= Moving to GPSD-NG: a Guide for Client Developers
Eric S. Raymond <esr@thyrsus.com>
v1.11, Jan 2021
:author: Eric S. Raymond
:date: 13 January 2021
:description: A Guide for Client Developers moving to GPSD-ND
:email: <esr@thyrsus.com>
:keywords: time, GPSD, gpsd, guide, developers, client
:robots: index,follow
:sectlinks:
:toc: macro

include::inc-menu.adoc[]

This document is mastered in asciidoc format.  If you are reading it in HTML,
you can find the original at the GPSD project website.

== Why a new protocol?

GPSD has moved to a new request/response protocol.  This move has been
forced upon us because the old one ran out of namespace.  It was
designed with case-insensitive single-character command/response
codes. 25 of 26 ASCII alphabetics were already in use by 2006, and
there have been functional challenges accumulating over the last three
years that will require several more request/response codes - things
like reporting AIS data, reporting raw pseudoranges, and reporting
RTCM3.

Yes, we could as a desperate expedient have pressed non-alphabetic
printables into service - but the result would have looked like
line noise and only delayed the day of reckoning.  Instead, we've
written a new protocol that is upward-compatible with the old one
in that you can mix old and new syntax.

There were other problems as well. The old command set encouraged
sloppy handling of data by supporting commands that return PVT and
fix-quality information in atomized partial form, without timestamps.

There was also no way to support returning more than single-line
responses to a client. This was a problem for returning things like
[RINEX] format, which we'd like to be able to do when a device can
report pseudoranges.

== The transition plan

We need to shed the code complexity and overhead of maintaining both
protocols at once. This will happen sooner than it otherwise might
have because gpsd is in part targeted for SBCs and other constrained
environments where shaving a few K off the runtime image can actually
matter.  When it comes to keeping the codebase lean and mean, we try
harder.

The old-protocol support was removed from the daemon in 2.91.  Old
protocol will be supported in the client-side library for somewhat
longer, giving your applications a bridge period when they can speak
both old and new protocol -- but the client-side support for old
protocol will be removed when 3.0 ships.

The 2.92 version, with the new protocol deployed, is be in Lucid
Lynx, the Ubuntu LTS release of April 2010.

If you follow our transition advice now, you will be able to talk to
all old-protocol and new-protocol versions of the daemon until the 3.0
shared client library is deployed, at which point your runtime will
silently get smaller but you may no longer be able to use 2.x daemon
versions any more.

We're counting on binary-package dependencies to make the transition
easier.  When you ship a release using the new interface library,
specify the GPSD package at version >= 2.90 to get the new shared
library; then, when the 3.0 interface library is deployed next year,
you shouldn't have to do anything.

We'll try to make the transition easy, but we cannot guarantee no
problems. The sooner you start adapting your code, the less pain you
are likely to experience.  The rest of this document will explain both
theory and practice, and give you specific pointers on how to fix
client code.

== Virtue is rewarded

Since 2004, the way you were *supposed* to be using gpsd was through
one of the client libraries (in C or Python). If you have been doing
it right, you have been telling gpsd to stream data at you with the
'w' command, via application code probably looked something like this:

-------------------------------------------------------------------

   gpsdata = gps_open(source.server, source.port);

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

   // This is in your application's main event loop somewhere.
   // It polls gpsd for new data.
   ... gps_poll(gpsdata) ...

   gps_close()

-------------------------------------------------------------------

If you have been virtuous, you need only to make four small changes to
your code.

. Give gps_open() a third argument that is the address of a struct gps_data_t.
(This interface changed to avoid malloc(3) and make it possible to write
re-entrant client code.)

. Replace the gps_query() call with:

-------------------------------------------------------------------
gps_stream(gpsdata, WATCH_ENABLE, NULL)
-------------------------------------------------------------------

This will tell whatever version of the client library your application
dynamically links to emit what it should under the hood, either old
or new protocol. Unless a target system carries a version of the
libgps shared library different from the gpsd version, everything
should work and continue to work through future updates.

. Change gps_poll calls to gps_read calls.

