Provided by: libgps-dev_3.25-4ubuntu1_amd64 
NAME
libgps - 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, char * message,
int message_size)
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)
int gps_close(struct gps_data_t * gpsdata)
int gps_stream(struct gps_data_t * gpsdata, unsigned int flags,
void * data)
int gps_mainloop(struct gps_data_t * gpsdata, int timeout,
void (* hook)(struct gps_data_t *gpsdata))
const char * gps_errstr(int err)
Python:
import gps
session = gps.gps(host="localhost", port="2947")
session.stream(flags=gps.WATCH_JSON)
while 0 == session.read():
process(session)
del session
DESCRIPTION
C
libgps is a service library which supports communicating with an instance of the gpsd(8),
link it with the linker option -lgps. Some systems may also require -lm.
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.
All the functions described here use the gps_data_t structure. Consult gps.h to learn more
about gps_data_t, its data members, associated structures. 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.
Warning
gps_data_t sets floating point variables to NaN when the actual variable value is
unknown. Check all floats and doubles with isfinite() before using them. isnan() is
not sufficient!
gps_open()
Calling gps_open() initializes a gps_data_t structure to hold the data collected by
the GPS, and sets up access to gpsd(8) 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()
gps_close() ends the session and should only be called after a successful gps_open().
It returns 0 on success, -1 on errors. The shared-memory interface close always
returns 0, whereas a socket close can result in an error. For a socket close error it
will have set an errno from the call to the system’s close().
gps_send()
gps_send() writes a command to the gpsd 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(8). 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()
gps_read() accepts a response, or sequence of responses, from the gpsd daemon and
decodes the response into a gps_data_t. By default, this function does either a
blocking read for data from the gpsd 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 gpsd daemon has closed or if the shared-memory
segment was unavailable, and 0 if no data is available.
The second argument to gps_read() is usually NULL, and the third argument is zero. If
your application wants to see the raw data from the gpsd daemon then set the second
argument to the address of your message buffer, and the third argument is the size of
your buffer. Use with care; this may not to be a NUL-terminated string if WATCH_RAW is
enabled.
gps_waiting()
gps_waiting() can be used to check whether there is new data from the gpsd daemon. The
second argument is the maximum amount of time to block (in microseconds) waiting on
input before returning. It returns true if there is input waiting, false on timeout
(no data waiting) or error condition. When using the socket export, 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.
Warning: under the shared-memory interface there is a tiny race window between
gps_waiting() and a following gps_read(); in that context, because the latter does not
block, it is probably better to write a simple read loop.
gps_mainloop()
gps_mainloop() enables the provided hook function to be continually called whenever
there is gpsd data. The second argument is the maximum amount of time to wait (in
microseconds) on input before exiting the loop (and return a value of -1). It will
also return a negative value on various errors.
gps_unpack()
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()
gps_data() returns the contents of the client data buffer (it returns NULL when using
the shared-memory export). Use with care; this may not to be a NUL-terminated string
if WATCH_RAW is enabled.
gps_stream()
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_DEVICE
Restrict watching to a specified device. The device path string is given as the
third argument (data).
WATCH_DISABLE
Disable the reporting modes specified by the other WATCH_ flags.
WATCH_ENABLE
Enable 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_NEWSTYLE
Force issuing a JSON initialization and getting new-style responses. This is the
default.
WATCH_NMEA
Enable generated pseudo-NMEA reporting on binary devices.
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_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.
gps_errstr()
gps_errstr() returns an ASCII string (in English) describing the error indicated by a
nonzero return value from gps_open().
Python
The Python implementation supports the same facilities as the the C library. gps_open() is
replaced by the initialization of a gps session object: session = gps.gps(...). The other
calls are methods of that object, and have the same names as the corresponding C
functions.
In addtion to using gps.read() to read messages from gpsd, you can use the session object
as an iterator, as in the code fragment given below. Python iterators implicitly call the
function gps.\_\_next() which is just a shim over gps.read(). There is other advantage to
using the implicit iterator and it does not allow the options that gps.read() does.
import gps
session = gps.gps(host="localhost", port="2947")
session.stream(flags=gps.WATCH_JSON)
for report in session:
process(report)
del session
Resources within the session object will be properly released when it is
garbage-collected.
For further information on the Python gps module, read the comments in the modules files.
There is a complete Python example in the file www/gpsd-client-example-code.adoc.
ENVIRONMENT VARIABLES
By setting the environment variable GPSD_SHM_KEY, you can control the key value used to
create shared-memory segment used for communication with gpsd. This will be useful mainly
when isolating test instances of gpsd from production ones.
EXAMPLES
The following is a fully functional minimal C client. Check the C source for the other
gpsd clients for more ideas.
// example gpsd client
// compile this way:
// gcc example1.c -o example1 -lgps -lm
#include <gps.h> .. for gps_*()
#include <math.h> // for isfinite()
#define MODE_STR_NUM 4
static char *mode_str[MODE_STR_NUM] = {
"n/a",
"None",
"2D",
"3D"
};
int main(int argc, char *argv[])
{
struct gps_data_t gps_data;
if (0 != gps_open("localhost", "2947", &gps_data)) {
printf("Open error. Bye, bye\n");
return 1;
}
(void)gps_stream(&gps_data, WATCH_ENABLE | WATCH_JSON, NULL);
while (gps_waiting(&gps_data, 5000000)) {
if (-1 == gps_read(&gps_data, NULL, 0)) {
printf("Read error. Bye, bye\n");
break;
}
if (MODE_SET != (MODE_SET & gps_data.set)) {
// did not even get mode, nothing to see here
continue;
}
if (0 > gps_data.fix.mode ||
MODE_STR_NUM <= gps_data.fix.mode) {
gps_data.fix.mode = 0;
}
printf("Fix mode: %s (%d) Time: ",
mode_str[gps_data.fix.mode],
gps_data.fix.mode);
if (TIME_SET == (TIME_SET & gps_data.set)) {
// not 32 bit safe
printf("%ld.%09ld ", gps_data.fix.time.tv_sec,
gps_data.fix.time.tv_nsec);
} else {
puts("n/a ");
}
if (isfinite(gps_data.fix.latitude) &&
isfinite(gps_data.fix.longitude)) {
// Display data from the GPS receiver if valid.
printf("Lat %.6f Lon %.6f\n",
gps_data.fix.latitude, gps_data.fix.longitude);
} else {
printf("Lat n/a Lon n/a\n");
}
}
// When you are done...
(void)gps_stream(&gps_data, WATCH_DISABLE, NULL);
(void)gps_close(&gps_data);
return 0;
}
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 gpsd 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 gpsd daemon after a command is shipped to it
will be the response to command.
If you must send commands to the gpsd daemon explicitly, use gps_send() but beware that
this ties your code to the GPSD wire protocol. It is not recommended.
In some versions of the API gps_read() is 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.
ACKNOWLEDGEMENTS
C sample code by Gary E. Miller gem@rellim.com and Charles Curley
charlescurley@charlescurley.com
SEE ALSO
gpsd(8), gps(1), gpsd_json(5), libgpsmm(3)
RESOURCES
• GPSD Client Example Code <https://gpsd.io/gpsd-client-example-code.html> An annotated
example client.
• GPSD Client HOWTO <https://gpsd.io/client-howto.html> A GPS client HOWTO.
• Project web site: https://gpsd.io/
COPYING
This file is Copyright 2013 by the GPSD project
SPDX-License-Identifier: BSD-2-clause
AUTHOR
Eric S. Raymond