Xprint — The "X print service" - a portable, network-transparent printing system based on the X11 protocol
Xprint is a very flexible, extensible, scaleable, client/server print system based on ISO 10175 (and some other specs) and the X11 rendering protocol. Using Xprint an application can search, query and use devices like printers, FAX machines or create documents in formats like PDF. In particular, an application can seek a printer, query supported attributes (like paper size, trays, fonts etc.), configure the printer device to match it’s needs and print on it like on any other X device reusing parts of the code which is used for the video card Xserver.
The "X Print Service" technology allows X rendering to devices such as printers and fax. Most of the service is available in the X11 technology stack as Xp, with the remainder in single toolkit stacks (e.g. DtPrint for CDE). Modifications have also been made to the LessTif/Motif/Qt technology stacks to support Xprint.
The Xp portion consists of:
Xp Extension for the X-Server (included in the X-Server Xprt)
Xp Extension API for the client side (libXp/libXprintUtils)
PCL ddx driver that converts core X to native PCL
PDF ddx driver that converts core X to native PDF
PostScript ddx driver that converts core X to native PostScript
Raster ddx driver that generates xwd rasters which can be converted to PCL, PDF or PostScript rasters
From an X clients perspective, it can attach to one of two nearly identical X-Servers, a "Video" X-Server, and a "Print" X-Server which has the additional Xp capability but otherwise looks and behaves the same.
The X Print Service expands on the traditional X-Server and Xlib world in four ways.
Most obvious is the use of "print ddx drivers" instead of "video ddx drivers". While a video ddx driver modifies pixels in a video frame buffer, a print ddx driver generates "page description language (PDL)" output (such as PCL, PDF or PostScript) or sends the print rendering instructions to a platform-specific print API (like Win32/GDI).
Once a print ddx driver generates PDL output, it can be sent to a spooler such as lp(1) or retrieved by the client (to implement functionality like "print-to-file").
Though not currently done, a single X-Server can support both print and video ddx drivers.
Since printers support "paged" output, unlike video, a portion of the Xp Extension supports APIs to delineate printed output. For example, XpStartPage and XpEndPage tell the X-Server where a physical page starts and ends in an otherwise continuous stream of X rendering primitives. Likewise, XpStartJob and XpEndJob determine when a collection of pages starts and ends. XpEndJob typically causes the generated PDL to be submitted to a spooler, such as lp(1).
Since printers have extensive capabilities, another portion of the Xp Extension supports APIs to manipulate "print contexts".
Once a printer is selected using the Xp Extension API, a print context to represent it can be created. A print context embodies the printer selected - it contains the printer's default capabilities, selectable range of capabilities, printer state, and generated output. Some "attributes" within the print context can be modified by the user, and the X-Server and print ddx driver will react accordingly. For example, the attribute "content-orientation" can be set to "landscape" or "portrait" (if the printer supports these values - which can be queried using the Xprint API as well).
Since printers can have "built in" fonts, the Xp Extension in the X-Server works with the print ddx drivers to make available (for printing only) additional fonts on a per print context basis.
When a print context is created and set for a given printer, the X font calls may be able to access additional printer fonts. To do this (typically), the X-Server must have access to "printer metric files" (.pmf) that describe at minimum the metrics of the built in fonts.
There are three tasks to start the X Print Service:
configuring the X Print Server,
starting the X Print Service
configuring the user session so that clients can find the running X Print Service
The tasks are described in detail below.
The X Print Server (Xprt) can read a number of configuration files which control its behavior and support for printers. Each vendor platform has a default location for this information. Xprt can also read the environment variable XPCONFIGDIR to locate alternate configuration directories. Common settings include:
| export XPCONFIGDIR=/X11/lib/X11/XpConfig/ |
| export XPCONFIGDIR=/proj/x11/xc/programs/Xserver/XpConfig/ |
Xprt has many built-in defaults, and lacking any configuration files, will immediately try to support all printers visible via lpstat(1).
