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Filter and Backend Programming

Filter and Backend Programming

Headers cups/backend.h
cups/sidechannel.h
Library -lcups
See Also Programming: Introduction to CUPS Programming
Programming: CUPS API
Programming: PPD API
Programming: Raster API
Programming: Developing PostScript Printer Drivers
Programming: Developing Raster Printer Drivers
Specifications: CUPS Design Description

Contents

    • Overview
    • Functions
    • Data Types
    • Constants

      • Overview

      Filters (which include printer drivers and port monitors) and backends are used to convert job files to a printable format and send that data to the printer itself. All of these programs use a common interface for processing print jobs and communicating status information to the scheduler. Each is run with a standard set of command-line arguments:

      argv[1]
      The job ID
      argv[2]
      The user printing the job
      argv[3]
      The job name/title
      argv[4]
      The number of copies to print
      argv[5]
      The options that were provided when the job was submitted
      argv[6]
      The file to print (first program only)

      The scheduler runs one or more of these programs to print any given job. The first filter reads from the print file and writes to the standard output, while the remaining filters read from the standard input and write to the standard output. The backend is the last filter in the chain and writes to the device.

      Filters are always run as a non-privileged user, typically "lp", with no connection to the user's desktop. Backends are run either as a non-privileged user or as root if the file permissions do not allow user or group execution. The file permissions section talks about this in more detail.

      Security Considerations

      It is always important to use security programming practices. Filters and most backends are run as a non-priviledged user, so the major security consideration is resource utilization - filters should not depend on unlimited amounts of CPU, memory, or disk space, and should protect against conditions that could lead to excess usage of any resource like infinite loops and unbounded recursion. In addition, filters must never allow the user to specify an arbitrary file path to a separator page, template, or other file used by the filter since that can lead to an unauthorized disclosure of information. Always treat input as suspect and validate it!

      If you are developing a backend that runs as root , make sure to check for potential buffer overflows, integer under/overflow conditions, and file accesses since these can lead to privilege escalations. When writing files, always validate the file path and never allow a user to determine where to store a file.

      Note:

      Never write files to a user's home directory. Aside from the security implications, CUPS is a network print service and as such the network user may not be the same as the local user and/or there may not be a local home directory to write to.

      In addition, some operating systems provide additional security mechanisms that further limit file system access, even for backends running as root. On Mac OS X, for example, no backend may write to a user's home directory.

      File Permissions

      For security reasons, CUPS will only run filters and backends that are owned by root and do not have world write permissions. The recommended permissions for filters and backends are 0555 - read and execute but no write. Backends that must run as root should use permissions of 0500 - read and execute by root, no access for other users. Write permissions can be enabled for the root user only.

      To avoid a warning message, the directory containing your filter(s) must also be owned by root and have world write disabled - permissions of 0755 or 0555 are strongly encouraged.

      Temporary Files

      Temporary files should be created in the directory specified by the "TMPDIR" environment variable. The cupsTempFile2 function can be used to safely create temporary files in this directory.

      Copy Generation

      The argv[4] argument specifies the number of copies to produce of the input file. In general, you should only generate copies if the filename argument is supplied. The only exception to this are filters that produce device-independent PostScript output, since the PostScript filter pstops is responsible for generating copies of PostScript files.

      Exit Codes

      Filters must exit with status 0 when they successfully generate print data or 1 when they encounter an error. Backends can return any of the cups_backend_t constants.

      Environment Variables

      The following environment variables are defined by the printing system when running print filters and backends:

