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<META NAME="COPYRIGHT" CONTENT="Copyright 1997-2005, All Rights Reserved">
<META NAME="DOCNUMBER" CONTENT="CUPS-SDD-1.1">
<META NAME="Author" CONTENT="Easy Software Products">
<TITLE>CUPS Software Design Description</TITLE>
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<H1>Scope</H1>
<H2>Identification</H2>
This software design description document provides general information
on the architecture and coding of the Common UNIX Printing System
("CUPS") Version 1.1.
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<H2>Document Overview</H2>
This software design description document is organized into the
following sections:
<UL>
<LI>1 - Scope
<LI>2 - References
<LI>3 - Design Overview
<LI>A - Glossary
</UL>
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<H1>Design Overview</H1>
CUPS is composed of 9 software sub-systems that operate together to
perform common printing tasks:
<UL>
<LI>Backends
<LI>Berkeley Commands
<LI>CGI
<LI>CUPS Application Programmers Interface
<LI>CUPS Imaging Library
<LI>Daemons
<LI>Filters
<LI>Scheduler
<LI>System V Commands
</UL>
<H2>Backends</H2>
The backends implement communications over a number of different interfaces.
All backends are called with a common set of arguments:
<UL>
<LI>Device URI - the Uniform Resource Identifier for the output device
(e.g. <CODE>parallel:/dev/plp</CODE>,
<CODE>ipp://hostname/resource</CODE>).
<LI>Job Identifier - the job identifier for this job (integer).
<LI>User Name - the user associated with this job (name string).
<LI>Title - the title/job-name associated with this job (name string).
<LI>Copies - the number of copies required (integer).
<LI>Options - the options associated with this job (space separated
option strings).
<LI>Filename (optional) - the file to print; if this option is not
specified, the backend must read the print file from the standard
input.
</UL>
<P>Backends are named using the scheme of the URI, so a URI of
"ipp://hostname/resource" would be processed by the "ipp" backend.
<H3>ipp</H3>
<P>The ipp backend sends the specified job to a network printer or host using
the Internet Printing Protocol. The URI is as specified by the
<CODE>printer-uri-supported</CODE> attribute from the printer or host.
<H3>lpd</H3>
<P>The lpd backend sends the specified job to a network printer or host using
the Line Printer Daemon protocol. The URI is of the form:
<UL><PRE>lpd://hostname/queue
</PRE></UL>
<H3>parallel</H3>
<P>The parallel backend sends the specified job to a local printer connected
via the specified parallel port device. The URI is of the form:
<UL><PRE>parallel:/dev/file
</PRE></UL>
<H3>serial</H3>
<P>The serial backend sends the specified job to a local printer connected
via the specified serial port device. The URI is of the form:
<UL><PRE>serial:/dev/file?option[+option+...]
</PRE></UL>
The options can be any combination of the following:
<UL>
<LI><CODE>baud=<I>rate</I></CODE> - Sets the baud rate for the device.
<LI><CODE>bits=<I>7 or 8</I></CODE> - Sets the number of data bits.
<LI><CODE>parity=<I>even</I></CODE> - Sets even parity checking.
<LI><CODE>parity=<I>odd</I></CODE> - Sets odd parity checking.
<LI><CODE>parity=<I>none</I></CODE> - Turns parity checking off.
<LI><CODE>flow=dtrdsr<I></I></CODE> - Turns DTR/DSR (hardware) flow
control on.
<LI><CODE>flow=hard<I></I></CODE> - Turns RTS/CTS
(hardware) flow control on.
<LI><CODE>flow=none<I></I></CODE> - Turns flow control off.
<LI><CODE>flow=rtscts<I></I></CODE> - Turns RTS/CTS
(hardware) flow control on.
<LI><CODE>flow=xonxoff<I></I></CODE> - Turns XON/XOFF
(software) flow control on.
</UL>
<H3>socket</H3>
<P>The socket backend sends the specified job to a network host using the
AppSocket protocol commonly used by Hewlett-Packard and Tektronix
printers. The URI is of the form:
<UL><PRE>socket://hostname[:port]
</PRE></UL>
The default port number is 9100.
<H3>usb</H3>
<P>The usb backend sends the specified job to a local printer connected
via the specified usb port device. The URI is of the form:
<UL><PRE>usb:/dev/file
</PRE></UL>
<H2>Berkeley Commands</H2>
<P>The Berkeley commands provide a simple command-line interface to CUPS
to submit and control print jobs. It is provided for compatibility with
existing software that is hardcoded to use the Berkeley commands.
<H3>lpc</H3>
The lpc command allows users and administrators to check the status and
control print queues. The version provided with CUPS supports the following
commands:
<UL>
<LI>quit - Quits the lpc command.
<LI>status - Shows the status of printers and jobs in the queue.
</UL>
<H3>lpq</H3>
<P>The lpq command shows the current queue status.
<H3>lpr</H3>
<P>The lpr command submits a job for printing. The CUPS version of lpr silently
ignores the "i", "t", "m", "h", and "s" options.
