3(3)								GPSD Documentation							      3(3)

NAME

       libgps - C service library for communicating with the GPS daemon

SYNOPSIS

       C:

       #include <gps.h>

       int gps_open(char *server, char * port, struct gps_data_t *gpsdata);

       int gps_send(struct gps_data_t *gpsdata, char *fmt...);

       int gps_read(struct gps_data_t *gpsdata);

       bool gps_waiting(const struct gps_data_t *gpsdata, int timeout);

       char *gps_data(const struct gps_data_t *gpsdata);

       int gps_unpack(char *buf, struct gps_data_t *gpsdata);

       void gps_close(struct gps_data_t *gpsdata);

       int gps_stream(struct gps_data_t *gpsdata, unsigned intflags, void *data);

       const char *gps_errstr(int err);

			      Python:

			      import gps

			      session = gps.gps(host="localhost", port="2947")

			      session.stream(flags=gps.WATCH_JSON)

			      for report in session:
				  process(report)

			      del session

DESCRIPTION

       libgps is a service library which supports communicating with an instance of the gpsd(8); link it with the linker option -lgps.

	   Warning
	   Take care to conditionalize your code on the major and minor API version symbols in gps.h; ideally, force a compilation failure if
	   GPSD_API_MAJOR_VERSION is not a version you recognize. See the GPSD project website for more information on the protocol and API
	   changes.

       Calling gps_open() initializes a GPS-data structure to hold the data collected by the GPS, and sets up access to gpsd(1) via either the
       socket or shared-memory export. The shared-memory export is faster, but does not carry information about device activation and deactivation
       events and will not allow you to monitor device packet traffic.

       gps_open() returns 0 on success, -1 on errors and is re-entrant. errno is set depending on the error returned from the socket or
       shared-memory interface; see gps.h for values and explanations; also see gps_errstr(). The host address may be a DNS name, an IPv4 dotted
       quad, an IPV6 address, or the special value GPSD_SHARED_MEMORY referring to the shared-memory export; the library will do the right thing
       for any of these.

       gps_close() ends the session.

       gps_send() writes a command to the daemon. It does nothing when using the shared-memory export. The second argument must be a format string
       containing elements from the command set documented at gpsd(1). It may have % elements as for sprintf(3), which will be filled in from any
       following arguments. This function returns a -1 if there was a Unix-level write error, otherwise 0. Please read the LIMITATIONS section for
       additional information and cautions. See gps_stream() as a possible alternative.

       gps_read() accepts a response, or sequence of responses, from the daemon and interprets. This function does either a nonblocking read for
       data from the daemon or a fetch from shared memory; it returns a count of bytes read for success, -1 with errno set on a Unix-level read
       error, -1 with errno not set if the socket to the daemon has closed or if the shared-memory segment was unavailable, and 0 if no data is
       available.

       gps_waiting() can be used to check whether there is new data from the daemon. The second argument is the maximum amount of time to wait (in
       microseconds) on input before returning. It returns true if there is input waiting, false on timeout (no data waiting) or error condition.
       This function is a convenience wrapper around a select(2) call, and zeros errno on entry; you can test errno after exit to get more
       information about error conditions.

       gps_unpack() parses JSON from the argument buffer into the target of the session structure pointer argument. Included in case your
       application wishes to manage socket I/O itself.

       gps_data() returns the contents of the client data buffer (it returns NULL when using the shared-memory export). Use with care; this may
       fail to be a NUL-terminated string if WATCH_RAW is enabled.

       gps_stream() asks gpsd to stream the reports it has at you, to be made available when you poll (not available when using the shared-memory
       export). The second argument is a flag mask that sets various policy bits; see the list below. Calling gps_stream() more than once with
       different flag masks is allowed.

       WATCH_DISABLE
	   Disable the reporting modes specified by the other WATCH_ flags.

       WATCH_ENABLE
	   Disable the reporting modes specified by the other WATCH_ flags. This is the default.

       WATCH_JSON
	   Enable JSON reporting of data. If WATCH_ENABLE is set, and no other WATCH flags are set, this is the default.