      APPLE_LANGUAGE
      The Apple language identifier associated with the job (Mac OS X only).
      CHARSET
      The job character set, typically "utf-8".
      CLASS
      When a job is submitted to a printer class, contains the name of the destination printer class. Otherwise this environment variable will not be set.
      CONTENT_TYPE
      The MIME type associated with the file (e.g. application/postscript).
      CUPS_CACHEDIR
      The directory where cache files can be stored. Cache files can be used to retain information between jobs or files in a job.
      CUPS_DATADIR
      The directory where (read-only) CUPS data files can be found.
      CUPS_FILETYPE
      The type of file being printed: "job-sheet" for a banner page and "document" for a regular print file.
      CUPS_SERVERROOT
      The root directory of the server.
      DEVICE_URI
      The device-uri associated with the printer.
      FINAL_CONTENT_TYPE
      The MIME type associated with the printer (e.g. application/vnd.cups-postscript).
      LANG
      The language locale associated with the job.
      PPD
      The full pathname of the PostScript Printer Description (PPD) file for this printer.
      PRINTER
      The queue name of the class or printer.
      RIP_CACHE
      The recommended amount of memory to use for Raster Image Processors (RIPs).
      TMPDIR
      The directory where temporary files should be created.

      Communicating with the Scheduler

      Filters and backends communicate with the scheduler by writing messages to the standard error file. The scheduler reads messages from all filters in a job and processes the message based on its prefix. For example, the following code sets the current printer state message to "Printing page 5":

      int page = 5;

fprintf(stderr, "INFO: Printing page %d\n", page);

      Each message is a single line of text starting with one of the following prefix strings:

      ALERT: message
      Sets the printer-state-message attribute and adds the specified message to the current error log file using the "alert" log level.
      ATTR: attribute=value [attribute=value]
      Sets the named printer or job attribute(s). Typically this is used to set the marker-colors, marker-levels, marker-message, marker-names, marker-types, printer-alert, and printer-alert-description printer attributes. Standard marker-types values are listed in Table 1.
      CRIT: message
      Sets the printer-state-message attribute and adds the specified message to the current error log file using the "critical" log level.
      DEBUG: message
      Sets the printer-state-message attribute and adds the specified message to the current error log file using the "debug" log level.
      DEBUG2: message
      Sets the printer-state-message attribute and adds the specified message to the current error log file using the "debug2" log level.
      EMERG: message
      Sets the printer-state-message attribute and adds the specified message to the current error log file using the "emergency" log level.
      ERROR: message
      Sets the printer-state-message attribute and adds the specified message to the current error log file using the "error" log level. Use "ERROR:" messages for non-persistent processing errors.
      INFO: message
      Sets the printer-state-message attribute. If the current log level is set to "debug2", also adds the specified message to the current error log file using the "info" log level.
      NOTICE: message
      Sets the printer-state-message attribute and adds the specified message to the current error log file using the "notice" log level.
      PAGE: page-number #-copies
      PAGE: total #-pages
      Adds an entry to the current page log file. The first form adds #-copies to the job-media-sheets-completed attribute. The second form sets the job-media-sheets-completed attribute to #-pages.
      PPD: keyword=value [keyword=value ...]
      Changes or adds keywords to the printer's PPD file. Typically this is used to update installable options or default media settings based on the printer configuration.
      STATE: printer-state-reason [printer-state-reason ...]
      STATE: + printer-state-reason [printer-state-reason ...]
      STATE: - printer-state-reason [printer-state-reason ...]
      Sets, adds, or removes printer-state-reason keywords to the current queue. Typically this is used to indicate persistent media, ink, toner, and configuration conditions or errors on a printer. Table 2 lists the standard state keywords - use vendor-prefixed ("com.acme.foo") keywords for custom states.
      Note:

      "STATE:" messages often provide visible alerts to the user. For example, on Mac OS X setting a printer-state-reason value with an "-error" or "-warning" suffix will cause the printer's dock item to bounce if the corresponding reason is localized with a cupsIPPReason keyword in the printer's PPD file.

      WARNING: message
      Sets the printer-state-message attribute and adds the specified message to the current error log file using the "warning" log level.

      Messages without one of these prefixes are treated as if they began with the "DEBUG:" prefix string.

