| Headers | cups/backend.h cups/sidechannel.h cups/snmp.h |
|---|---|
| Library | -lcups |
| See Also | Programming: Introduction to CUPS Programming Programming: CUPS API Programming: PPD API Programming: Raster API |
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.
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.
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.
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.
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); }
Re-write for side-channel-based SNMP queries.
Read data from the backchannel.
ssize_t cupsBackChannelRead (
char *buffer,
size_t bytes,
double timeout
);
Bytes read or -1 on error
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.
Write data to the backchannel.
ssize_t cupsBackChannelWrite (
const char *buffer,
size_t bytes,
double timeout
);
Bytes written or -1 on error
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.
Get the device URI for a backend.
const char *cupsBackendDeviceURI (
char **argv
);
Device URI or NULL
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.
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
);
Status of command
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.
Read a side-channel message.
int cupsSideChannelRead (
cups_sc_command_t *command,
cups_sc_status_t *status,
char *data,
int *datalen,
double timeout
);
0 on success, -1 on error
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.
Write a side-channel message.
int cupsSideChannelWrite (
cups_sc_command_t command,
cups_sc_status_t status,
const char *data,
int datalen,
double timeout
);
0 on success, -1 on error
This function is normally only called by backend programs to send responses to a filter, driver, or port monitor program.
Backend exit codes
typedef enum cups_backend_e cups_backend_t;
Bidirectional capabilities
typedef enum cups_sc_bidi_e cups_sc_bidi_t;
Request command codes
typedef enum cups_sc_command_e cups_sc_command_t;
Printer state bits
typedef enum cups_sc_state_e cups_sc_state_t;
Response status codes
typedef enum cups_sc_status_e cups_sc_status_t;
Backend exit codes
Bidirectional capabilities
Request command codes
Printer state bits
Response status codes