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 Specifications: CUPS Design Description |
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:
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 when running print filters and backends:
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:
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.Messages without one of these prefixes are treated as if they began with the "DEBUG:" prefix string.
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 |
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 |
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';
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); }
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);
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/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.
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/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.
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.
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
);
NULL
NULL
NULL
NULL
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.
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.
Query a SNMP OID's value.
cups_sc_status_t cupsSideChannelSNMPGet (
const char *oid,
char *data,
int *datalen,
double timeout
);
Query status
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.
Query multiple SNMP OID values.
cups_sc_status_t cupsSideChannelSNMPWalk (
const char *oid,
double timeout,
cups_sc_walk_func_t cb,
void *context
);
Status of first query of CUPS_SC_STATUS_OK
on success
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.
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;
SNMP walk callback
typedef void (*cups_sc_walk_func_t)(const char *oid, const char *data, int datalen, void *context);
Backend exit codes
Bidirectional capability values
Request command codes
Printer state bits
Response status codes