106 lines
4.5 KiB
Plaintext
106 lines
4.5 KiB
Plaintext
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MACPRINT SPOOLER CDD
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Design Goals:
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MacPrint is ported from the OS/2 implementation. The primary design
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goal is to simplify the porting process as much as possible. The
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original structure of MacPrint was retained and modified only where
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necessary. There are 3 main areas of modification:
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* configuration
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* synchronization
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* removal of the despooler component
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* support for cached dictionaries
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configuration changes -
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Where the OS/2 implementation depended on being able to determine
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at service start time the configuration for all possible shared
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print queues (this was read from lanman.ini), the NT implementation
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allows for the dynamic addition and removal of printer objects
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without specifying configuration information in a data file. This
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forced the modification of the main queue list structure to be
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a single linked list instead of an array. In the NT implementation,
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there is now a single data structure for NT printer objects, and
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being in this structure implies the existence of a validly configured
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NT Printer Object.
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All of the code to read and parse configuration information from
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LANMAN.INI at service start time was removed and replaced by code
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to get configuration information from the EnumPrinters() API as new
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printers are discovered.
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synchronization changes -
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NT provides a more robust synchronization mechanism than OS/2.
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WaitForMultipleObjects() actually works correctly under NT, and
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it is possible to wait on objects besides semaphores. Mutual
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exclusion to data is achieved through critical sections as opposed
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to mutexes or semaphores. Critical sections require fewer resources
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than either, but can only be shared between threads of the same
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process. Events are used to process jobs asynchronously. Instead
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of clearing a semaphore when a read or open occurs, an event is
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signaled. Semaphores in NT are used as a gating mechanism,
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and are not designed to be used as a signaling mechanism. Syncronization
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with the termination of threads is achieved by waiting on the thread
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object as opposed to waiting on an event that the thread signals
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just before it terminates.
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removal of the despooler -
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The spooler and despooler were somewhat integrated in the OS/2
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implementation - especially with regards to configuration information.
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Data structures pertinent to the despooler only were removed, and
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the configuration routines are no longer contained in a DLL shared
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by the spooler and despooler. The only connection between the two
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components is now through a PostScript comment that is included by
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the spooler to indicate if ctrl-d's should be stripped by the
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despooler.
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support for cached dictionaries -
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As the caching of PostScript dictionaries achieved only modest performance
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improvements at best, and since it added significant complexity to the
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code, support for cached dictionaries will most likely be removed in
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the NT implementation. If at coding time it becomes clear that more
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work is involved to remove the feature than to port it, it may stay
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in.
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Thread/Process Structure:
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MacPrint is an NT Service, hence it follows the NT Service Model. main()
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is the service control dispatcher for the MacPrint service. This thread
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is used by the NT Service Controller to communicate with the MacPrint
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Service.
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MacPrintMain() is the main thread for MacPrint. It is responsible for
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monitoring the NT Print Manager for configuration changes (creation
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and removal of printer objects) and for starting and stopping threads
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to manage each NT Printer Object. Essentially, MacPrintMain polls
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until a service stop request is received. While polling, it enumerates
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printers from the NT Print Manager and either creates a control thread
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for a newly discovered printer or terminates the control thread for
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each printer no longer configured.
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QueueServiceThread() is the thread for each active NT Printer Object.
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This thread listens for jobs directed to the NT printer and processes
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the job. One thread supports multiple jobs for the printer. This is
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done by using two events per queue - ReadEvent and OpenEvent. The
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PAP API have the property that they take an address to store a completion
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code when the event completes, and this address is specified to be in
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a per job data structure. The event is common for all jobs in the
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queue. When a particular event is signaled, the list of pending jobs
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in that queue is walked to find one that is in the correct state
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(read_pending or open_pending) and that has it's completion code set.
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The event is processed for that job, and the job is moved to the end
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of the list so that events for jobs are processed in round-robin
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order.
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