Filter and Backend APIs
Keywords: Programming

## Introduction

The CUPS filter and backend APIs define standard exit codes and provide access to the backchannel data stream. They are only used when writing backends, filters, and port monitors.

## General Usage

The <cups/backend.h> and <cups/cups.h> header files must be included to use the `CUPS_BACKEND_` constants and `cupsBackChannel` functions, respectively.

The <cups/sidechannel.h> header file must be included to use the `CUPS_SC_` constants and `cupsSideChannel` functions.

Programs using these functions must be linked to the CUPS library: libcups.a, libcups.so.2, libcups.2.dylib, libcups_s.a, or libcups2.lib depending on the platform. The following command compiles myprogram.c using GCC and the CUPS library:

    gcc -o myprogram myprogram.c -lcups

## Compatibility

The `cupsBackChannel` functions require CUPS 1.2 or higher. The `cupsSideChannel` functions require CUPS 1.3 or higher.

## Using the cupsBackChannel APIs

The `cupsBackChannel` APIs allow your filters, drivers, and port monitors to read data back from a printer and your backends to send data from a printer to the filters, drivers, and port monitors associated with the current job. Back-channel data is normally sent by the printer in response to a command sent from your program to the printer via `stdout`.

The `cupsBackChannelRead()` function reads data from the printer via the backend. You provide a timeout in seconds along with a buffer pointer and the size of that buffer. It returns the number of bytes or -1 if there was an error. The following code example shows how to poll for back-channel data in your program:

    #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);

If you are writing a backend, the `cupsBackChannelWrite()` function sends any back-channel data you have received from the printer to upstream filters in the print filter chain. We recommend using a timeout of 1.0 seconds:

    #include <cups/cups.h>

    char buffer[8192];
    ssize_t bytes;

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

## Using the cupsSideChannel APIs

The `cupsSideChannel` APIs allow your filters, drivers, port monitors, and backend to send and receive the following out-of-band commands:

- `CUPS_SC_CMD_SOFT_RESET` - Do a soft reset
- `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

### Sending Commands from a Filter, Driver, or Port Monitor

The `cupsSideChannelDoRequest()` function is used by filters, drivers, and port monitors to send a command to the backend and read back a response:

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

The `CUPS_SC_CMD_SOFT_RESET` and `CUPS_SC_CMD_DRAIN_OUTPUT` commands do not return any data values, while the others return one or more bytes. The `timeout` parameter allows your program to poll or wait for the command to complete - use a timeout of 30 seconds for `CUPS_SC_CMD_SOFT_RESET` and `CUPS_SC_CMD_DRAIN_OUTPUT` and a timeout of 1 second for all other commands.

`CUPS_SC_CMD_GET_BIDI` returns a single `char` value that tells you whether the backend supports bidirectional communications:

    #include <cups/sidechannel.h>

    char data;
    int datalen;
    cups_sc_bidi_t bidi;
    cups_sc_status_t status;

    /* Tell cupsSideChannelDoRequest() how big our buffer is... */
    datalen = 1;

    /* Get the bidirectional capabilities, waiting for up to 1 second */
    status  = cupsSideChannelDoRequest(CUPS_SC_CMD_GET_BIDI, &data, &datalen, 1.0);

    /* Use the returned value if OK was returned and the length is still 1 */
    if (status == CUPS_SC_STATUS_OK && datalen == 1)
      bidi = (cups_sc_bidi_t)data;
    else
      bidi = CUPS_SC_BIDI_NOT_SUPPORTED;

`CUPS_SC_CMD_GET_DEVICE_ID` returns a string of characters containing the IEEE-1284 device ID for the connected printer:

    #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';

`CUPS_SC_CMD_GET_STATE` returns a single `char` value that tells you the current device state:

    #include <cups/sidechannel.h>

    char data;
    int datalen;
    cups_sc_state_t state;
    cups_sc_status_t status;

    /* Tell cupsSideChannelDoRequest() how big our buffer is... */
    datalen = 1;

    /* Get the bidirectional capabilities, waiting for up to 1 second */
    status  = cupsSideChannelDoRequest(CUPS_SC_CMD_GET_STATE, &data, &datalen, 1.0);

    /* Use the returned value if OK was returned and the length is still 1 */
    if (status == CUPS_SC_STATUS_OK && datalen == 1)
      state = (cups_sc_state_t)data;
    else
      state = CUPS_SC_STATE_OFFLINE;

### Handling Commands in your Backend

The `cupsSideChannelRead()` function is used by backends to read a command from a filter, driver, or port monitor:

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

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. Backends can pass `NULL` for the `data` and `datalen` parameters, since none of the commands sent by upstream filters contain any data at this time.

Once a command is processed, the backend uses the `cupsSideChannelWrite()` function to send its response:

    #include <cups/sidechannel.h>

    cups_sc_command_t command;
    cups_sc_status_t status;

    /* Poll for a command... */
    if (!cupsSideChannelRead(&command, &status, NULL, NULL, 0.0))
    {
      char data[2048];
      int datalen;

      switch (command)
      {
        ... handle supported commands, file 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);
    }

## Contents

- Functions

## Functions

- `cupsBackChannelRead()` CUPS 1.2
- `cupsBackChannelWrite()` CUPS 1.2
- `cupsSideChannelDoRequest()` CUPS 1.3
- `cupsSideChannelRead()` CUPS 1.3
- `cupsSideChannelWrite()` CUPS 1.3

### CUPS 1.2 cupsBackChannelRead()

#### Description

Read data from the backchannel.

Reads up to "bytes" bytes from the backchannel. 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.

#### Syntax

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

#### Arguments

Name | Description
--- | ---
`buffer` | Buffer to read
`bytes` | Bytes to read
`timeout` | Timeout in seconds

#### Returns

Bytes read or -1 on error

### CUPS 1.2 cupsBackChannelWrite()

#### Description

Write data to the backchannel.

Writes "bytes" bytes to the backchannel. 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.

#### Syntax

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

#### Arguments

Name | Description
--- | ---
`buffer` | Buffer to write
`bytes` | Bytes to write
`timeout` | Timeout in seconds

#### Returns

Bytes written or -1 on error

### CUPS 1.3 cupsSideChannelDoRequest()

#### Description

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

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.

#### Syntax

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

#### Arguments

Name | Description
--- | ---
`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

#### Returns

Status of command

### CUPS 1.3 cupsSideChannelRead()

#### Description

Read a side-channel message.

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.

#### Syntax

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

#### Arguments

Name | Description
--- | ---
`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

#### Returns

0 on success, -1 on error

### CUPS 1.3 cupsSideChannelWrite()

#### Description

Write a side-channel message.

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

#### Syntax

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

#### Arguments

Name | Description
--- | ---
`command` | Command code
`status` | Status code
`data` | Data buffer pointer
`datalen` | Number of bytes of data
`timeout` | Timeout in seconds

#### Returns

0 on success, -1 on error