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<h1 id='bug-reporting'>How do I report bugs in GPSD?</h1>
<blockquote>
<p>When you have a problem with gpsd, here are some steps you can take to
gelp us resolve the bug as quickly as possible.</p>
<h3>Read this whole FAQ first</h3>
<p>First, read this whole FAQ before reporting apparent misbehavior as a
bug. You may find a solution here.</p>
<h3>Make sure it's not a problem in your client software</h3>
<p>Make sure it is a real gpsd bug and not a problem with
your client software. A good way to do this is to run your client and
the gpsd test client (xgps) side by side. If xgps seems to report
good numbers but your client does not, you have a client problem.
If xgps reports the same sort of bad numbers as your client, you
have a real gpsd bug.</p>
<h3>Check the latest version of <code>gpsd</code> for the bug.</h3>
<p>If you are using an old version of <code>gpsd</code>, it is
possible your bug has already been fixed. Download the latest public
version from the <a
href='http://developer.berlios.de/projects/gpsd/'>project page</a> and
test it. To be really helpful, check out the <a
href='http://developer.berlios.de/svn/?group_id=2116'>Subversion
head</a> and test that. We don't mind getting bug reports that say
"I saw version foo had the following bug, but you've fixed it."</p>
<h3>Capture a log that triggers the problem</h3>
<p>If we can reproduce your gpsd problem, we can usually fix it very
rapidly. If we can't reproduce it, you might get lucky or you might
not — and we try hard, but all too often the result is 'not'.</p>
<p>Therefore the most important step you can take is to capture a log
of some GPS output that reproduces the bug. If you're using a SiRF
chip, the 'l' command of sirfmon will help you. If you're using
a GPS that emits NMEA, telnetting to port 2947 and enabling raw
mode will do the trick. For non-SiRF binary protocols, you will
have to cat direct from the serial device into a file.</p>
<h3>Trim the log that reproduces the problem</h3>
<p>Your next step should be to feed the log you just captured to a
<code>gpsd</code> instance through <code>gpsfake</code> to verify that
the log does in fact reproduce the bug.</p>
<p>Once you have the log, trim it to the smallest span of data that
reproduces the bug. A systematic way to do this is to cut the log in
half at the middle and test each half. If one half doesn't reproduce
the bug but the other does, throw away the half that doesn't. Repeat
this procedure on each half that tickles the bug until you can't make
it any smaller. Then send us that.</p>
<p>If possible, use the -l option of gpsfake to pin down the sentence
or packet that produces the bug, and tell us that.</p>
<h3>Look at <code>gpsd</code> log output to see if it gives you a clue</h3>
<p>You may get a better handle on your problem by running gpsd in
foreground (-N option) with the -D option set to a high level (-D 4 is
often good). If the transcript has anything that looks like a clue
in it, send it along with your bug report. When in doubt about
whether it holds a clue, send it.</p>
<p>One of the things this should tell you, if the chip reports it at
all, is the firmware version. You will want that for your report.</p>
<h3>If it's a dual-mode GPS, see if the problem reproduces in NMEA mode</h3>
<p>If you're using a SiRF, Evermore, or iTalk GPS in binary mode
(which you can tell from the -D 4 output), switch back to NMEA mode
using the N command and see if the bug still reproduces.</p>
<h3>If your bug core-dumps gpsd, send us a stack trace.</h3>
<p>Though it happens seldom, badly-formed NMEA from a device with poor
standards compliance has been known to core-dump gpsd. If your gpsd
has core-dumped, try to use gdb or whatever your local symbolic
debugger is to generate a stack trace of the crash, and send us
that.</p>
<h3>Try to determine what release introduced the bug</h3>
<p>If you have upgraded from a previous version of <code>gpsd</code>,
and the upgrade broke something that was working previously, the
most useful thing you can do is pin down the release in which the
bug was introduced.</p>
<p>How efficiently you can do this depends on whether or not you have
a client for the Subversion version control system. If you don't, all
you can do is download and test named releases. If you do, you can
pin down the exact change that introduced the bug. The latter is
far more helpful to us and will get your bug fixed faster, so we'll
describe that procedure here.</p>
<ol>
<li><p>Follow <a
href='http://developer.berlios.de/svn/?group_id=2116'>these
instructions</a> to check out a copy of the software.</p></li>
<li><p>Do "svn log *" in the top-level directory of the checked-out tree.
