Ubuntu Manpage: libgps - C service library for communicating with the GPS daemon

Provided by: libgps-dev_3.15-2build1_amd64

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. 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,
beause the latter does not block, it is probably better to write a simple read loop.

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.

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) == -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.

AUTHOR

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