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Technical Note TN1097
Desktop Printing Revealed


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This Technote, originally written in February 1997, describes how a printer vendor might add desktop printing support in their printer driver. Support for third-party desktop printing has been added to Mac OS 8.5 with specification changes, so we encourage you to read the revisions in this updated Technote, and to try adding desktop printing support to your printer driver.

 Updated: [Aug 10 1998]

With the introduction of System 6 and MultiFinder in 1989, Apple introduced background printing. For the first time, a user could regain control of the Macintosh almost immediately after printing a document, while the printer driver processed the document in the background. Classic background printing has been far more robust than anyone might have imagined, and it's still the basis of much of today's printing technology, including desktop printing. Unfortunately for developers interested in supporting desktop printing, the interfaces to background printing have remained largely undocumented.

In 1994, the desktop printing architecture was introduced with the LaserWriter 8.3 and the StyleWriter 1200 driver. From its initial release to the present, desktop printing has existed as a collection of extensions and invisible applications. While this has not created a clear and simple picture for developers interested in supporting desktop printing, this technology shows every sign of thriving for a long time to come, so understanding its architecture is more vital than ever.

While the complete story is far from written, and things will certainly change again with the introduction of Mac OS X, this article will cover background printing for all pre-Mac OS X versions of the Mac OS. This includes the significant changes that make Desktop Printing accessible to third-party developers. If you currently have a printer driver, this updated article describes everything you need to add support for desktop printing.

Classic Background Printing

In early background printing, printer drivers would save each page as a QuickDraw picture in the data fork of a temporary file (a spool file) in a special folder (the Spool Folder) within the System Folder. There was also information stored in the resource fork describing the page format, document name, and other job information, as well as offsets to the beginning of the data for each page. A special application (Backgrounder) launched by MultiFinder at startup time (in System 7, Backgrounder was incorporated into Finder) would see the document created by the printer driver and would then launch PrintMonitor, which would run in the background, feeding the spool file to the printer driver.

PrintMonitor performs its magic by calling the printer driver in much the same way as an application would, except rather than your driver's 'PDEF' 0 resource getting called, its 'PDEF' 126 resource--which has the same format as the 'PDEF' 0 resource--is called. A special PrGeneral call is sent just after PrOpen has been called (before PrOpenDoc has been called) to provide the printer driver with the pointers to notification functions it will need to call. PrintMonitor then prints each page by replaying the stored page data to the driver. This method of background printing is still used by the Printing Manager today in two cases: when desktop printing isn't installed (or has been disabled) and when the Finder isn't running (usually because At Ease is running instead).

Getting to know classic background printing means learning the structure of its spool file. The resource fork of the spool file contains the following resources (note that all structures mentioned in this document have 68k alignment):

  • 'PREC' 3 -- the print record
  • 'alis' -8192 -- an alias to the driver that created the spool file
  • 'ics#' 131 -- the small icon to display for the spool file in the PrintMonitor window
  • 'PREC' 124 -- the printer name
  • 'PREC' 126 -- the job information
typedef {
    short version;    // always 1
    short flags;      // always 0
    short numPages;   // total number of pages in the spool file
    short numCopies;  // total number of copies for the spool file
    OSType creator;   // the creator type of the driver used to
                      // create the spool file
    Str31 appName;    // the application name used to print the
                      // spool file
} PREC126Record, *PREC126Ptr, **PREC126Handle;
  • 'STR ' -8192 -- the filename of the printer driver
  • 'STR ' -8189 -- the document name [always padded to 80 bytes]
The data fork of the spool file with a SpoolFileHeader structure, followed by the pages.

typedef struct {
    short version;      // should always be 1
    long fileLen;       // length of file including spool file header
    long fileFlags;     // should always be 0
    short numPages;     // total number of pages in the spool file
    TPrint printRecord; // used only if PREC 3 can't be read
} SpoolFileHeader, *SpoolFileHeaderPtr, **SpoolFileHeaderHandle;

typedef struct {
    long pictFlags;      // should always be 0
    Picture thePict;     // variable length
    long pageOffset;     // offset to the beginning of this page's
                         // PICT
} SpoolPage;

The spool file is created by the driver in the Spool Folder or PrintMonitor Documents folder within the System Folder (or in the current default desktop printer folder if Mac OS 8 desktop printing is active-- you can find the correct folder with FindFolder and the kPrintMonitorDocsFolderType selector). Spool files that are being written have a type of '?job' and a creator of 'prmt'. Once the file is completely written, the driver changes the type to 'pjob'. When the version of Desktop Printing that supports third-party drivers is available, your driver also needs to send an Apple event to the Finder telling it that a new spool file was created (more on this below).