Save the results. This is your map of dates to revision levels.</p></li>
<li><p>Determine the revision levels of the first known bad release
and the last known good release. You can do this by looking for the
release dates from gpsd.spec in the log output. Note: pick the
<em>first</em> revision matching the known-good date and the <em>last</em>
revision matching the known-bad date.</p></li>
<li><p>Use the -r option of svn up to check out the revision level in
the middle of that range. Thus, if the last known good release was
2502 and the known bad one is 2760, check out release (2760 + 2502) /
2 = 2631. The command you want would be "svn -r 2631 up" in this
case.</p></li>
<li><p>Build and test the revision. Whether it works or not, you will
eliminate half the range. If it works, you know the bug was introduced
somewhere in the upper half range. If it doesn't, the bug was introduced
in the lower half.</p></li>
<li><p>Do the same test on the half-range where the error was introduced;
e.g. check out the middle version, test that, and narrow the range by another
factor of 2.</p></li>
<li><p>Repeat this procedure until you know the exact revision that
introduced your bug. This will happen very quickly, as the number of
tests required will be the log to the base 2 of the number of
revisions in your original span. Even if there are (say) 500
revisions in the span you should only require 9 tests to nail down the
exact change in question.</p></li>
</ol>
<h3>Include the vendor, mode, and firmware version in your report.</h3>
<p>Always include with your bug report the GPS vendor and model. If
your GPS is SiRF-based, include the firmware version as well. You can
find out what that is by running at the daemon at -D 4.</p>
</blockquote>
<h1>Why does the first A,E,O,P,T,U, or V command to a device always return "?"</h1>
<p>To understand what's going on, you need to know that
<code>gpsd</code> does not immediately assign a client a GPS from its
pool of known devices when the client connects. Rather, it waits
until the client issues a command that requires GPS information.</p>
<p>The reason for this goes back to when multi-device support was
added in 2.21. In a multi-device world, what the client might want to
do is list available GPSes (with K) and choose one (with F). There is
also at least one other command, L, that doesn't require a GPS. And
in general, waiting until a GPS is really needed to wake one up is a
good idea — it saves power, which can be important because
GPS-equipped computers are more than likely running off a battery.</p>
<p>So <code>gpsd</code> now defers binding the device. Your first
request for fix data triggers the action of binding a GPS to your
channel, but <em>at that time</em> no GPS is yet bound. The GPS
doesn't have a fix, so you get ?. But by the time of your
<em>next</em> request <code>gpsd</code> has polled the daemon and has
a fix.</p>
<p>Generally only human beings testing <code>gpsd</code> via telnet/ssh
ever notice this bug. <code>gpsd</code>-using applications poll the
daemon repeatedly; the delay before the second response comes in
normally is not noticeable.</p>
<p>We haven't fixed this because the test clients all use watcher mode. In
watcher mode, you get 'O' updates whenever the GPS ships a recognized
sentence. The old-style individual requests are obsolete, really.
