3(3)

NAME
libgps - C service library for communicating with the GPS daemon

SYNOPSIS

C:
#include <gps.h>

struct gps_data_t *gps_open(intaf, char *server, char * port);

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

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

void gps_set_raw_hook(struct gps_data_t *gpsdata, void (*hook)(struct gps_data_t *, char *buf, size_t len));

int gps_poll(struct gps_data_t *gpsdata);

bool gps_waiting(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);

char *gps_errstr(int err);

Python:
import gps
session = gps.gps(host="localhost", port="2947")
session.set_raw_hook(raw_hook)
session.stream(flags=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 returns a socket attached to gpsd(1). gps_open() returns NULL on errors. errno is set depending on the error returned from the the socket layer; see gps.h for values and explanations. The host address may be a DNS name, an IPv4 dotted quad, or an IPV6 address; the library will do the right thing for any of these.

gps_open_r() is a reentrent-friendly version that puts the session storage where you wish to allocate it. It returns 0 on success and -1 on failure, with errno set appropriately.

gps_close() ends the session.

gps_send() writes a command to the daemon. 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.

gps_poll() accepts a response, or sequence of responses, from the daemon and interprets it as though it were a query response (the return value is as for a query). gps_poll() returns the validity mask of the received structure. This function does a blocking read waiting for data from the daemon; it returns 0 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.

gps_waiting() can be used to check whether there is data from the daemon. It returns true if there is, false on no data waiting or error condition. It does not block waiting for input.

gps_stream() asks gpsd to stream the reports it has at you, to be made available whenn you poll. It is preferable to the older-style (pre-2.90) way of doing this, gps_query() with a "w+" argument, because it insulates your code from whether your client library and your gpsd are using old or new protocol. The second argument is a flag mask that sets various policy bits; see trhe 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. Cannot be used to disable POLL_NONBLOCK.

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 passtrough of binary packets.

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

WATCH_NEWSTYLE
    Force issuing a JSON initialization and getting new-style responses. This will become the default in a future release.

WATCH_OLDSTYLE
    Force issuing a W or R command and getting old-style responses. This is now the default behavior, but will be removed in a future release.

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

POLL_NONBLOCK
    Normally gps_poll() blocks until either there is a read error or some data is received from tha daemon. In this mode, gps_poll() returns immediately with a value of 0 if there is no input waiting.

gps_set_raw_hook() takes a function you specify and run it (synchronously) on the raw data pulled by a gps_query() or gps_poll() call. The arguments passed to this hook will be a pointer to a structure containing parsed data, and a buffer containining the raw gpsd response.

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 poll. 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 C library. 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. Resources within the session object will be properly released when it is garbage-collected. Note one limitation: POLL_NOBLOCK is not yet supported in Python; use the waiting() method instead.

CODE EXAMPLE

The following is an excerpted and simplified version of the libgps interface code from xgps(1). The function handle_input() is a trivial piece of code that calls gps_poll(gpsdata).

        gpsdata = gps_open(server, port);
        build_gui(toplevel);
        gps_set_raw_hook(gpsdata, update_panel);

        (void)gps_stream(gpsdata, WATCH_ENABLE, NULL);
        (void)XtAppAddInput(app, gpsdata->gps_fd,
    		  (XtPointer)XtInputReadMask, handle_input, NULL);
        (void)XtAppMainLoop(app);
        (void)gps_close(gpsdata);

LIMITATIONS

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, and AIS; 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 damon after a command is shipped to it will be the reponse to command.

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

This API has been stable since GPSD 2.90, except that gps_waiting() was added in 2.91.

SEE ALSO

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

AUTHOR

Eric S. Raymond <esr@thyrsus.com>, Thread-callback methods in the C binding added by Alfredo Pironti <alfredo@users.sourceforge.net>.