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gpsd_json.xml
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<?xml version="1.0" encoding="ISO-8859-1"?>
<!--
This file is Copyright (c) 2010-2018 by the GPSD project
SPDX-License-Identifier: BSD-2-clause
-->
<!DOCTYPE refentry PUBLIC
"-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
<!ENTITY gpsdsock "/var/run/gpsd.sock">
]>
<refentry id='gpsd_json.5'>
<refentryinfo><date>17 Jun 2018</date></refentryinfo>
<refmeta>
<refentrytitle>gpsd_json</refentrytitle>
<manvolnum>5</manvolnum>
<refmiscinfo class="source">The GPSD Project</refmiscinfo>
<refmiscinfo class="manual">GPSD Documentation</refmiscinfo>
</refmeta>
<refnamediv id='name'>
<refname>gpsd_json</refname>
<refpurpose>gpsd request/response protocol</refpurpose>
</refnamediv>
<refsect1 id='overview'><title>OVERVIEW</title>
<para><application>gpsd</application> is a service daemon that can be used
to monitor GPSes, DGPS receivers, Marine AIS broadcasts, and various other
location-related and kinematic sensors.</para>
<para>Clients may communicate with <application>gpsd</application> via
textual requests and responses over a socket. It is a bad idea for
applications to speak the protocol directly: rather, they should use
the <application>libgps</application> client library (for C; bindings
also exist for other languages) and take appropriate care to
conditionalize their code on the major and minor protocol version
symbols.</para>
<para>The GPSD protocol is built on top of JSON, JavaScript Object
Notation, as specified in RFC 7159: <citetitle>The JavaScript Object
Notation (JSON) Data Interchange Format</citetitle>. GPSD's use of
JSON is restricted in some ways that make parsing it in fixed-extent
languages (such as C) easier.</para>
<para>A request line is introduced by "?" and may include multiple
commands. Commands begin with a command identifier, followed either
by a terminating ';' or by an equal sign "=" and a JSON object treated
as an argument. Any ';' or newline indication (either LF or CR-LF)
after the end of a command is ignored. All request lines must be
composed of US-ASCII characters and may be no more than 80 characters
in length, exclusive of the trailing newline.</para>
<para>Responses are JSON objects all of which have a "class" attribute
the value of which is either the name of the invoking command. There
are reports (including but not limited to as "TPV", "SKY", "DEVICE",
and "ERROR") which are not direct responses to commands.</para>
<para> The order of JSON attributes within a response object is never
significant, and you may specify attributes in commands in any
order. Responses never contain the special JSON value null; instead,
attributes with empty or undefined values are omitted. The length
limit for responses and reports is 1536 characters, including trailing
newline; longer responses will be truncated, so client code must be
prepared for the possibility of invalid JSON fragments.</para>
<para>In JSON reports, if an attribute is present only if the parent
attribute is present or has a particular range, then the parent
attribute is emitted first.</para>
<para>There is one constraint on the order in which attributes will
be omitted. If an optional attribute is present only when a parent
attribute has a specified value or range of values, the parent
attribute will be emitted first to make parsing easier.</para>
<para>The next subsection section documents the core GPSD protocol.
Extensions are documented in the following subsections. The extensions
may not be supported in your <application>gpsd</application> instance
if it has been compiled with a restricted feature set.</para>
</refsect1>
<refsect1 id='core-protocol'><title>CORE SOCKET PROTOCOL</title>
<para>Here are the core-protocol responses:</para>
<variablelist>
<varlistentry>
<term>TPV</term>
<listitem>
<para>A TPV object is a time-position-velocity report. The "class" and "mode"
fields will reliably be present. The "mode" field will be emitted
before optional fields that may be absent when there is no fix. Error
estimates will be emitted after the fix components they're associated with.
Others may be reported or not depending on the fix quality.</para>
<table frame="all" pgwide="0"><title>TPV object</title>
<tgroup cols="3" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Name</entry>
<entry>Always?</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>class</entry>
<entry>Yes</entry>
<entry>string</entry>
<entry>Fixed: "TPV"</entry>
</row>
<row>
<entry>device</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Name of originating device.</entry>
</row>
<row>
<entry>status</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>GPS status: %d, 2=DGPS fix, otherwise not present.</entry>
</row>
<row>
<entry>mode</entry>
<entry>Yes</entry>
<entry>numeric</entry>
<entry>NMEA mode: %d, 0=no mode value yet seen, 1=no fix, 2=2D, 3=3D.</entry>
</row>
<row>
<entry>time</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Time/date stamp in ISO8601 format, UTC. May have a
fractional part of up to .001sec precision. May be absent if mode
is not 2 or 3.</entry>
</row>
<row>
<entry>ept</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Estimated timestamp error (%f, seconds, 95% confidence).