You should fix your application to use watcher mode — or better
yet, the libgps client library.</p>
<h1 id='speed'>Why does my receiver report wildly fluctuating speed?</h1>
<p>If your problem is wildly fluctuating speed reports on a SiRF, switching
on static navigation mode using the 'c' command in sirfmon. Static navigation
mode will freeze your position if your speed is below 1.2 m/s for three
seconds, and will begin updating your position again when speed exceeds 1.4
m/s. This prevents multipath, weak signals, poor constellation geometry from
dragging your solutions around too much. Other receivers may suffer the same
problem and may have a similar solution.</p>
<h1 id='gpsdrive'>Why do I get implausibly low speeds when using gpsdrive?</h1>
<p>This is a gpsdrive bug, as you can verify by running xgps alongside it.</p>
<h1 id='flicker'>Why do the date and other fields in <code>xgps</code> flicker
to "n/a" part of the time even when there's a fix?</h1>
<p>The sentence or packet your GPS uses to report satellite
bearing/elevation has no timestamp. The <code>xgps</code>
date display flickers to "n/a" when it has just seen this report.</p>
<p>This is a known problem. It's not a bug — or, at least, not
a bug in the GPSD code. Blame the idiot protocol designers who
saw fit not to timestamp their satellite-data packet. (NMEA and Garmin
GPSes don't. SiRF GPSes do. Score one for SiRF.)</p>
<p><code>gpsd</code> is faithfully reporting the information it is
getting from the GPS, including the fact that the Y sentence contains
no date. That's its job. The libgps library is doing its bit by
passing everything from <code>gpsd</code> on to the client application
as it arrives, including the lack of date.</p>
<p>Other fields may flicker as well; latitude is prone to this on NMEA
devices. It's the same problem. In theory, a client could accumulate
data through an entire send cycle from the GPS, and change the display
only once at end of cycle. The problem with this is that there is
so much variation in the order GPS sends sentences that there is no
no way to spot end-of-cycle — the best you could do would be
to wait until a sentence-timestamp change signals the start of a new
cycle, at which time the buffered data is by definition obsolete.</p>
<p>It's the <em>client's</em> job to interpret/interpolate/fill in
gaps, to do policy. What you're seeing as a bug only looks like one
because <code>xgps</code>, as is proper for a test client, has as
little policy as possible.</p>
<h1 id='kismet'>Why do I get flaky results when I try to use <code>gpsd</code> with Kismet?</h1>
<p>Kismet's interface was designed for a much older version of
<code>gpsd</code>, and tends to fight with the autobauding code in the
newer versions. The Kismet maintainer has promised to fix this. Until
he does, the workaround is to start <code>gpsd</code> and make sure
it has synced up to a GPS before running Kismet.</p>
<p>You can <a href='start-kismet'>download a shellscript</a> that does
this.</p>
<h1 id='bluetooth'>Why do I have to restart <code>gpsd</code> whenever I power-cycle my Bluetooth device?</h1>
<p>The Bluetooth stack returns 0 for a read from a missing device,
rather than -1, and doesn't set errno. This is wrong and needs to be
fixed at OS level.</p>
<h1 id='lockup'>My <code>gpsd</code> sometimes stops responding overnight</h1>
<p>At one point in the development of <code>gpsd</code> we got a
report of the daemon ceasing to respond to queries when run for
more than a day or so; the user, quite reasonably, suspected some sort
of resource leak in the daemon. On the other hand, other users reported
good operation over much longer periods with the same version of
the software. That suggests a bug at the level of the user's operating
system or local site configuration.</p>
<p>Nevertheless, the possibility of a resource-leak bug alarmed us
enough that after 2.26 one of us (ESR) built an entire test framework
for auditing the code's dynamic behavior and used it to apply <a
href="http://valgrind.org">Valgrind</a>. You can look at the
resulting script, valgrind-audit, in the source distribution. This
turned up a couple of minor leaks, but nothing sufficient to explain
the report.</p>
<p>One of our senior developers, Rob Janssen, has seen
<code>gpsd</code> interact badly with overnight backups, pushing the
system load average through the roof. He says: "when you copy many
gigabytes of data from disk to disk, the [Linux] kernel's buffer
management goes completely haywire. [...] I think this is caused both
by allocation of many buffers for reading files, and by accumulation
of many dirty buffers that still have to be written. At some point,
programs like gpsd (but also all interactive programs and the X
display manager) come to a complete standstill while the system is
swapping like mad."</p>
<p>If Rob's analysis is correct, <code>gpsd</code> is a canary in a
coal mine. If your <code>gpsd</code> locks up after a long period of
operation, you should look at your logs and see if you can connect the
point at which it stopped responding to some kind of resource crisis
brought on by lots of I/O activity.</p>
<p>Another thing to try is running <code>gpsd</code> under Valgrind overnight
and seeing if it reports any leaks.</p>
<h1 id='why_migrate'>Why use this version of <code>gpsd</code>?</h1>