When PrintMonitor (or Desktop PrintMonitor) prints a job, it calls the driver's PrOpen routine and then calls PrGeneral with the structure shown below (see the "DesktopPrinting.h" header file for specifics). PrintMonitor then calls PrOpenDoc with a pIdleProc that the driver needs to call periodically. PrOpenPage and PrClosePage will get called for each page of the document, and the page will be printed to the driver.

// Notification Procs
typedef pascal void (*DTPAsyncErrorNotificationProcPtr) (StringHandle string);
typedef pascal void (*DTPEndNotificationProcPtr) ();
typedef pascal Boolean (*DTPInForegroundProcPtr) ();
typedef pascal void (*DTPStatusMessageProcPtr) (StringHandle string);

// PrGeneral call that PrintMonitor/Desktop PrintMonitor
// use to set up the notification process
#define kPrintMonitorPrGeneral = -3;

// TPrintMonitorPrintingData:
// for classic background printing and desktop printing
// that does not support third-party drivers
typedef struct {
    short iOpCode;    // kPrintMonitorPrGeneral
    short iError;
    long iReserved;   // 0 - classic PrintMonitor is running
    THPrint hPrint;
    short noProcs;    // number of notification procs
    long iReserved2;
        pAsyncNotificationProc; // UPP to put up a notification
        pAsyncEndnotifyProc;    // UPP to take down
                                // the notification
        pDTPInForegroundProc;   // UPP to check if PrintMonitor is
                                // in foreground
} TPrintMonitorPrintingData;

// TDesktopPrintMonitorPrintingData:
// for desktop printing that supports third-party print drivers
typedef struct {
    short iOpCode;      // kPrintMonitorPrGeneral
    short iError;
    long iReserved;     // 1 - Desktop PrintMonitor is running
    THPrint hPrint;
    short noProcs;      // number of notification procs
    long iReserved2;
        pAsyncNotificationProc; // UPP to put up a notification
        pAsyncEndnotifyProc; // UPP to take down the notification
        pInForegroundProc; // UPP to check if desktop printing is
                           // in foreground
        pStatusMessageProc; // UPP to update the printing status
                            // message in the desktop printer window
} TDesktopPrintMonitorPrintingData;

When printing with background printing, both PrintMonitor and Desktop PrintMonitor will put a DialogPtr into the low-memory global ApplScratch just before calling PrOpenDoc. The driver should put status messages into the first item in that dialog using GetDialogItem and SetDialogItemText.

If the PrGeneral call says that printing is occurring from Desktop PrintMonitor (which is a faceless background application), no dialogs or alerts should be displayed. The one exception is that Desktop PrintMonitor patches StopAlert and ParamText. If you call ParamText and then StopAlert, the Finder will display an alert for you with the text you've set. However, the filter proc passed into StopAlert will not be called.

A printer driver can provide the user the options of putting a job on hold, stopping the print queue, trying to print again, or canceling the job. After user selects an option in the alert, the printer driver should set the print manager error (SetPrError) to one of the following error codes that corresponds to user selection:

// desktop printing error codes
#define kDTPHoldJobErr -4200
#define kDTPStopQueueErr -4201
#define kDTPTryAgainErr -4202
#define kDTPAbortJobErr 128

Desktop PrintMonitor will then put the job on hold, stop the print queue, try to print the job again, or cancel the job accordingly.

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Desktop Printing Today

Desktop printing was introduced with the LaserWriter 8.3 and the StyleWriter 1200 printer drivers, and it uses much the same approach as classic background printing. Currently, desktop printing only supports Apple print drivers, but the additions described in this section are things you will need to incorporate into your drivers so they will work with desktop printing in the future.

