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, char *message, int message_size);

       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);

       int gps_close(struct gps_data_t *gpsdata);

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

       int gps_mainloop(struct gps_data_t *gpsdata, int timeout,
                        void (*hook)(struct gps_data_t *gpsdata));

       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 and should only be called after a successful gps_open(). It
       returns 0 on success, -1 on errors. The shared-memory interface close always returns 0,
       whereas a socket close can result in an error. For a socket close error it will have set
       an errno from the call to the system's close().

       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. When using the socket export, 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. Warning: under the shared-memory interface there
       is a tiny race window between gps_waiting() and a following gps_read(); in that context,
       because the latter does not block, it is probably better to write a simple read loop.

       gps_mainloop() enables the provided hook function to be continually called whenever there
       is gpsd data. The second argument is the maximum amount of time to wait (in microseconds)
       on input before exiting the loop (and return a value of -1). It will also return a
       negative value on various errors.

       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
           Enable 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. The device path string is given as the third
           argument (data).

       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.

ENVIRONMENT VARIABLES

       By setting the environment variable GPSD_SHM_KEY, you can control the key value used to
       create shared-memory segment used for communication with gpsd. This will be useful mainly
       when isolating test instances of gpsd from production ones.

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, NULL, 0) == -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), libgpsmm(3).

AUTHOR

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