       WATCH_NMEA
	   Enable generated pseudo-NMEA reporting on binary devices.

       WATCH_RARE
	   Enable reporting of binary packets in encoded hex.

       WATCH_RAW
	   Enable literal passthrough of binary packets.

       WATCH_SCALED
	   When reporting AIS or Subframe data, scale integer quantities to floats if they have a divisor or rendering formula associated with
	   them.

       WATCH_NEWSTYLE
	   Force issuing a JSON initialization and getting new-style responses. This is the default.

       WATCH_OLDSTYLE
	   Force issuing a W or R command and getting old-style responses. Warning: this flag (and the capability) will be removed in a future
	   release.

       WATCH_DEVICE
	   Restrict watching to a specified device, patch given as second argument.

       gps_errstr() returns an ASCII string (in English) describing the error indicated by a nonzero return value from gps_open().

       Consult gps.h to learn more about the data members and associated timestamps. Note that information will accumulate in the session
       structure over time, and the 'valid' field is not automatically zeroed by each gps_read(). It is up to the client to zero that field when
       appropriate and to keep an eye on the fix and sentence timestamps.

       The Python implementation supports the same facilities as the socket-export calls in the C library; there is no shared-memory interface.
       gps_open() is replaced by the initialization of a gps session object; the other calls are methods of that object, and have the same names
       as the corresponding C functions. However, it is simpler just to use the session object as an iterator, as in the example given below.
       Resources within the session object will be properly released when it is garbage-collected.

CODE EXAMPLE

       The following is an excerpted and simplified version of the libgps interface code from cgps(1).

	       struct gps_data_t gps_data;

	       ret = gps_open(hostName, hostPort, &gps_data);

	       (void) gps_stream(&gps_data, WATCH_ENABLE | WATCH_JSON, NULL);

	       /* Put this in a loop with a call to a high resolution sleep () in it. */
	       if (gps_waiting (&gps_data, 500)) {
		   errno = 0;
		   if (gps_read (&gps_data) == -1) {
		       ...
		   } else {
		       /* Display data from the GPS receiver. */
		       if (gps_data.set & ...
		   }
	       }

	       /* When you are done... */
	       (void) gps_stream(&gps_data, WATCH_DISABLE, NULL);
	       (void) gps_close (&gps_data);

LIMITATIONS

       On some systems (those which do not support implicit linking in libraries) you may need to add -lm to your link line when you link libgps.
       It is always safe to do this.

       In the C API, incautious use of gps_send() may lead to subtle bugs. In order to not bloat struct gps_data_t with space used by responses
       that are not expected to be shipped in close sequence with each other, the storage for fields associated with certain responses are
       combined in a union.

       The risky set of responses includes VERSION, DEVICELIST, RTCM2, RTCM3, SUBFRAME, AIS, GST, and ERROR; it may not be limited to that set.
       The logic of the daemon's watcher mode is careful to avoid dangerous sequences, but you should read and understand the layout of struct
       gps_data_t before using gps_send() to request any of these responses.

COMPATIBILITY

       The gps_query() supported in major versions 1 and 2 of this library has been removed. With the new streaming-oriented wire protocol behind
       this library, it is extremely unwise to assume that the first transmission from the daemon after a command is shipped to it will be the
       response to command.

       If you must send commands to the daemon explicitly, use gps_send() but beware that this ties your code to the GPSD wire protocol. It is not
       recommended.

       In earlier versions of the API gps_read() was a blocking call and there was a POLL_NONBLOCK option to make it nonblocking.  gps_waiting()
       was added to reduce the number of wrong ways to code a polling loop.

       See the comment above the symbol GPSD_API_MAJOR_VERSION in gps.h for recent changes.

SEE ALSO

       gpsd(8), gps(1), libgpsd(3).  libgpsmm(3).

AUTHOR

       Eric S. Raymond <esr@thyrsus.com>, C sample code Charles Curley <charlescurley@charlescurley.com>

The GPSD Project						    14 Aug 2004 							      3(3)