In order of importance for configuration by a system administrator, the configuration files for a "C" locale are as follows (see Xprt(7) for more details (including support for non-"C" locales)):
'Xprinters' is the top most configuration file. It tells Xprt which specific printer names (e.g. mylaser) should be supported, and whether lpstat(1) or other commands should be used to automatically supplement the list of printers.
The 'printer' file maps printer names to model configurations (see 'model-config' below). For example, "mylaser" could be mapped to a "HPDJ1600C", and all other arbitrary printers could be mapped to a default, such as "HPLJ4SI". When depending on lpstat(1) in the Xprinters file, setting up defaults in 'printer' becomes all the more important.
The 'document' file specifies the initial document values for any print jobs. For example, which paper tray to use, what default resolution, etc.
The 'job' file specifies the initial job values for any print jobs. For example, "notification-profile" can be set so that when a print job is successfully sent to a printer, e-mail is sent to the user.
The 'model-config' file has attributes that describe the printer model’s capabilities and default settings. Printer model fonts may also be present. The model-config file also identifies the print ddx driver to be used. For each printer model supported, a complete hierarchy of files should exist. In most cases, these files do not need to be modified.
The print ddx drivers can have highly specific configuration files to control their behavior. In most cases, these files do not need to be modified.
More information in how to configure and customize the X print server can be found in the Xprt(7) manual page.
The summary checklist for starting the X Print Service is as follows:
Choose an execution model for the X Print Service. The X Print Service can be run on a per-user session basis, per machine basis, or can be run on a few machines globally available to a number of users.
If print jobs are to be submitted to a spooler (almost always the case), make sure all needed printers are available to the spooler subsystem (most often lp(1)) on the same machine running the X Print Service.
Configure the X Print Server. See ``X Print Server Configuration''.
Depending on #1, start the X Print Server process "Xprt", and then the toolkit-specific Print Dialog Manager Daemon process (such as CDEnext's "dtpdmd") at the appropriate times. Note that libXprintUtils-based applications/toolkits do not need a Print Dialog Manager Daemon process to use Xprint.
The details are described below.
Because the X Print Service is based on X, it can be easily distributed. The most significant factors in which execution model to choose will be driven by:
how many printers will be accessable through the printer subsystem on any given machine. A system administrator may choose to cluster printers on a few given machines, or scatter them across an organization and possibly make extensive use of remote spoolers to make them globally available.
how many machines will need a copy of the X Print Server configuration files. The files have been architected so that one super-set version of them can be maintained and distributed (e.g. via NFS), and a per-machine or per-user version of the `Xprinters' is all that is needed to have the appropriate information in them utilized or ignored.
how many users can demand services from a given X Print Service.
With the above in mind, some obvious execution models include:
Global - in this model, the system administrator is choosing to run the X Print Service on a *few* select machines with appropriate printers configured, and allow clients access to the global resource. This can centralize the administration of printers and configuration files, but may have to be monitored for performance loading.
Startup would likely be done by boot-up scripts (such as /etc/init.d/xprint).
Per-machine - every machine with potential X Print Service users would run the service. Printer and configuration file administration is decentralized, and usage would be limited to the users on the machine.
Startup would likely be done by boot-up scripts (such as /etc/init.d/xprint).
Per-user session - every user would run an entire X Print Service for themselves. In the future, the Video X Server normally started may contain Print X Server capability, so this model becomes very natural.
Startup would likely be done at session login or by launching actions or processes manually once the user logs in. Note: Deamons like "dtpdmd" must be started after Xprt.
Starting of the processes is straight forward. In strict order (example is for manually starting the X print server for CDEnext usage):
[machineA] % Xprt [-XpFile Xprinters file] [:dispNum]
Note that Xprt will look for configuration files in either a default location or where XPCONFIGDIR points.
-XpFile specifies an alternate `Xprinters' file, rather than the default one or `${XPCONFIGDIR}/C/print/Xprinters'.