<H3>lprm</H3>
<P>The lprm removes one or more print jobs.
<H2>CGI</H2>
<P>The Common Gateway Interface (CGI) programs provide a web-based
status interface to monitor the status of printers, classes, and jobs.
Each of the CGIs utilize HTML template files that can be customized to
provide alternate appearances.
<H3>admin.cgi</H3>
<P>The admin CGI provides administration interfaces for printers and
classes. The user can add, modify, delete, start, stop, and configure
printers and classes using "wizard" interfaces.
<H3>classes.cgi</H3>
<P>The classes CGI lists the available printer classes and any pending
jobs for the class. The user can click on individual classes to limit
the display and click on jobs to see the job status.
<H3>jobs.cgi</H3>
<P>The jobs CGI lists the queued print jobs in order of priority. The
list can be limited by printer or job.
<H3>printers.cgi</H3>
<P>The printers CGI lists the available printer queues and any pending
jobs for the printer. The user can click on individual printers to
limit the display and click on jobs to see the job status.
<H2>CUPS Application Programmers Interface</H2>
<P>The CUPS Application Programmers Interface ("API") provides common
convenience, HTTP, IPP, language, and PPD functions used by the CUPS
software.
<H3>Convenience Functions</H3>
<P>Convenience functions are provided to submit an IPP request, send a
print file, cancel a job, get a list of available printers, get a list
of available classes, get the default printer or class, get the default
server name, get the local username, and get a password string.
<H3>HTTP Functions</H3>
<P>The HTTP functions provide functions to connect to HTTP servers,
issue requests, read data from a server, and write data to a server.
<H3>IPP Functions</H3>
<P>The IPP function provide functions to manage IPP request data and
attributes, read IPP responses from a server, and write IPP requests to
a server.
<H3>Language Functions</H3>
<P>The language functions provide a standard interface for retrieving
common textual messages for a particular locale and determining the
correct encoding (e.g. US ASCII, UTF-8, ISO-8859-1, etc.)
<H3>PPD Functions</H3>
<P>The PostScript Printer Description functions manage PPD files,
select options, check for option conflicts, and emit selected options
in the correct order.
<H2>CUPS Imaging Library</H2>
<P>The CUPS imaging library provides colorspace conversion, color
management, image management, scaling, image file, and raster functions
used by the CUPS raster filters.
<H3>Colorspace Conversion Functions</H3>
<P>The colorspace conversion functions handle conversion of grayscale
and RGB colors to grayscale, RGB, K, CMY, CMYK, and CMYKcm colorspaces.
<H3>Color Management Functions</H3>
<P>The color management functions handle gamut mapping and density
correction. These are integrated with the colorspace conversion
functions so that colorspace conversion and color management are
processed in a single step.
<H3>Image Management Functions</H3>
<P>The image management functions manage a tiled image database that is
swapped to/from disk as needed.
<H3>Scaling Functions</H3>
<P>The scaling functions provide image scaling services using
nearest-neighbor sampling and bilinear interpolation as appropriate.
<H3>Image File Functions</H3>
<P>The image file functions handle loading of all image file formats.
<H3>Raster Functions</H3>
<P>The raster functions manage streams of CUPS raster data (described
in the Interface Design Document) used by non-PostScript printer
drivers and raster filters.
<H2>Daemons</H2>
<P>The daemons provide additional network functions for the scheduler.
Currently only two daemons are provided with CUPS.
<H3>Line Printer Daemon</H3>
<P>The line printer daemon provides remote LPD client support and is
run by the <CODE>inetd(8)</CODE> daemon as needed.
<H3>Polling Daemon</H3>
<P>The polling daemon is used to poll a remote server for a list of
available printers and provide it to the scheduler for addition. A
separate polling daemon is run by the scheduler for every remote
system listed for polling in the scheduler configuration file.
<H2>Filters</H2>
<P>The filters implement file conversion services for CUPS. All filters
are called with a common set of arguments:
<UL>
<LI>Printer name - the name of the destination printer (name string).
<LI>Job Identifier - the job identifier for this job (integer).
<LI>User Name - the user associated with this job (name string).
<LI>Title - the title/job-name associated with this job (name string).
<LI>Copies - the number of copies required (integer).
<LI>Options - the options associated with this job (space separated
option strings).
<LI>Filename (optional) - the file to print; if this option is not
specified, the filter must read the input file from the standard
input.
</UL>
<P>Filters are added to the MIME conversion data file and implement all
necessary conversions from one file type to another.
<H3>hpgltops</H3>
<P>The hpgltops filter converts HP-GL/2 files into PostScript.
<H3>imagetops</H3>
<P>The imagetops filter converts image files into PostScript.
<H3>imagetoraster</H3>
<P>The imagetoraster filter converts image files into CUPS raster data.
<H3>pdftops</H3>
<P>The pdftops filter converts PDF files into PostScript.
<H3>pstops</H3>
<P>The pstops filter inserts printer-specific commands from PPD files and
performs page filtering as requested by the user.
