| 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); }
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 CUPS SNMP functions provide a simple API for querying network printers.
Queries are made using a datagram socket that is created using
cupsSNMPOpen and destroyed using
cupsSNMPClose:
#include <cups/snmp.h> int snmp = cupsSNMPOpen(AF_INET); /* do some queries */ cupsSNMPClose(snmp);
OIDs are simple C arrays of integers, terminated by a value of -1. For example, the page counter OID .1.3.6.1.2.1.43.10.2.1.4.1.1 would be:
int page_counter_oid[] = { 1, 3, 6, 1, 2, 1, 43, 10, 2, 1, 4, 1, 1, -1 };
You send a query using
cupsSNMPWrite and read the value back
using cupsSNMPRead. The value is read
into a structure called cups_snmp_t:
#include <cups/snmp.h>
int page_counter_oid[] = { 1, 3, 6, 1, 2, 1, 43, 10, 2, 1, 4, 1, 1, -1 };
http_addrlist_t *host = httpAddrGetList("myprinter", AF_UNSPEC, "161");
int snmp = cupsSNMPOpen(host->addr.addr.sa_family);
cups_snmp_t packet;
cupsSNMPWrite(snmp, &(host->addr), CUPS_SNMP_VERSION_1,
cupsSNMPDefaultCommunity(), CUPS_ASN1_GET_REQUEST, 1,
page_counter_oid);
if (cupsSNMPRead(snmp, &packet, 5000))
{
/* Do something with the value */
printf("Page counter is: %d\n", packet.object_value.integer);
}
The cupsSNMPWalk function allows you
to query a whole group of OIDs, calling a function of your choice for each OID
that is found:
#include <cups/snmp.h> void my_callback(cups_snmp_t *packet, void *data) { /* Do something with the value */ } int printer_mib_oid[] = { 1, 3, 6, 1, 2, 1, 43, -1 }; http_addrlist_t *host = httpAddrGetList("myprinter", AF_UNSPEC, "161"); int snmp = cupsSNMPOpen(host->addr.addr.sa_family); void *my_data; cupsSNMPWalk(snmp, &(host->addr), CUPS_SNMP_VERSION_1, cupsSNMPDefaultCommunity(), printer_mib_oid, 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. 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.
Close a SNMP socket.
void cupsSNMPClose (
int fd
);
Copy an OID.
int *cupsSNMPCopyOID (
int *dst,
const int *src,
int dstsize
);
New OID
The array pointed to by "src" is terminated by the value -1.
Get the default SNMP community name.
const char *cupsSNMPDefaultCommunity (void);
Default community name
The default community name is the first community name found in the snmp.conf file. If no community name is defined there, "public" is used.
Test whether a SNMP response contains the specified OID.
int cupsSNMPIsOID (
cups_snmp_t *packet,
const int *oid
);
1 if equal, 0 if not equal
The array pointed to by "oid" is terminated by the value -1.
Test whether a SNMP response uses the specified OID prefix.
int cupsSNMPIsOIDPrefixed (
cups_snmp_t *packet,
const int *prefix
);
1 if prefixed, 0 if not prefixed
The array pointed to by "prefix" is terminated by the value -1.
Open a SNMP socket.
int cupsSNMPOpen (
int family
);
AF_INET or AF_INET6SNMP socket file descriptor
Read and parse a SNMP response.
cups_snmp_t *cupsSNMPRead (
int fd,
cups_snmp_t *packet,
int msec
);
SNMP packet or NULL if none
If "timeout" is negative, cupsSNMPRead will wait for a response
indefinitely.
Enable/disable debug logging to stderr.
void cupsSNMPSetDebug (
int level
);
Enumerate a group of OIDs.
int cupsSNMPWalk (
int fd,
http_addr_t *address,
int version,
const char *community,
const int *prefix,
int msec,
cups_snmp_cb_t cb,
void *data
);
Number of OIDs found or -1 on error
This function queries all of the OIDs with the specified OID prefix,
calling the "cb" function for every response that is received.
The array pointed to by "prefix" is terminated by the value -1.
Send an SNMP query packet.
int cupsSNMPWrite (
int fd,
http_addr_t *address,
int version,
const char *community,
cups_asn1_t request_type,
const unsigned request_id,
const int *oid
);
1 on success, 0 on error
The array pointed to by "oid" is terminated by the value -1.
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.
ASN1 request/object types
typedef enum cups_asn1_e cups_asn1_t;
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;
Prototypes...
typedef void (*cups_snmp_cb_t)(cups_snmp_t *packet, void *data);
SNMP data packet
typedef struct cups_snmp_s cups_snmp_t;
Hex-STRING value
struct cups_snmp_hexstring_s {
unsigned char bytes[CUPS_SNMP_MAX_STRING];
int num_bytes;
};
SNMP data packet
struct cups_snmp_s {
http_addr_t address;
char community[CUPS_SNMP_MAX_STRING];
const char *error;
int error_index;
int error_status;
int object_name[CUPS_SNMP_MAX_OID];
cups_asn1_t object_type;
union cups_snmp_value_u object_value;
int request_id;
cups_asn1_t request_type;
int version;
};
Object value
union cups_snmp_value_u {
int boolean;
unsigned counter;
unsigned gauge;
struct cups_snmp_hexstring_s hex_string;
int integer;
int oid[CUPS_SNMP_MAX_OID];
char string[CUPS_SNMP_MAX_STRING];
unsigned timeticks;
};
ASN1 request/object types
Backend exit codes
Bidirectional capabilities
Request command codes
Printer state bits
Response status codes