Present if time is present.</entry>
</row>
<row>
<entry>lat</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Latitude in degrees: +/- signifies North/South. Present
when mode is 2 or 3.</entry>
</row>
<row>
<entry>lon</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Longitude in degrees: +/- signifies East/West. Present
when mode is 2 or 3.</entry>
</row>
<row>
<entry>alt</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Altitude in meters. Present if mode is 3.</entry>
</row>
<row>
<entry>epx</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Longitude error estimate in meters, 95% confidence. Present
if mode is 2 or 3 and DOPs can be calculated from the satellite
view.</entry>
</row>
<row>
<entry>epy</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Latitude error estimate in meters, 95% confidence. Present
if mode is 2 or 3 and DOPs can be calculated from the satellite
view.</entry>
</row>
<row>
<entry>epv</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Estimated vertical error in meters, 95% confidence. Present
if mode is 3 and DOPs can be calculated from the satellite
view.</entry>
</row>
<row>
<entry>track</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Course over ground, degrees from true north.</entry>
</row>
<row>
<entry>magtrack</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Course over ground, degrees magnetic.</entry>
</row>
<row>
<entry>speed</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Speed over ground, meters per second.</entry>
</row>
<row>
<entry>climb</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Climb (positive) or sink (negative) rate, meters per
second.</entry>
</row>
<row>
<entry>epd</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Direction error estimate in degrees, 95% confidence.</entry>
</row>
<row>
<entry>eps</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Speed error estinmate in meters/sec, 95% confidence.</entry>
</row>
<row>
<entry>epc</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Climb/sink error estimate in meters/sec, 95% confidence.</entry>
</row>
<row>
<entry>ecefx</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>ECEF X position in meters.</entry>
</row>
<row>
<entry>ecefy</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>ECEF Y position in meters.</entry>
</row>
<row>
<entry>ecefz</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>ECEF Z position in meters.</entry>
</row>
<row>
<entry>ecefpAcc</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>ECEF position error in meters.</entry>
</row>
<row>
<entry>ecefvx</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>ECEF X velocity in meters/second.</entry>
</row>
<row>
<entry>ecefvy</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>ECEF Y velocity in meters/second.</entry>
</row>
<row>
<entry>ecefvz</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>ECEF Z velocity in meters/second.</entry>
</row>
<row>
<entry>ecefvAcc</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>ECEF velocity error in meters/second.</entry>
</row>
</tbody>
</tgroup>
</table>
<para>When the C client library parses a response of this kind, it
will assert validity bits in the top-level set member for each
field actually received; see gps.h for bitmask names and values.</para>
<para>Here's an example:</para>
<programlisting>
{"class":"TPV","device":"/dev/pts/1",
"time":"2005-06-08T10:34:48.283Z","ept":0.005,
"lat":46.498293369,"lon":7.567411672,"alt":1343.127,
"eph":36.000,"epv":32.321,
"track":10.3788,"speed":0.091,"climb":-0.085,"mode":3}
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>SKY</term>
<listitem>
<para>A SKY object reports a sky view of the GPS satellite positions.