<p>If you have written a <code>gpsd</code>-aware application using one
of the old 1.x versions, or a fork such as ngpsd or tgpsd, here are
some good functional reasons to migrate to 2.0:</p>
<ol>
<li>Hotplug- and reconnect support. Your application does not have to be
aware of GPS device connects and disconnects, but can choose to be by
watching for X commands.</li>
<li>Your application can now query whether or not the GPS is online
and get an authoritative answer.</li>
<li>Timestamps are now no longer truncated to seconds, but reported to
whatever resolution the GPS ships. Often (notably on SiRF-II GPSes)
this is milliseconds.</li>
<li>There is a new "watcher" mode. It is like raw mode in that the
GPS streams updates at you, but unlike it in that the updates are in
the simpler GPSD format rather than the more complex NMEA one.</li>
<li>The daemon now automatically tries to reconnect to the GPS once
a second when it is offline but clients are connected.</li>
<li>Writes to clients are nonblocking, so new <code>gpsd</code> cannot
be stalled by a wedged client.</li>
<li>The code in the new version has been carefully audited for quality,
static-checked using <a href='http://www.splint.org'>splint</a>, and
is regression-tested before every release.</li>
</ol>
<h1 id='why_not_parse_nmea'>Why use the <code>gpsd</code> protocol rather than parsing raw NMEA?</h1>
<p>Some applications that use <code>gpsd</code> start raw mode with
the 'r' command and parse the NMEA directly. This is not a good idea.</p>
<p>One problem with raw mode is that NMEA is a poorly specified
standard. There are, for example, two different and incompatible
variants of GPVTG. Another issue is that implementations vary as to
whether they leave fields they don't supply empty or fill them in with
a special value such as 0.0. Interpretation of the different NMEA
status fields is a black art.</p>
<p>It is all too easy to write an NMEA parser that works well on one
variant but breaks on another, delivering subtly incorrect results or
even crashing your application. Because <code>gpsd</code> specializes
in the job, we collect knowledge on all variants and do parsing that
is much less likely to get tripped up.</p>
<p>Another issue is that some of the reports your application would
like to have are not generated by all GPSes. Estimated position error
in meters is the most obvious example; climb/sink is another. When a GPS
doesn't supply these, <code>gpsd</code> can fill them in using the
same sorts of computation that more capable GPSes use.</p>
<h1 id='interfacing'>How should I interface my application with <code>gpsd</code>?</h1>
<p>The <code>gpsd</code> package provides two ways for C code to get
data from a GPS. Both go through the libgps.a library, which supports
two sets of entry points. The <a href="libgpsd.html">low-level
interface</a> talks directly to the GPS. The <a
href="libgps.html">high-level interface</a> communicates with an
instance of <code>gpsd</code>, which uses its own copy of libgps.a to
talk to the device.</p>
<p>A third way would be to open a socket to <code>gpsd</code> and
interpret <code>gpsd</code> protocol or raw NMEA in your application.
Before 2.0, all <code>gpsd</code>-aware applications had to do this
because libgps.a didn't exist. Now that it does, the exercise is
rather pointless. Using libgps.a will probably simplify your code a
lot.</p>
<p>You will almost always want to use the high-level interface and go
through the daemon; among other things, this means more than one
application will be able to query the GPS without causing confusion.
The only exception will be in very space-constrained single-user
scenarios, perhaps on embedded systems or PDAs. On those it may be
appropriate to use the low-level interface directly, probably with a
build from source that conditions out all but one of the drivers.</p>
<p>For Python programmers, there is a gps.py module the high-level
interface. It exports a class that encapsulates a GPS session.</p>
<h1 id='changes'>How has the <code>gpsd</code> interface changed since 1.x?</h1>
<p>There are three minor incompatibilities with <code>gpsd</code> 1.x:</p>
<p>First, <code>gpsd</code>-2's command-line options have been changed
and simplified. If your <code>gpsd</code>-using application is
starting up <code>gpsd</code> directly you may find you have to modify
the invocation. However, we don't recommend this. New
<code>gpsd</code> is designed to be started by hotplug scripts when
a USB device wakes up, or started at boot time and run
continuously just like any normal daemon. It will do nothing, and be
swapped out, unless there are clients trying to query the GPS.</p>
<p>Second, <code>gpsd</code> now returns "?" as the contents for a
field when it doesn't have valid data for that field (e.g. latitude
or longitude before the first fix). This is only an issue if you are
interpreting GPSD responses yourself rather than using libgps.a or the
gps.py Python module.</p>
<p>Third, the format of the timestamp returned by the D command has
changed, from "%m/%d/%Y %H:%M:%S" to ISO-8601: "%Y-%m-%dT%H:%M:%SZ".
No more U.S.-centric date-format assumptions! Also, as previously
noted, the seconds part may have one or more digits of decimal fractional
seconds.</p>
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