<H3>pstoraster</H3>
<P>The pstoraster filter converts PostScript program data into CUPS
raster data.
<H3>rastertoepson</H3>
<P>The rastertoepson filter handles converting CUPS raster data to
ESC/P and supports both color and black-and-white printers.
<H3>rastertohp</H3>
<P>The rastertohp filter handles converting CUPS raster data to HP-PCL
and supports both color and black-and-white printers.
<H3>texttops</H3>
<P>The texttops filter converts text files into PostScript.
<H2>Scheduler</H2>
<P>The scheduler is a fully-functional HTTP/1.1 and IPP/1.1 server that
manages the printers, classes, and jobs in the system. It also handles
a simple broadcast-based directory service so that remote print queues
and classes can be accessed transparently from the local system.
<H3>Authorization</H3>
<P>The authorization module is responsible for performing access
control and authentication for all HTTP and IPP requests entering the
system.
<H3>Classes</H3>
<P>The classes module is responsible for managing printer classes in
the system. Each class is a collection of local and/or remote
printers. The classes module also reads and writes the classes
configuration file.
<H3>Client</H3>
<P>The client module is responsible for all HTTP client
communications. It handles listening on selected interfaces, accepting
connections from prospective clients, processing incoming HTTP
requests, and sending HTTP responses to those requests. The client
module also is responsible for executing the external CGI programs as
needed to support web-based printer, class, and job status monitoring
and administration.
<P>Once authorized, all IPP requests are sent to the IPP module.
<H3>Configuration</H3>
<P>The configuration module is responsible for reading the CUPS
configuration file and initializing the appropriate data structures and
values. The configuration module also stops CUPS services before
reading the configuration file and restarts them after the
configuration file has been read.
<H3>Devices</H3>
<P>The devices module is responsible for managing the list of available
devices for the CUPS-Get-Devices operation.
<H3>Directory Services</H3>
<P>The directory services module sends and recieves printer state
information over a broadcast socket. Remote printers and classes are
automatically added to or removed from the local printer and class
lists as needed.
<P>The directory services module can only recieve printer state information
over a single UDP port, however it can broadcast to multiple addresses and
ports as needed.
<H3>IPP</H3>
<P>The IPP module handles IPP requests and acts accordingly. URI
validation is also performed here, as a client can post IPP data to any
URI on the server which might sidestep the access control or
authentication of the HTTP server.
<H3>Jobs</H3>
<P>The jobs module manages print jobs, starts filter and backend
processes for jobs to be printed, and monitors status messages from
those filters and backends.
<H3>Logging</H3>
<P>The logging module manages the access, error, and page log files
that are generated by the scheduler.
<H3>Main</H3>
<P>The main module is responsible for timing out and dispatching input
and output for client connections. It also watches for incoming
<CODE>SIGHUP</CODE> and <CODE>SIGCHLD</CODE> signals, reloads the
server configuration files as needed, and handles child process errors
and exits.
<H3>MIME</H3>
<P>The Multimedia Internet Mail Exchange module manages a MIME type and
conversion database that supports file typing by extension and content
and least-cost file filtering from a source to a destination file type.
<H3>PPDs</H3>
<P>The PPDs module is responsible for managing the list of available
PPD files for the CUPS-Get-PPDs operation.
<H3>Printers</H3>
<P>The printers module is responsible for managing printers and PPD
files in the system. The printers module also reads and writes the
printers configuration file.
<H2>System V Commands</H2>
<P>The System V commands provide a robust command-line interface to
CUPS to submit and control printers and jobs.
<H3>accept</H3>
<P>The accept command tells the scheduler to accept new jobs for specific
printers.
<H3>cancel</H3>
<P>The cancel command tells the scheduler to cancel one or more jobs that are
queued for printing.
<H3>disable</H3>
<P>The disable command tells the scheduler to stop printing jobs on the
specified printers.
<H3>enable</H3>
<P>The enable command tells the scheduler to start printing jobs on the
specified printers.
<H3>lp</H3>
<P>The lp command submits submits files for printing. Unlike the standard
System V lp command, a single CUPS lp command will generate a separate
job ID for each file that is printed. Also, the Solaris "f", "H", "P", "S",
and "y" options are silently ignored.
<H3>lpadmin</H3>
<P>The lpadmin command manages printer queues and classes. The Solaris
"A", "F", "I", "M", "P", "Q", "S", "T", "U", "W", "f", "l", "m", "o",
"s", "t", and "u" options are not supported, and new options "P" (PPD
file) and "E" (enable and accept) are provided to configure
CUPS-specific features.
<H3>lpinfo</H3>
<P>The lpinfo command lists the available PPD files or devices as selected
by the user.
<H3>lpmove</H3>
<P>The lpmove command moves a print job to a new destination.
<H3>lpoptions</H3>
<P>The lpoptions command manages user-defined printers and options.
<H3>lpstat</H3>
<P>The lpstat command lists printers, classes, and jobs as requested by the
user.
<H3>reject</H3>
<P>The reject command tells the scheduler not to accept new jobs for specific
printers.
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