. If you have references to the 'satellites' member of the structure,
those need to change them to 'satellites_visible'.

There. You're probably done, unless you relied on some parts of
struct gpsdata_t that application developers usually don't or issued
unusual configuration commands. Here are the exceptions:

* You issued other gps_query() or gps_send() commands, such as "J=1".
  If so, you'll need to learn how to say them in the new API (the J
  command itself is dead, and you can just remove it entirely). That
  is not difficult, and this document will cover what you need to
  know.

* Your application code referenced struct gpsdata_t members that no
  longer exist. This will be obvious because your compilation will
  fail.  Later in this document you will learn how to fix these
  problems.

* You set a per-packet raw hook.  This feature is gone; code
  can now just look at the response buffer directly.

* You set a thread hook.  We have deleted the thread-hook portion of
  the API; for discussion, see "Why threads are gone" below.

You can probably ignore the rest of this document, unless
either (a) you want to learn about gpsd's new capabilities so you
can use them in creative ways, or (b) you want to caper with unholy glee
as you contemplate the trials awaiting the non-virtuous.

If you are non-virtuous -- that is, you rolled your own client-side
interface -- you've had years of warning that this choice would
fail to insulate you from protocol details and cost you pain in the
future. That future is now.

In the remainder of this document we will try to help you minimize the
pain.  The main strategy for doing so is to *use libgps* (or its
functional equivalents in languages other than C).  Scrap your
hand-rolled interface code!  When you use libgps, compatibility issues
become *our* problem to solve rather than *yours*.

== Binary stability

In the past, the GPSD project has not been very good about preserving
stability of the binary structure layout for struct gpsdata_t between
releases. There was a reason for this -- we were very focused
on reducing memory footprint for SBCs and embedded devices, so we
conditioned out various pieces of the structure depending on what
features were or were not compiled in.

We're not going to do this any more. It has been pointed out to us
that the friction costs of breaking shared-library compatibility are
higher than we were reckoning. The new layout has no sections
conditionalized; instead, we have moved a number of fields into
a union. From 2.90 on, the structure layout will change rarely,
only at major version bumps.

== When the bough breaks

Even virtuous clients have to worry about version skew. Supposing you
have used libgps and not done anything exotic, you will still have
problems if the client library you linked and the instance of gpsd it
speaks to are using different protocols.

The possible failure modes are pretty obvious.  Transitions are
difficult.  We're essentially relying on the distribution integrators
to ship libgps and gpsd updates at the same time, with sane
package dependencies. If that goes smoothly, applications may
not even notice the changes.  We can hope...

== Why threads are gone

We have deleted the two functions in the API that managed a
library-internal thread hook.  Here's why:

1. Actual use of it has been at best very rare and possibly nonexistent.

2. Applications that want location handing to run in a thread are in
   a better position to manage thread locks and mutexes themselves
   than our client library can possibly be -- after all, the
   application knows what all the other threads and mutex locks
   are, and our library doesn't.

3. We don't like to ship code we can't test, we didn't have a
   regression test for the thread stuff, and writing one would
   have been a painful expenditure of time better spent elsewhere.

== On not doing things by halves

At the same time that pressure has been building to redesign the
protocol, we've been gaining experience in gpsd's application domain
that has made us rethink some of the assumptions behind the old one.

Since we knew we were going to have a forced compatibility break at the
wire-protocol level anyway, we decided not to do things by halves.  One
big break -- in the application model, struct gpsdata_t, and the
wire protocol behind it -- is better than three or four spread out
over a period of time.

As a result, the new protocol is not an exact superset of the old one.
It reflects a different way of carving up the behavior space in gpsd's
application domain.  And the shape of struct gpsdata_t, the
client-side interface structure, has changed in corresponding ways.

Accordingly, there are three things a client developer will need to
understand about the new protocol.  The first is theory: how its model
of the gpsd application domain is different. The second is practice:
how to issue new-style commands and interpret responses. The third, if
you have relied on the structure in a way that now breaks your
compile, is how that structure has changed.

