libgps - C service library for communicating with the GPS daemon
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,
char *gps_errstr(int err);
session = gps.gps(host="localhost", port="2947")
for report in session:
libgps is a service library which supports communicating with an
instance of the gpsd(8); link it with the linker option -lgps.
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
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 roight thing for any of
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.
Disable the reporting modes specified by the other WATCH_ flags.
Cannot be used to disable POLL_NONBLOCK.
Disable the reporting modes specified by the other WATCH_ flags.
This is the default.
Enable JSON reporting of data. If WATCH_ENABLE is set, and no other
WATCH flags are set, this is the default.
Enable generated pseudo-NMEA reporting on binary devices.
Enable reporting of binary packets in encoded hex.
Enable literal passtrough of binary packets.
When reporting AIS data, scale integer quantities to floats if they
have a divisor or rendering formula assosiated with them.
Force issuing a JSON initialization and getting new-style
responses. This will become the default in a future release.
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
Restrict watching to a speciied device, patch given as second
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
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.
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);
(void)gps_stream(gpsdata, WATCH_ENABLE, NULL);
(XtPointer)XtInputReadMask, handle_input, NULL);
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
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.
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
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
This API has been stable since GPSD 2.90, except that gps_waiting() was
added in 2.91.
gpsd(8), gps(1), libgpsd(3). libgpsmm(3).
Eric S. Raymond <firstname.lastname@example.org>, Thread-callback methods in the C
binding added by Alfredo Pironti <email@example.com>.