= libgps(3)
:author: Eric S. Raymond
:date: 21 September 2022
:email: <esr@thyrsus.com.>
:keywords: gps, gpsd, libgps
:manmanual: GPSD Documentation
:mansource: GPSD, Version {gpsdver}
:robots: index,follow
:sectlinks:
:source-highlighter: rouge
:toc: macro
:type: manpage

include::../www/inc-menu.adoc[]

== NAME

libgps - C service library for communicating with the GPS daemon

== SYNOPSIS

$$C:$$

[source%nowrap,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:

[source%nowrap,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 addition 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.

[source%nowrap,python]
----
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.

[source%nowrap,c,numbered]
----
include::../www/example1.c.txt[]
----

== 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

* {gpsdweb}gpsd-client-example-code.html[GPSD Client Example Code] An
annotated example client.

* {gpsdweb}client-howto.html[GPSD Client HOWTO] A GPS client HOWTO.

* *Project web site:* {gpsdweb}

== COPYING

This file is Copyright 2013 by the GPSD project +
SPDX-License-Identifier: BSD-2-clause