      Table 1: Standard marker-types Values
      marker-type Description
      developer Developer unit
      fuser Fuser unit
      fuserCleaningPad Fuser cleaning pad
      fuserOil Fuser oil
      ink Ink supply
      opc Photo conductor
      solidWax Wax supply
      staples Staple supply
      toner Toner supply
      transferUnit Transfer unit
      wasteInk Waste ink tank
      wasteToner Waste toner tank
      wasteWax Waste wax tank

      Table 2: Standard State Keywords
      Keyword Description
      connecting-to-device Connecting to printer but not printing yet
      cover-open A cover is open on the printer
      input-tray-missing An input tray is missing from the printer
      marker-supply-empty Out of ink
      marker-supply-low Low on ink
      marker-waste-almost-full Waste tank almost full
      marker-waste-full Waste tank full
      media-empty Out of media
      media-jam Media is jammed in the printer
      media-low Low on media
      paused Stop the printer
      timed-out Unable to connect to printer
      toner-empty Out of toner
      toner-low Low on toner

      Communicating with the Backend

      Filters can communicate with the backend via the cupsBackChannelRead and cupsSideChannelDoRequest functions. The cupsBackChannelRead function reads data that has been sent back from the device and is typically used to obtain status and configuration information. For example, the following code polls the backend for back-channel data:

      #include <cups/cups.h>

char buffer[8192];
ssize_t bytes;

/* Use a timeout of 0.0 seconds to poll for back-channel data */
bytes = cupsBackChannelRead(buffer, sizeof(buffer), 0.0);

      Filters can also use select() or poll() on the back-channel file descriptor (3 or CUPS_BC_FD) to read data only when it is available.

      The cupsSideChannelDoRequest function allows you to get out-of-band status information and do synchronization with the device. For example, the following code gets the current IEEE-1284 device ID string from the backend:

      #include <cups/sidechannel.h>

char data[2049];
int datalen;
cups_sc_status_t status;

/* Tell cupsSideChannelDoRequest() how big our buffer is, less 1 byte for
   nul-termination... */
datalen = sizeof(data) - 1;

/* Get the IEEE-1284 device ID, waiting for up to 1 second */
status = cupsSideChannelDoRequest(CUPS_SC_CMD_GET_DEVICE_ID, data, &datalen, 1.0);

/* Use the returned value if OK was returned and the length is non-zero */
if (status == CUPS_SC_STATUS_OK && datalen > 0)
  data[datalen] = '\0';
else
  data[0] = '\0';

      Communicating with Filters

      Backends communicate with filters using the reciprocal functions cupsBackChannelWrite, cupsSideChannelRead, and cupsSideChannelWrite. We recommend writing back-channel data using a timeout of 1.0 seconds:

      #include <cups/cups.h>

char buffer[8192];
ssize_t bytes;

/* Obtain data from printer/device */
...

/* Use a timeout of 1.0 seconds to give filters a chance to read */
cupsBackChannelWrite(buffer, bytes, 1.0);

      The cupsSideChannelRead function reads a side-channel command from a filter, driver, or port monitor. Backends can either poll for commands using a timeout of 0.0, wait indefinitely for commands using a timeout of -1.0 (probably in a separate thread for that purpose), or use select or poll on the CUPS_SC_FD file descriptor (4) to handle input and output on several file descriptors at the same time.

      Once a command is processed, the backend uses the cupsSideChannelWrite function to send its response. For example, the following code shows how to poll for a side-channel command and respond to it:

      #include <cups/sidechannel.h>

cups_sc_command_t command;
cups_sc_status_t status;
char data[2048];
int datalen = sizeof(data);

/* Poll for a command... */
if (!cupsSideChannelRead(&command, &status, data, &datalen, 0.0))
{
  switch (command)
  {
    /* handle supported commands, fill data/datalen/status with values as needed */

    default :
        status  = CUPS_SC_STATUS_NOT_IMPLEMENTED;
	datalen = 0;
	break;
  }

  /* Send a response... */
  cupsSideChannelWrite(command, status, data, datalen, 1.0);
}

      Doing SNMP Queries with Network Printers

      The Simple Network Management Protocol (SNMP) allows you to get the current status, page counter, and supply levels from most network printers. Every piece of information is associated with an Object Identifier (OID), and every printer has a community name associated with it. OIDs can be queried directly or by "walking" over a range of OIDs with a common prefix.