[machineA] % dtpdmd -d machineA[:dispNum] [-l /tmp/dtpdmd.log]
The dtpdmd will maintain an X-Selection on the X-Server, and will start dtpdm's as required to service requests.
In all but the per-user session model, the machine running the dtpdmd (thus dtpdm's) will need display authorization to the users video display.
Once a X Print Server and dtpdmd have been started -- many of them in some cases -- clients will need to find and use them. There are two mechanisms that allow clients to discover X Print Servers and printers.
"X Print Specifier" - assuming usage of the DtPrint/XprintUtils-based print applications, the following notation is understood:
printer_name@machine[:dispNum]
For example:
colorlj7@printhub:2
In the above example, the X Print Server running at `printhub:2' is assumed to support the printer named `colorlj7'.
${XPSERVERLIST} - assuming usage of the DtPrint print dialogs, the environment variable ${XPSERVERLIST} can contain a list of X Print Servers. For example:
XPSERVERLIST="printhub:2 printhub:3 otherdept:0"
Then in the dialogs, only a printer name needs to be entered. The dialog will then search the X Print Servers in ${XPSERVERLIST} for a server than supports the printer, and then establish contact.
From most CDEnext applications, printing is accomplished by bringing down the File menu and selecting Print.... This will result in the DtPrintSetupBox dialog, which will request the name of a printer, and offer limited capability to configure print options (e.g. number of copies). If the user wishes, they can select Setup..., which will start a dtpdm capable of modifying additional print options. Finally, the user should select Print.
This environment variable points to the root of the Xprint server configuration directory hierarchy. If the variable is not defined, the default path is be assumed. The default path may be /usr/X11R6/lib/X11/xserver/, /usr/lib/X11/xserver/, /usr/share/Xprint/xserver/ or /usr/openwin/server/etc/XpConfig, depending on the system, and may be configured in /etc/init.d/xprint.
This environment variable selects the locale settings used by the Xprint server. Xprt allows language-specific settings (stored in ${XPCONFIGDIR}/${LANG}/print/) which will override the default settings (stored in ${XPCONFIGDIR}/C/print/). If ${LANG} is not set "C" is assumed.
The environment variable ${XPSERVERLIST} contains a list of display identifiers (separated by whitespace) which tell an application where it can find the Xprint servers. Usually ${XPSERVERLIST} is set by the profile startup scripts (e.g. /etc/profile or /etc/profile.d/xprint.sh) using the output of /etc/init.d/xprint get_xpserverlist.
Example:
export XPSERVERLIST="`/etc/init.d/xprint get_xpserverlist`"
Alternatively ${XPSERVERLIST} can be set manually. Example:
export XPSERVERLIST="littlecat:80 bitdog:72"
instructs an application to find an Xprint server at display 80 on the machine "littlecat" and at display 72 on the machine bigdog.
The environment variable ${XPRINTER} defines the default printer used by print applications. The syntax is either printername or printername@display.
Examples:
tells an application to look for the first printer named "ps003" on all Xprint servers.
tells an application to use the printer "hplaser19" on the Xprint server at display "littlecat:80".
If ${XPRINTER} is not set the applications will examine the values of the ${PDPRINTER}, ${LPDEST}, and ${PRINTER} environment variables (in that order).
X11(7), xplsprinters(7), xprehashprinterlist(7), xphelloworld(7), xpxmhelloworld(7), xpawhelloworld(7), xpxthelloworld(7), xpsimplehelloworld(7), Xserver(7), Xprt(7), libXp(3x), libXprintUtils(3x), libXprintAppUtils(3x), XmPrintShell(3x), XawPrintShell(3x), Xprint FAQ (http://xprint.mozdev.org/docs/Xprint_FAQ.html), Xprint main site (http://xprint.mozdev.org/)
This manual page was written by Roland Mainz <roland.mainz@nrubsig.org> based on the original X11R6.6 xc/programs/Xserver/XpConfig/README.