The additional resources a driver needs to add to support desktop printing are the icons for the desktop printers. As a driver developer you'll need to supply a full 'BNDL' resource with your driver's creator. Beyond the types and icons you've probably already got in your driver (for the driver itself and any preferences file), you'll need to add icons for the types 'dpnn', 'dpcn', and 'dpna', which correspond to a "normal" desktop printer, a default desktop printer (with the heavy line around it), and an inactive (or unavailable) desktop printer. As an example, the bundle from the Color StyleWriter 2500 is shown below. LaserWriter 8 has a slightly different scheme for setting the icons for desktop printers; it retrieves them from the PostScript printer description (PPD) file for a given printer.

The StyleWriter 2500 bundle and icons
Figure 1.The StyleWriter 2500 bundle and icons

When an Apple driver creates a spool file for desktop printing, it places the file in the same folder used by classic background printing. The Desktop Printing Extension will take care of moving the file to the desktop printer and beginning the process of actually printing the document. When desktop printing is active, the following resources are added to the spool file:

  • 'PINX' -8200 -- the page index resource
  • 'jobi' 1 -- the print job information

// PINX -8200 (page index resource)
typedef struct {
    short count;         // number of elements in the pageOffset array
    long pageOffset[1];  // the offset from the beginning of the file
                         // to the page record
                         // e.g., it would be sizeof(SpoolFileHeader)
                         // for the first page.
} SpoolPageIndex,  *SpoolPageIndexPtr, **SpoolPageIndexHandle;

// jobi 1 (DTP print job information resource)
// print priorities
#define kDTPPrintJobUrgent 0x00000001
#define kDTPPrintJobAtTime 0x00000002
#define kDTPPrintJobNormal 0x00000003
#define kDTPPrintJobHolding 0x00001003

typedef struct
    short firstPageToPrint;  // first page in the spool file to print
    short priority;          // print priority (e.g.,  kDTPPrintJobNormal)
    short numCopies;         // total number of copies
    short numPages;          // total number of pages in the spool file
    unsigned long  timeToPrint;  // time to print (in seconds) when
                                 // priority is kDTPPrintJobAtTime
    Str31 documentName;     // name of the document
    Str31 applicationName;  // name of the application that's used
                            // to create this spool file
    Str32 printerName;      // name of the target printer
                            // (should be the same as what's in PREC 124)
} DTPPrintJobInfo, *DTPPrintJobInfoPtr, **DTPPrintJobInfoHandle;

Another addition is a new way to change the default desktop printer. An application or driver can send an Apple event to the Finder as shown below. (Note that SendAEToFinder just sends the event to the Finder--see "DTPSample.c" for info.)

#define kDTPSignature  'dtpx'
#define aeDTPSetDefaultEventType 'pfsd'

// event data
typedef struct {
    OSType dtpSignature; // kDTPSignature
    OSType dtpEventType; // aeDTPSetDefaultEventType or aeDTPSyncEventType
    FSSpec dtpSpec; // the file spec of the target dtp
} DTPAppleEventData;

OSErr SetDefaultDTP(const FSSpec* dtpSpec) {
    OSErr err;
    DTPAppleEventData myEvent;

    myEvent.dtpsignature = kDTPSignature;
    myEvent.dtpEventType = aeDTPSetDefaultEventType;
    BlockMove((Ptr)dtpSpec, (Ptr)&myEvent.dtpSpec, sizeof(FSSpec));

    err = SendAEToFinder((Ptr)&myEvent, sizeof(DTPAppleEventData));

    return err;

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Desktop Printing Tomorrow

With the introduction of Mac OS 8, desktop printing will no longer be a separate extension. It will be integrated into the Finder and therefore available in most cases. A user can still disable desktop printing by disabling the Desktop PrintMonitor and Desktop Printer Spooler in Extensions Manager, though. In Mac OS 8.5, changes have been made to make it possible for third parties to integrate support.

Desktop printing installs a Gestalt selector to tell you if third-party drivers can be supported. If desktop printing is available, but this selector does not report that third-party drivers are supported by desktop printing, you will need to use the methods described in the "Classic Background Printing" section above.