If there is no GPS device available, or no skyview has been reported
yet, only the "class" field will reliably be present.</para>
<table frame="all" pgwide="0"><title>SKY object</title>
<tgroup cols="3" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Name</entry>
<entry>Always?</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>class</entry>
<entry>Yes</entry>
<entry>string</entry>
<entry>Fixed: "SKY"</entry>
</row>
<row>
<entry>device</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Name of originating device</entry>
</row>
<row>
<entry>time</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Time/date stamp in ISO8601 format, UTC. May have a
fractional part of up to .001sec precision.</entry>
</row>
<row>
<entry>xdop</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Longitudinal dilution of precision, a dimensionless
factor which should be multiplied by a base UERE to get an
error estimate.</entry>
</row>
<row>
<entry>ydop</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Latitudinal dilution of precision, a dimensionless
factor which should be multiplied by a base UERE to get an
error estimate.</entry>
</row>
<row>
<entry>vdop</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Altitude dilution of precision, a dimensionless
factor which should be multiplied by a base UERE to get an
error estimate.</entry>
</row>
<row>
<entry>tdop</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Time dilution of precision, a dimensionless
factor which should be multiplied by a base UERE to get an
error estimate.</entry>
</row>
<row>
<entry>hdop</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Horizontal dilution of precision, a dimensionless
factor which should be multiplied by a base UERE to get a
circular error estimate.</entry>
</row>
<row>
<entry>pdop</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Spherical dilution of precision, a dimensionless
factor which should be multiplied by a base UERE to get an
error estimate.</entry>
</row>
<row>
<entry>gdop</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Hyperspherical dilution of precision, a dimensionless
factor which should be multiplied by a base UERE to get an
error estimate.</entry>
</row>
<row>
<entry>satellites</entry>
<entry>Yes</entry>
<entry>list</entry>
<entry>List of satellite objects in skyview</entry>
</row>
</tbody>
</tgroup>
</table>
<para>Many devices compute dilution of precision factors but do not
include them in their reports. Many that do report DOPs report only
HDOP, two-dimensional circular error. <application>gpsd</application>
always passes through whatever the device actually reports, then
attempts to fill in other DOPs by calculating the appropriate
determinants in a covariance matrix based on the satellite view. DOPs
may be missing if some of these determinants are singular. It can even
happen that the device reports an error estimate in meters when the
corresponding DOP is unavailable; some devices use more sophisticated
error modeling than the covariance calculation.</para>
<para>The satellite list objects have the following elements:</para>
<table frame="all" pgwide="0"><title>Satellite object</title>
<tgroup cols="3" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Name</entry>
<entry>Always?</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>PRN</entry>
<entry>Yes</entry>
<entry>numeric</entry>
<entry>PRN ID of the satellite. 1-63 are GNSS satellites,
64-96 are GLONASS satellites, 100-164 are SBAS satellites</entry>
</row>
<row>
<entry>az</entry>
<entry>Yes</entry>
<entry>numeric</entry>
<entry>Azimuth, degrees from true north.</entry>
</row>
<row>
<entry>el</entry>
<entry>Yes</entry>
<entry>numeric</entry>
<entry>Elevation in degrees.</entry>
</row>
<row>
<entry>ss</entry>
<entry>Yes</entry>
<entry>numeric</entry>
<entry>Signal strength in dB.</entry>
</row>
<row>
<entry>used</entry>
<entry>Yes</entry>
<entry>boolean</entry>
<entry>Used in current solution? (SBAS/WAAS/EGNOS satellites
may be flagged used if the solution has corrections from them,
but not all drivers make this information available.)</entry>
</row>
</tbody>
</tgroup>
</table>
<para>Note that satellite objects do not have a "class" field, as
they are never shipped outside of a SKY object.</para>
<para>When the C client library parses a SKY response, it
will assert the SATELLITE_SET bit in the top-level set member.</para>
<para>Here's an example:</para>
<programlisting>
{"class":"SKY","device":"/dev/pts/1",
"time":"2005-07-08T11:28:07.114Z",
"xdop":1.55,"hdop":1.24,"pdop":1.99,
"satellites":[
{"PRN":23,"el":6,"az":84,"ss":0,"used":false},
{"PRN":28,"el":7,"az":160,"ss":0,"used":false},
{"PRN":8,"el":66,"az":189,"ss":44,"used":true},
{"PRN":29,"el":13,"az":273,"ss":0,"used":false},
{"PRN":10,"el":51,"az":304,"ss":29,"used":true},
{"PRN":4,"el":15,"az":199,"ss":36,"used":true},
{"PRN":2,"el":34,"az":241,"ss":43,"used":true},
{"PRN":27,"el":71,"az":76,"ss":43,"used":true}]}
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>GST</term>
<listitem>
<para>A GST object is a pseudorange noise report.</para>
<table frame="all" pgwide="0"><title>GST object</title>
<tgroup cols="3" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Name</entry>
<entry>Always?</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>class</entry>
<entry>Yes</entry>
<entry>string</entry>
<entry>Fixed: "GST"</entry>
</row>
<row>