== How the theory has changed

=== Channels are gone

In old protocol, when you requested data from the daemon, it would
search for a device supplying the kind of data you had told it you
wanted (GPS, by default) and connect your listening channel to *that
single device*. The association between channel and device was set
when channel was first bound to device and implicit; reports weren't
labeled with the device name. You could request a specific device if
you wanted to.

In the new protocol, channels are gone.  You tell gpsd to stream
reports at you; thereafter, every time an attached GPS or other device
generates a report, you'll get it.  There may be multiple devices
reporting; each report will come labeled with the name of the
originating device, and that name will be left in your client
structure along with the rest of the new data.

In both protocols, when you poll gpsd and get data the client library
takes care of interpreting what comes up the wire from the daemon, and
merges the resulting data into your client structure (struct
gpsdata_t).

The difference is that before, the API locked you to one device during
the life of the session.  Now it potentially has to deal with a *set*
of devices, treated symmetrically.

There are multiple reasons this change is a good idea.  One is that it
makes coping with devices being hotplugged in or out completely
trivial from the client's point of view - it can just choose to ignore
the fact that the device IDs in the reports have changed.  Also, the
channel-management hair in the interface goes away. Also, it means
that clients can treat identically the cases where (a) you have one
device reporting different kinds of data (e.g. a marine navigation
system reporting both GPS and AIS) and (b) you have several devices
reporting different kinds of data.

=== From lockstep to streaming

A subtler change has to do with the difference between a lockstep
or conversational interface and a streaming, stateless one.

In the earliest versions of GPSD, clients requested various pieces of
data by command. After each request, they would need to wait until a
response came back.  Then, watcher mode was added.  By saying "w+",
you could ask gpsd to stream GPS reports at you whenever it got them.

In the new protocol, streaming is all there is.  Every report coming
up from the daemon is tagged with its device and type.  Instead of
issuing commands and then waiting for specific responses, clients
should expect any kind of report at any time and merge it into
client-local storage (libgps does this for you).

This change is necessary to cope with devices that may send (for
example) mixed GPS and AIS data. In the future, the stream from
gpsd could include other kinds of data, such as the take from
a digital compass, water-temperature sensors, or even aircraft
transponders.

=== Asynchronous-write handling

The old client code had an assumption baked into it that gps_poll()
can do one read call end expect the daemon to hand it an entire
\n-terminated packet. 99.9% of the time this is true, but socket
layers can do some remarkably perverse things.

In 2.91 and later, what was gps_poll() and is now gps_read() behaves
in a subtly different way.  Each call does exactly one read() call as
before, but the incoming data is now buffered; the logic to interpret
the buffer and empty it is called only when the read() contains a \n.
When that happens, the validity flags include the PACKET_SET mask.

== How the command set has changed

If your code issues old-protocol commands 'A', 'D', 'E', 'M', 'P',
'T', 'U', or 'V', it is a wretched hive of scum and villainy that
probably hasn't changed since before the introduction of 'W' in
2004-2005.  You are using the oldest single-shot commands and will
have to rewrite your interface significantly, as the new protocol does
not support equivalents.  Use libgps.

If your code issues B, C, or N commands, they need to change to
?DEVICE commands.  See the protocol reference for details.

The 'F' command has no equivalent in 2.90; consider teaching your
client to ignore fix updates when they don't have a specified "device"
or "class" tag, respectively. In 2.91 and later versions, use the "device"
option of the ?WATCH command for similar effect.

The old 'G' command does not have an equivalent.  It would be possible
to implement one, but we probably won't do it unless there is actual
demand (and we don't expect any).

The old 'I' command has no equivalent. You probably issued it as part
of an initialization string, hoping that a subtype string would later
show up in gps_id so you could post it somewhere.  In the new
protocol, when a device sends back subtype information the daemon
ships the client an object of class DEVICE with a device tag and
subtype fields.  Watch for that and process appropriately.

The old 'J' command is dead. gpsd now detects the end of the reporting
cycle reliably and ships on that, buffering data during the individual
reporting cycle.

The old 'K' command is replaced by ?DEVICES.

The old 'L' command is replaced by ?VERSION.  Note that the daemon now
ships a version response to each client on connect, so it will
probably never be necessary for you to issue a ?VERSION request.