      The two CUPS SNMP functions provide a simple API for querying network printers through the side-channel interface. Each accepts a string containing an OID like ".1.3.6.1.2.1.43.10.2.1.4.1.1" (the standard page counter OID) along with a timeout for the query.

      The cupsSideChannelSNMPGet function queries a single OID and returns the value as a string in a buffer you supply:

      #include <cups/sidechannel.h>

char data[512];
int datalen = sizeof(data);

if (cupsSideChannelSNMPGet(".1.3.6.1.2.1.43.10.2.1.4.1.1", data, &datalen, 5.0)
        == CUPS_SC_STATUS_OK)
{
  /* Do something with the value */
  printf("Page counter is: %s\n", data);
}

      The cupsSideChannelSNMPWalk function allows you to query a whole group of OIDs, calling a function of your choice for each OID that is found:

      #include <cups/sidechannel.h>

void
my_callback(const char *oid, const char *data, int datalen, void *context)
{
  /* Do something with the value */
  printf("%s=%s\n", oid, data);
}

...

void *my_data;

cupsSNMPSideChannelWalk(".1.3.6.1.2.1.43", 5.0, my_callback, my_data);

      Functions

       CUPS 1.2/Mac OS X 10.5 cupsBackChannelRead

      Read data from the backchannel.

      ssize_t cupsBackChannelRead (
          char *buffer,
          size_t bytes,
          double timeout
      );

      Parameters

      buffer
      Buffer to read into
      bytes
      Bytes to read
      timeout
      Timeout in seconds, typically 0.0 to poll

      Return Value

      Bytes read or -1 on error

      Discussion

      Reads up to "bytes" bytes from the backchannel/backend. The "timeout" parameter controls how many seconds to wait for the data - use 0.0 to return immediately if there is no data, -1.0 to wait for data indefinitely.

       CUPS 1.2/Mac OS X 10.5 cupsBackChannelWrite

      Write data to the backchannel.

      ssize_t cupsBackChannelWrite (
          const char *buffer,
          size_t bytes,
          double timeout
      );

      Parameters

      buffer
      Buffer to write
      bytes
      Bytes to write
      timeout
      Timeout in seconds, typically 1.0

      Return Value

      Bytes written or -1 on error

      Discussion

      Writes "bytes" bytes to the backchannel/filter. The "timeout" parameter controls how many seconds to wait for the data to be written - use 0.0 to return immediately if the data cannot be written, -1.0 to wait indefinitely.

       CUPS 1.2/Mac OS X 10.5 cupsBackendDeviceURI

      Get the device URI for a backend.

      const char *cupsBackendDeviceURI (
          char **argv
      );

      Parameters

      argv
      Command-line arguments

      Return Value

      Device URI or NULL

      Discussion

      The "argv" argument is the argv argument passed to main(). This function returns the device URI passed in the DEVICE_URI environment variable or the device URI passed in argv[0], whichever is found first.

       CUPS 1.4/Mac OS X 10.6 cupsBackendReport

      Write a device line from a backend.

      void cupsBackendReport (
          const char *device_scheme,
          const char *device_uri,
          const char *device_make_and_model,
          const char *device_info,
          const char *device_id,
          const char *device_location
      );

      Parameters

      device_scheme
      device-scheme string
      device_uri
      device-uri string
      device_make_and_model
      device-make-and-model string or NULL
      device_info
      device-info string or NULL
      device_id
      device-id string or NULL
      device_location
      device-location string or NULL

      Discussion

      This function writes a single device line to stdout for a backend. It handles quoting of special characters in the device-make-and-model, device-info, device-id, and device-location strings.

       CUPS 1.3/Mac OS X 10.5 cupsSideChannelDoRequest

      Send a side-channel command to a backend and wait for a response.

      cups_sc_status_t cupsSideChannelDoRequest (
          cups_sc_command_t command,
          char *data,
          int *datalen,
          double timeout
      );

      Parameters

      command
      Command to send
      data
      Response data buffer pointer
      datalen
      Size of data buffer on entry, number of bytes in buffer on return
      timeout
      Timeout in seconds

      Return Value

      Status of command

      Discussion

      This function is normally only called by filters, drivers, or port monitors in order to communicate with the backend used by the current printer. Programs must be prepared to handle timeout or "not implemented" status codes, which indicate that the backend or device do not support the specified side-channel command.

