″ Apple MacBook with Ce...
Apple has Certified Refurbished 2017 12″ Retina MacBooks available for $200-$240 off the cost of new models. Apple will include a standard one-year warranty with each MacBook, and shipping is free.... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Information Security - Security Data...
# Apple Information Security - Security Data Analyst Job Number: 113119545 Austin, Texas, United States Posted: 10-Nov-2017 Weekly Hours: 40.00 **Job Summary** This Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.