<entry>device</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Name of originating device</entry>
</row>
<row>
<entry>time</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Time/date stamp in ISO8601 format, UTC. May have a
fractional part of up to .001sec precision.</entry>
</row>
<row>
<entry>rms</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Value of the standard deviation of the range inputs to the
navigation process (range inputs include pseudoranges and DGPS
corrections).</entry>
</row>
<row>
<entry>major</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Standard deviation of semi-major axis of error ellipse, in meters.</entry>
</row>
<row>
<entry>minor</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Standard deviation of semi-minor axis of error ellipse, in meters.</entry>
</row>
<row>
<entry>orient</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Orientation of semi-major axis of error ellipse, in degrees from true north.</entry>
</row>
<row>
<entry>lat</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Standard deviation of latitude error, in meters.</entry>
</row>
<row>
<entry>lon</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Standard deviation of longitude error, in meters.</entry>
</row>
<row>
<entry>alt</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Standard deviation of altitude error, in meters.</entry>
</row>
</tbody>
</tgroup>
</table>
<para>Here's an example:</para>
<programlisting>
{"class":"GST","device":"/dev/ttyUSB0",
"time":"2010-12-07T10:23:07.096Z","rms":2.440,
"major":1.660,"minor":1.120,"orient":68.989,
"lat":1.600,"lon":1.200,"alt":2.520}
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>ATT</term>
<listitem>
<para>An ATT object is a vehicle-attitude report. It is returned by
digital-compass and gyroscope sensors; depending on device, it may
include: heading, pitch, roll, yaw, gyroscope, and magnetic-field
readings. Because such sensors are often bundled as part of
marine-navigation systems, the ATT response may also include
water depth.</para>
<para>The "class" and "mode" fields will reliably be present. Others
may be reported or not depending on the specific device type.</para>
<table frame="all" pgwide="0"><title>ATT object</title>
<tgroup cols="3" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Name</entry>
<entry>Always?</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>class</entry>
<entry>Yes</entry>
<entry>string</entry>
<entry>Fixed: "ATT"</entry>
</row>
<row>
<entry>device</entry>
<entry>Yes</entry>
<entry>string</entry>
<entry>Name of originating device</entry>
</row>
<row>
<entry>time</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Time/date stamp in ISO8601 format, UTC. May have a
fractional part of up to .001sec precision.</entry>
</row>
<row>
<entry>heading</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Heading, degrees from true north.</entry>
</row>
<row>
<entry>mag_st</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Magnetometer status.</entry>
</row>
<row>
<entry>pitch</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Pitch in degrees.</entry>
</row>
<row>
<entry>pitch_st</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Pitch sensor status.</entry>
</row>
<row>
<entry>yaw</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Yaw in degrees</entry>
</row>
<row>
<entry>yaw_st</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Yaw sensor status.</entry>
</row>
<row>
<entry>roll</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Roll in degrees.</entry>
</row>
<row>
<entry>roll_st</entry>
<entry>No</entry>
<entry>string</entry>
<entry>Roll sensor status.</entry>
</row>
<row>
<entry>dip</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Local magnetic inclination, degrees, positive when the magnetic
field points downward (into the Earth).</entry>
</row>
<row>
<entry>mag_len</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Scalar magnetic field strength.</entry>
</row>
<row>
<entry>mag_x</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>X component of magnetic field strength.</entry>
</row>
<row>
<entry>mag_y</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Y component of magnetic field strength.</entry>
</row>
<row>
<entry>mag_z</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Z component of magnetic field strength.</entry>
</row>
<row>
<entry>acc_len</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Scalar acceleration.</entry>
</row>
<row>
<entry>acc_x</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>X component of acceleration.</entry>
</row>
<row>
<entry>acc_y</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Y component of acceleration.</entry>
</row>
<row>
<entry>acc_z</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Z component of acceleration.</entry>
</row>
<row>
<entry>gyro_x</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>X component of acceleration.</entry>
</row>
<row>
<entry>gyro_y</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Y component of acceleration.</entry>
</row>
<row>
<entry>depth</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Water depth in meters.</entry>
</row>
<row>
<entry>temp</entry>
<entry>No</entry>
<entry>numeric</entry>
<entry>Temperature at sensor, degrees centigrade.</entry>
</row>
</tbody>
</tgroup>
</table>
<para>The heading, pitch, and roll status codes (if present) vary by device.
