3(3) GPSD Documentation 3(3)NAMElibgps - 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
DESCRIPTIONlibgps 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.
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.
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.
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 ALSOgpsd(8), gps(1), libgpsd(3). libgpsmm(3).
AUTHOR
Eric S. Raymond <esr@thyrsus.com>, C sample code Charles Curley
<charlescurley@charlescurley.com>
The GPSD Project 14 Aug 2004 3(3)
