Filters, printer drivers, port monitors, and backends use a common interface for processing print jobs and communicating status information to the scheduler. Each filter is run with a standard set of command-line arguments:
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 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.
The following environment variables are defined by the printing system:
Filters and backends communicate wih the scheduler by writing messages to the standard error file. 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:
marker-colors, marker-levels,
marker-names, marker-types,
printer-alert, and printer-alert-description
printer attributes.Messages without one of these prefixes are treated as if they began with the "DEBUG:" prefix string.
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);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';
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; /* 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. 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. 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; /* 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); }