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.
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);