// Desktop Printing Feature Gestalt
// Use this gestalt to check if third-party printer driver support is available
enum { gestaltDTPFeatures = 'dtpf' };
#define kDTPThirdPartySupported 0x00000004  // mask for checking if third-
                                            // party drivers are supported

Boolean ThirdPartyDriverSupported(void) {
    long response;
    Boolean result = false;

    OSErr err = Gestalt(gestaltDTPFeatures, &response);
    if (err == noErr) {
        result = !!(response & kDTPThirdPartySupported);


When non-Apple drivers are supported by desktop printing, your driver needs to write your spool files directly to the desktop printer folder with the type of '?job'. When you are done spooling, you need to change the file's type to 'pjob'. The driver can determine the current default desktop printer folder, and many other things, by calling the Gestalt routine with the desktop printing extension selector. The selector is 'dtpx', and the information is returned to you in a handle. When you're done with the handle, do not call DisposeHandle on theDTPList and the GestaltDTPInfoHdle.

// Desktop Printer Info Gestalt
// Use this gestalt to gather information for all of the
// active desktop printers

enum { gestaltDTPInfo = 'dtpx' };
        // version 2.0f0 (for Mac OS 8.0, 8.1 and 7.x)
    kDTPGestaltStructVersion2 = 0x02008000,
        // version 3.0 (for Mac OS 8.5)
    kDTPGestaltStructVersion3 = 0x03000000

// DTPInfo
typedef struct {
    short vRefNum;      // vRefNum of the DTP folder
    long dirID;         // directory ID of the DTP folder
    Str31 dtpName;      // name of the DTP folder
    OSType driverType;  // creator type of the print driver for this DTP
    Boolean current;    // is this DTP currently the default printer?
    Str32 printerName;  // name of the actual printer on the net
                        // (only for LaserWriter 8 dtps)
    Str32 zoneName;     // zone where this printer resides
                        // (only for LaserWriter 8 dtps)
} DTPInfo;

// data associated with the desktop printer info gestalt
typedef struct {
    long version; // kDTPGestaltStructVersion3 or kDTPGestaltStructVersion2
    short numDTPs; // number of the active dtps
    Handle theDTPList; // handle to a list of DTPInfo for the active dtps
    Handle theDTPDriverList; // handle to a list of print driver file specs for
                             // each of the active dtp in theDTPList
    long reserved;
} GestaltDTPInfo, **GestaltDTPInfoHdle;


When you're done writing the spool file, you need to send an Apple event to tell the Finder about the new spool file (see SyncDTP function in the file "DTPSample.c").

There are also added calls for this version of desktop printing. Specifically, there are three new PrGeneral selectors that a driver needs to support: kDTPIsSamePrinterInfo, kDTPGetPrinterInfo, and kDTPSetDefaultPrinterInfo. These selectors enable the desktop printing extension to decide which of the driver's desktop printers is the current default printer, to determine whether it needs to create a new desktop printer, and to inform the driver that a desktop printer has been selected as the default. The structures you'll need to use to support these selectors are shown below:

// PrGeneral opcodes
    kDTPGetPrinterInfo        = 23,
    kDTPIsSamePrinterInfo     = 24,
    kDTPSetDefaultPrinterInfo = 25

// DTP printer types (address types)
    kDTPUnknownPrinterType      = -1,   // unknown address type
    kDTPSerialPrinterType       = 0,    // serial printer
    kDTPAppleTalkPrinterType    = 1,    // AppleTalk printer
    kDTPTCPIPPrinterType        = 2,    // TCP/IP printer
    kDTPSCSIPrinterType         = 3,    // SCSI printer
    kDTPUSBPrinterType          = 4     // USB printer

// serial ports
    kDTPUnknownPort = -1, // for drivers that support serial connection by the
                          // Comm Toolbox other than modem and printer port
    kDTPPrinterPort = 0,  // printer port
    kDTPModemPort = 1     // modem port

// serial printer address
typedef struct
    short    port;        // kDTPPrinterPort, kDTPModemPort or kDTPUnknownPort
    Str31    portName;    // name of the port specified in the port field
} DTPSerialAddress;

// AppleTalk printer address
typedef struct
    Str32    nbpName;
    Str32    nbpZone;
    Str32    nbpType;
} DTPAppleTalkAddress;

// TCP/IP printer address
typedef struct
    Str255    TCPIPAddress;
    Str255    queueName;
} DTPTCPIPAddress;

// SCSI printer address
typedef struct
    short    id;         // SCSI id
} DTPSCSIAddress;

