libgps - C service library for communicating with the GPS daemon

C:

#include <gps.h>

Python:

import gps

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

session.set_raw_hook(raw_hook)

session.query(commands)

session.poll()

del session

libgps is a service library which supports querying GPS devices; link it with the linker option -lgps. There are two interfaces supported in it; one high-level interface that goes through gpsd(1) and is intended for concurrent use by several applications, and one low-level interface that speaks directly with the serial or USB device to which the GPS is attached. This page describes the high-level interface that is safe for multiple applications to use simultaneously; it is probably the one you want. The low-level interface is documented at libgps(3).

Calling gpsd_open() initializes a GPS-data structure to hold the data collected by the GPS, and returns a socket attached to gpsd(1). gpsd_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.

gpsd_close() ends the session.

gpsd_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). gpsd_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, or -1 on a Unix-level read error.

gpsd_query() writes a command to the daemon, accepts a one-line response, and updates parts of the GPS-data structure that correspond to data changed since the last call. The second argument must be a string containing letters from the command set documented at gpsd(1). This function returns a 0 on success, or a -1 if there was a Unix-level read error.

gps_set_raw_hook() takes a function you specify and run it (synchronously) on the raw data pulled by a gpsd_query() or gpsd_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_set_callback() takes a function you specify and run it asynchronously each time new data arrives from gpsd, using POSIX threads. For example, you can call gps_set_callback(gpsdata, my_function, handler) once in your program, and from there on your gpsdata structure will be parsed by your my_function() each time new data are available. my_function() could change some global variables in your program based on received data; it is your responsibility to ensure that your program uses mutexes or other mechanisms to avoid race conditions.

gps_del_callback() deregisters the callback function previously set with gps_set_callback(). After the invocation of this funcion no operation will be done when new data arrives.

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 rtcm_unpack() will be useful when you are connected to an RTCM-104 source in raw mode. Use it as part of a raw hook, calling it with the address of the struct rtcm_t element of your session structure buffer as first argument and the buffer as the second. It will unpack a line of RTCM data into the structure. This function returns 0 when it has read the last line of an RTCM-104 message, a positive int when it expects more dump lines, and a negative int on parse failure. You must zero out the struct rtcm_t each time before this function is called on a new header (H) line, as it relies on the message type field being initially zero and uses it to track what kind of following line is expected.

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.

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

gpsdata = gps_open(server, port);

build_gui(toplevel);

gps_set_raw_hook(gpsdata, update_panel); (void)gps_query(gpsdata, "w+x\n");

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

(void)gps_close(gpsdata);

gpsd(8), xgps(1), libgps(3). libgpsmm(3).

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