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3(3) | GPSD Documentation | 3(3) |
NAME¶
libgps - C service library for communicating with the GPS daemonSYNOPSIS¶
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.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>14 Aug 2004 | The GPSD Project |