      The "datalen" parameter must be initialized to the size of the buffer pointed to by the "data" parameter. cupsSideChannelDoRequest() will update the value to contain the number of data bytes in the buffer.

       CUPS 1.3/Mac OS X 10.5 cupsSideChannelRead

      Read a side-channel message.

      int cupsSideChannelRead (
          cups_sc_command_t *command,
          cups_sc_status_t *status,
          char *data,
          int *datalen,
          double timeout
      );

      Parameters

      command
      Command code
      status
      Status code
      data
      Data buffer pointer
      datalen
      Size of data buffer on entry, number of bytes in buffer on return
      timeout
      Timeout in seconds

      Return Value

      0 on success, -1 on error

      Discussion

      This function is normally only called by backend programs to read commands from a filter, driver, or port monitor program. The caller must be prepared to handle incomplete or invalid messages and return the corresponding status codes.

      The "datalen" parameter must be initialized to the size of the buffer pointed to by the "data" parameter. cupsSideChannelDoRequest() will update the value to contain the number of data bytes in the buffer.

       CUPS 1.4/Mac OS X 10.6 cupsSideChannelSNMPGet

      Query a SNMP OID's value.

      cups_sc_status_t cupsSideChannelSNMPGet (
          const char *oid,
          char *data,
          int *datalen,
          double timeout
      );

      Parameters

      oid
      OID to query
      data
      Buffer for OID value
      datalen
      Size of OID buffer on entry, size of value on return
      timeout
      Timeout in seconds

      Return Value

      Query status

      Discussion

      This function asks the backend to do a SNMP OID query on behalf of the filter, port monitor, or backend using the default community name.

      "oid" contains a numeric OID consisting of integers separated by periods, for example ".1.3.6.1.2.1.43". Symbolic names from SNMP MIBs are not supported and must be converted to their numeric forms.

      On input, "data" and "datalen" provide the location and size of the buffer to hold the OID value as a string. HEX-String (binary) values are converted to hexadecimal strings representing the binary data, while NULL-Value and unknown OID types are returned as the empty string. The returned "datalen" does not include the trailing nul. CUPS_SC_STATUS_NOT_IMPLEMENTED is returned by backends that do not support SNMP queries. CUPS_SC_STATUS_NO_RESPONSE is returned when the printer does not respond to the SNMP query.

       CUPS 1.4/Mac OS X 10.6 cupsSideChannelSNMPWalk

      Query multiple SNMP OID values.

      cups_sc_status_t cupsSideChannelSNMPWalk (
          const char *oid,
          double timeout,
          cups_sc_walk_func_t cb,
          void *context
      );

      Parameters

      oid
      First numeric OID to query
      timeout
      Timeout for each query in seconds
      cb
      Function to call with each value
      context
      Application-defined pointer to send to callback

      Return Value

      Status of first query of CUPS_SC_STATUS_OK on success

      Discussion

      This function asks the backend to do multiple SNMP OID queries on behalf of the filter, port monitor, or backend using the default community name. All OIDs under the "parent" OID are queried and the results are sent to the callback function you provide.

      "oid" contains a numeric OID consisting of integers separated by periods, for example ".1.3.6.1.2.1.43". Symbolic names from SNMP MIBs are not supported and must be converted to their numeric forms.

      "timeout" specifies the timeout for each OID query. The total amount of time will depend on the number of OID values found and the time required for each query.

      "cb" provides a function to call for every value that is found. "context" is an application-defined pointer that is sent to the callback function along with the OID and current data. The data passed to the callback is the same as returned by cupsSideChannelSNMPGet. CUPS_SC_STATUS_NOT_IMPLEMENTED is returned by backends that do not support SNMP queries. CUPS_SC_STATUS_NO_RESPONSE is returned when the printer does not respond to the first SNMP query.

