# 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
  - Security Considerations
  - File Permissions
  - Temporary Files
  - Copy Generation
  - Exit Codes
  - Environment Variables
  - Communicating with the Scheduler
  - Communicating with the Backend
  - Communicating with Filters
  - Doing SNMP Queries with Network Printers
- Functions
  - cupsBackChannelRead
  - cupsBackChannelWrite
  - cupsBackendDeviceURI
  - cupsBackendReport
  - cupsSideChannelDoRequest
  - cupsSideChannelRead
  - cupsSideChannelSNMPGet
  - cupsSideChannelSNMPWalk
  - cupsSideChannelWrite
- Data Types
  - cups_backend_t
  - cups_sc_bidi_t
  - cups_sc_command_t
  - cups_sc_state_t
  - cups_sc_status_t
  - cups_sc_walk_func_t
- Constants
  - cups_backend_e
  - cups_sc_bidi_e
  - cups_sc_command_e
  - cups_sc_state_e
  - cups_sc_status_e

## 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-privileged 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 or group 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 and group 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