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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. *


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