       CUPS 1.3/Mac OS X 10.5 cupsSideChannelWrite

      Write a side-channel message.

      int cupsSideChannelWrite (
          cups_sc_command_t command,
          cups_sc_status_t status,
          const char *data,
          int datalen,
          double timeout
      );

      Parameters

      command
      Command code
      status
      Status code
      data
      Data buffer pointer
      datalen
      Number of bytes of data
      timeout
      Timeout in seconds

      Return Value

      0 on success, -1 on error

      Discussion

      This function is normally only called by backend programs to send responses to a filter, driver, or port monitor program.

      Data Types

      cups_backend_t

      Backend exit codes

      typedef enum cups_backend_e cups_backend_t;

      cups_sc_bidi_t

      Bidirectional capabilities

      typedef enum cups_sc_bidi_e cups_sc_bidi_t;

      cups_sc_command_t

      Request command codes

      typedef enum cups_sc_command_e cups_sc_command_t;

      cups_sc_state_t

      Printer state bits

      typedef enum cups_sc_state_e cups_sc_state_t;

      cups_sc_status_t

      Response status codes

      typedef enum cups_sc_status_e cups_sc_status_t;

      cups_sc_walk_func_t

      SNMP walk callback

      typedef void (*cups_sc_walk_func_t)(const char *oid, const char *data, int datalen, void *context);

      Constants

      cups_backend_e

      Backend exit codes

      Constants

      CUPS_BACKEND_AUTH_REQUIRED
      Job failed, authentication required
      CUPS_BACKEND_CANCEL
      Job failed, cancel job
      CUPS_BACKEND_FAILED
      Job failed, use error-policy
      CUPS_BACKEND_HOLD
      Job failed, hold job
      CUPS_BACKEND_OK
      Job completed successfully
      CUPS_BACKEND_STOP
      Job failed, stop queue

      cups_sc_bidi_e

      Bidirectional capability values

      Constants

      CUPS_SC_BIDI_NOT_SUPPORTED
      Bidirectional I/O is not supported
      CUPS_SC_BIDI_SUPPORTED
      Bidirectional I/O is supported

      cups_sc_command_e

      Request command codes

      Constants

      CUPS_SC_CMD_DRAIN_OUTPUT
      Drain all pending output
      CUPS_SC_CMD_GET_BIDI
      Return bidirectional capabilities
      CUPS_SC_CMD_GET_DEVICE_ID
      Return the IEEE-1284 device ID
      CUPS_SC_CMD_GET_STATE
      Return the device state
      CUPS_SC_CMD_SNMP_GET  CUPS 1.4/Mac OS X 10.6 
      Query an SNMP OID
      CUPS_SC_CMD_SNMP_GET_NEXT  CUPS 1.4/Mac OS X 10.6 
      Query the next SNMP OID
      CUPS_SC_CMD_SOFT_RESET
      Do a soft reset

      cups_sc_state_e

      Printer state bits

      Constants

      CUPS_SC_STATE_BUSY
      Device is busy
      CUPS_SC_STATE_ERROR
      Other error condition
      CUPS_SC_STATE_MARKER_EMPTY
      Toner/ink out condition
      CUPS_SC_STATE_MARKER_LOW
      Toner/ink low condition
      CUPS_SC_STATE_MEDIA_EMPTY
      Paper out condition
      CUPS_SC_STATE_MEDIA_LOW
      Paper low condition
      CUPS_SC_STATE_OFFLINE
      Device is offline
      CUPS_SC_STATE_ONLINE
      Device is online

      cups_sc_status_e

      Response status codes

      Constants

      CUPS_SC_STATUS_BAD_MESSAGE
      The command/response message was invalid
      CUPS_SC_STATUS_IO_ERROR
      An I/O error occurred
      CUPS_SC_STATUS_NONE
      No status
      CUPS_SC_STATUS_NOT_IMPLEMENTED
      Command not implemented
      CUPS_SC_STATUS_NO_RESPONSE
      The device did not respond
      CUPS_SC_STATUS_OK
      Operation succeeded
      CUPS_SC_STATUS_TIMEOUT
      The backend did not respond
      CUPS_SC_STATUS_TOO_BIG
      Response too big
 
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