For the TNT Revolution digital compasses, they are coded as follows: </para>
<table frame="all" pgwide="0"><title>Device flags</title>
<tgroup cols="2" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Code</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>C</entry>
<entry>magnetometer calibration alarm</entry>
</row>
<row>
<entry>L</entry>
<entry>low alarm</entry>
</row>
<row>
<entry>M</entry>
<entry>low warning</entry>
</row>
<row>
<entry>N</entry>
<entry>normal</entry>
</row>
<row>
<entry>O</entry>
<entry>high warning</entry>
</row>
<row>
<entry>P</entry>
<entry>high alarm</entry>
</row>
<row>
<entry>V</entry>
<entry>magnetometer voltage level alarm</entry>
</row>
</tbody>
</tgroup>
</table>
<para>When the C client library parses a response of this kind, it
will assert ATT_IS.</para>
<para>Here's an example:</para>
<programlisting>
{"class":"ATT","time":1270938096.843,
"heading":14223.00,"mag_st":"N",
"pitch":169.00,"pitch_st":"N", "roll":-43.00,"roll_st":"N",
"dip":13641.000,"mag_x":2454.000}
</programlisting>
</listitem>
</varlistentry>
</variablelist>
<para>And here are the commands:</para>
<variablelist>
<varlistentry>
<term>?VERSION;</term>
<listitem><para>Returns an object with the following attributes:</para>
<table frame="all" pgwide="0"><title>VERSION object</title>
<tgroup cols="4" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Name</entry>
<entry>Always?</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>class</entry>
<entry>Yes</entry>
<entry>string</entry>
<entry>Fixed: "VERSION"</entry>
</row>
<row>
<entry>release</entry>
<entry>Yes</entry>
<entry>string</entry>
<entry>Public release level</entry>
</row>
<row>
<entry>rev</entry>
<entry>Yes</entry>
<entry>string</entry>
<entry>Internal revision-control level.</entry>
</row>
<row>
<entry>proto_major</entry>
<entry>Yes</entry>
<entry>numeric</entry>
<entry>API major revision level.</entry>
</row>
<row>
<entry>proto_minor</entry>
<entry>Yes</entry>
<entry>numeric</entry>
<entry>API minor revision level.</entry>
</row>
<row>
<entry>remote</entry>
<entry>No</entry>
<entry>string</entry>
<entry>URL of the remote daemon reporting this version. If
empty, this is the version of the local daemon.</entry>
</row>
</tbody>
</tgroup>
</table>
<para>The daemon ships a VERSION response to each client when the
client first connects to it.</para>
<para>When the C client library parses a response of this kind, it
will assert the VERSION_SET bit in the top-level set member.</para>
<para>Here's an example:</para>
<programlisting>
{"class":"VERSION","version":"2.40dev",
"rev":"06f62e14eae9886cde907dae61c124c53eb1101f",
"proto_major":3,"proto_minor":1
}
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>?DEVICES;</term>
<listitem><para>Returns a device list object with the
following elements:</para>
<table frame="all" pgwide="0"><title>DEVICES object</title>
<tgroup cols="3" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Name</entry>
<entry>Always?</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>class</entry>
<entry>Yes</entry>
<entry>string</entry>
<entry>Fixed: "DEVICES"</entry>
</row>
<row>
<entry>devices</entry>
<entry>Yes</entry>
<entry>list</entry>
<entry>List of device descriptions</entry>
</row>
<row>
<entry>remote</entry>
<entry>No</entry>
<entry>string</entry>
<entry>URL of the remote daemon reporting the device set. If
empty, this is a DEVICES response from the local daemon.</entry>
</row>
</tbody>
</tgroup>
</table>
<para>When the C client library parses a response of this kind, it
will assert the DEVICELIST_SET bit in the top-level set member.</para>
<para>Here's an example:</para>
<programlisting>
{"class"="DEVICES","devices":[
{"class":"DEVICE","path":"/dev/pts/1","flags":1,"driver":"SiRF binary"},
{"class":"DEVICE","path":"/dev/pts/3","flags":4,"driver":"AIVDM"}]}
</programlisting>
<para>The daemon occasionally ships a bare DEVICE object to the client
(that is, one not inside a DEVICES wrapper). The data content of these
objects will be described later as a response to the ?DEVICE command.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>?WATCH;</term>
<listitem>
<para>This command sets watcher mode. It also sets or elicits a report
of per-subscriber policy and the raw bit. An argument WATCH object
changes the subscriber's policy. The response describes the
subscriber's policy. The response will also include a DEVICES
object.</para>
<para>A WATCH object has the following elements:</para>
<table frame="all" pgwide="0"><title>WATCH object</title>
<tgroup cols="4" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Name</entry>
<entry>Always?</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>class</entry>
<entry>Yes</entry>
<entry>string</entry>
<entry>Fixed: "WATCH"</entry>
</row>
<row>
<entry>enable</entry>
<entry>No</entry>
<entry>boolean</entry>
<entry>Enable (true) or disable (false) watcher mode. Default
is true.</entry>