The old 'M' command has no equivalent.  Mode is reported in the TPV response.

The old 'O' and 'Y' commands are gone.  Use ?WATCH and sample the
stream instead.

The old 'Q' command has no equivalent.  DOPs are reported in the SKY response.

The 'S' command has no equivalent, because it is not well defined what
value should be presented for binary protocols.

The old 'R' command has been replaced by three optional attributes in
?WATCH.  Include the WATCH_RARE, WATCH_RAW and/or WATCH_NMEA masks in
the argument of gps_stream(), or set a raw hook before alling
gps_stream().

The old 'W' command has been replaced by ?WATCH. Call gps_stream()
with whatever options you want to set.

The old 'X' command is gone.  Instead, you will see an object of
class DEVICE from the daemon whenever a device is opened or closed.

The old 'Z' and '$' commands, used by the developers for profiling,
have equivalents, which are presently considered unstable and thus
are undocumented.

== How the C API and gps_data_t structure has changed

gps_open() now takes a third argument and is re-entrant - it's the
old undocumented gps_open_r().

The gps_query() entry point is gone.  With the new streaming-oriented
wire protocol, it is extremely unwise to assume that the first
transmission from the damon after a command is shipped to it will be
the response to command.  If you must send explicit
commands to the daemon, use gps_send() and handle the response in
your main event-polling loop -- but beware, as using gps_send()
ties your code to the GPSD wire protocol and is not recommended.

gps_poll() is renamed gps_read().

The client library's reporting structure, struct gpsdata_t, has a new
substructure (struct devconfig_t) named "dev" that groups together
information about the device that shipped the last update to the
client.  The members of this structure replace several top-level
struct gpsdata members in older versions.

Most notably, the gps_device member has been replaced by dev.path.
It is valid after every response with a device tag (DEVICE, TPV, SKY,
AIS, RTCM2, RTCM3).

The top-level gps_id member is replaced by dev.subtype.  This data
should be considered valid only when DEVICEID_SET is on in the
top-level set member.

The dev members baudrate, parity, stopbits, cycle, mincycle, and
driver_mode replace older top-level members.  They should be
considered valid only when DEVICE_SET is on in the top-level set
member.

The top-level members ndevices and devicelist (used only on the client
side) have been replaced by an array of struct devconfig_t structures.
Data in this structure should be considered valid only when
DEVICELIST_SET is on in the top-level set member.  Storage for
pathnames is no longer dynamically allocated, but static; to save
space, it lives in a union with several other substructures.

The top-level member "satellites" has been changed to
"satellites_visible". The ambiguity in that name had actually induced
a bug or two.

There is a new substructure, dop, which holds the
dilution-of-precision factors that were previously individual members
of the gpsdata structure. Two new DOPs, xdop and ydop, are available;
these express dilution of precision in longitude and latitude,
respectively.

There is a gps_waiting() method analogous to the waiting() method in
the Python class -- a way to check if input is waiting from the
daemon. It blocks but takes a timeout value.

The raw_hook member is gone.

== C++  client library changes

In API version 5, the C++ library defines a single object using RAII.
There are no explicit open() and close() methods; instead, you initialize
the object handing it a host and server, and the connection is shut down
when the object is deleted or goes out of scope.

== Python client library changes

There is a new stream() method analogous to the gps_stream() call in
the C library.  As in the C library, the query() method is gone, for
the same reasons.  The gps_send() entry point, new in version 3 of the C API,
has had a corresponding Python gps-class send() method all along.

The pre-existing interface using the poll() method and self.valid is
still available and should work compatibly with a daemon speaking
JSON. One new feature has been added; after a VERSION response (which
a JSON-speaking instance of gpsd should emit when a connection is
opened) the version member of the session will be an object containing
version information. However, data from the new responses (WATCH,
VERSION, AIS, and TIMING in particular) will be available only through
the self.data member.

The preferred way to use the new gps class is as an iterator
factory, like this:

----------------------------------------------------------------------

for report in gps(mode=WATCH_ENABLE):
    process(report)

----------------------------------------------------------------------

See the Client HOWTO for a more detailed example.