// USB printer address
typedef struct
    Str255    name;       // printer name
} DTPUSBAddress;

// data passed into the PrGeneral calls
typedef struct
    Str31 dtpDefaultName; // default name for the desktop printer.
    short printerType;    // kDTPSerialPrinterType, kDTPAppleTalkPrinterType,
                          // kDTPTCPIPPrinterType, kDTPSCSIPrinterType,
                          // kDTPUSBPrinterType or kDTPUnknownPrinterType

    // Info specific to each type of printers
        DTPSerialAddress    serial;
        DTPAppleTalkAddress appleTalk;
        DTPTCPIPAddress     tcpip;
        DTPSCSIAddress      scsi;
        DTPUSBAddress       usb;
    } u;

    // optional driver-specific information can be appended here

} DTPPrinterInfo, **DTPPrinterInfoHandle;

typedef struct {
    short iOpCode; // kDTPGetPrinterInfo, kDTPIsSamePrinterInfo
                   // or kDTPSetDefaultPrinterInfo
    short iError;
    long iCommand;
    DTPPrinterInfoHandle printerInfo;
} TDTPPrGeneralData;
In order for the Finder to recognize whether or not your device supports desktop printing, you need to add the 'dtpi' resource to the printer driver's resource fork.
// desktop printer info resource
#define kDTPInfoResType 'dtpi'
#define kDTPInfoResID -8192

// connection types supported
#define kDTPUnknownConnection 0x00000000 // unknown connection type
#define kDTPSerialConnection 0x00000001 // serial connection
#define kDTPSCSIConnection 0x00000002 // SCSI connection
#define kDTPAppleTalkConnection 0x00000004 // AppleTalk connection
#define kDTPTCPIPConnection 0x00000008 // TCP/IP connection
#define kDTPUSBConnection 0x00000010 // USB connection

// dtp extra features supported
#define kDTPBasicFeatures 0x00000000 // only basic dtp funtionalities
                                     // are supported

typedef struct
    long features; // kDTPBasicFeatures (only basic features are supported
                   // for MacOS 8.5)
    long connectionType; // can be kDTPUnknownConnection or any combination
                         // of kDTPSerialConnection, kDTPSCSIConnection,
                         // kDTPAppleTalkConnection, kDTPTCPIPConnection,
                         // and kDTPUSBConnection
} DTPInfoResource;

When your driver is selected in the Chooser, desktop printing will call your driver via a series of PrGeneral calls. First, you'll get called with the kIsSamePrinterInfo selector for each of the desktop printers created by your driver in order to determine which is the currently selected printer. Your driver responds by filling in the iError field of the TPrinterInfoPrGeneralData record. If the printer that your driver thinks is current matches the information passed in with the kIsSamePrinterInfo selector, the driver responds by setting the iError field to noErr. If it's not a match, the driver sets the iError field to -1.

If the printer selected in the Chooser is not among the desktop printers owned by your driver, desktop printing will create a new desktop printer and call PrGeneral with the kGetPrinterInfo selector to get the information for the selected printer. At this point, your driver should resize the printerInfo handle and fill in the printer information. You need to fill in the dtpDefaultName field with the name you'd like to see assigned to the newly created desktop printer. Note also that your driver can append as much (or as little) extra printer information as you'd like. Once you've returned the information, the desktop printing extension will save this information into the desktop printer.

If a user selects one of your desktop printers via the Set Default Printer menu item in the Printing menu (which appears in the Finder when you've clicked on a desktop printer), your driver will get a PrGeneral call with the kSetDefaultPrinterInfo selector. When you receive this call, you should change any internal settings your driver maintains so that the printer pointed to by the DTPPrinterInfo handle you received is now the currently selected printer for your driver.

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The Future Is Now

If you've currently got a driver that supports classic background printing, your best bet for future compatibility is to add support for desktop printing. However, if you're just starting to tackle background printing now, and while you could implement support for Mac OS 8 desktop printing only, we strongly recommend that you support the current desktop printing architecture and classic background printing as well. That way, your users will have a more consistent experience when printing, regardless of which version of the Mac OS they're running.

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Acrobat gif

Acrobat version of this Note (100K)


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DesktopPrinting.h (144K)


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DTPSample.c (140K)


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