path: root/libgps.xml

<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
                   "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd">
<refentry>
<refentryinfo><date>14 Aug 2004</date></refentryinfo>
<refmeta>
<refentrytitle>3</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo class="source">The GPSD Project</refmiscinfo>
<refmiscinfo class="manual">GPSD Documentation</refmiscinfo>
</refmeta>
<refnamediv id='name'>
<refname>libgps</refname>
<refpurpose>C service library for communicating with the GPS daemon</refpurpose>
</refnamediv>
<refsynopsisdiv id='synopsis'>
<funcsynopsis>
<funcsynopsisinfo>

C:

#include &lt;gps.h&gt;

</funcsynopsisinfo>
<funcprototype>
<funcdef>int <function>gps_open</function></funcdef>
    <paramdef>char *<parameter>server</parameter></paramdef>
    <paramdef>char * <parameter>port</parameter></paramdef>
    <paramdef>struct gps_data_t *<parameter>gpsdata</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>gps_send</function></funcdef>
    <paramdef>struct gps_data_t *<parameter>gpsdata</parameter></paramdef>
    <paramdef>char *<parameter>fmt</parameter>...</paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>gps_read</function></funcdef>
    <paramdef>struct gps_data_t *<parameter>gpsdata</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>bool <function>gps_waiting</function></funcdef>
    <paramdef>struct gps_data_t *<parameter>gpsdata</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>void <function>gps_close</function></funcdef>
    <paramdef>struct gps_data_t *<parameter>gpsdata</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>gps_stream</function></funcdef>
    <paramdef>struct gps_data_t *<parameter>gpsdata</parameter></paramdef>
    <paramdef>unsigned int<parameter>flags</parameter></paramdef>
    <paramdef>void *<parameter>data</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>const char *<function>gps_errstr</function></funcdef>
    <paramdef>int <parameter>err</parameter></paramdef>
</funcprototype>
<funcsynopsisinfo>

Python:

import gps

session = gps.gps(host="localhost", port="2947")

session.stream(flags=WATCH_JSON)

for report in session:
    process(report)

del session

</funcsynopsisinfo>
</funcsynopsis>
</refsynopsisdiv>

<refsect1 id='description'><title>DESCRIPTION</title>

<para><emphasis remap='B'>libgps</emphasis> is a service library which
supports communicating with an instance of the
<citerefentry><refentrytitle>gpsd</refentrytitle><manvolnum>8</manvolnum></citerefentry>; link it with the linker option -lgps.</para>

<warning><para>Take care to conditionalize your code on the major and
minor API version symbols in <filename>gps.h</filename>; 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.</para></warning>

<para>Calling <function>gps_open()</function> initializes a GPS-data
structure to hold the data collected by the GPS, and returns a socket
attached to
<citerefentry><refentrytitle>gpsd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.

<function>gps_open()</function> returns 0 on success, -1 on errors and is
re-entrant.  errno is set depending on the error returned from the the
socket layer; see <filename>gps.h</filename> for values and
explanations; also see <function>gps_errstr()</function>. The host
address may be a DNS name, an IPv4 dotted quad, or an IPV6 address;
the library will do the right thing for any of these.</para>

<para><function>gps_close()</function> ends the session.</para>

<para><function>gps_send()</function> writes a command to the daemon.
The second argument must be a format string containing elements from
the command set documented at
<citerefentry><refentrytitle>gpsd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
It may have % elements as for
<citerefentry><refentrytitle>sprintf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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 <function>gps_stream()</function> as a possible
alternative.</para>

<para><function>gps_read()</function> accepts a response, or sequence
of responses, from the daemon and interprets it as though it were a
query response (the return value is as for a query).  This function
does a blocking read waiting for data from the daemon (but see
POLL_NONBLOCK below); 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 daemon has closed, and 0 if POLL_NONBLOCK was
passed to gps_stream() and no data is yet available.</para>

<para><function>gps_waiting()</function> can be used to check
whether there is data from the daemon. It returns true if there is,
false on no data waiting or error condition. It does not block
waiting for input.</para>

<para><function>gps_stream()</function> asks
<application>gpsd</application> to stream the reports it has at you,
to be made available when you poll.  It is preferable to the
older-style (pre-2.90) way of doing this,
<function>gps_query()</function> with a "w+" argument, because it
insulates your code from whether your client library and your
<application>gpsd</application> are using old or new protocol.
The second argument is a flag mask that sets various policy bits;
see the list below.  Calling <function>gps_stream()</function>
more than once with different flag masks is allowed.</para>

<variablelist>
<varlistentry>
<term>WATCH_DISABLE</term>
<listitem>
<para>Disable the reporting modes specified by the other WATCH_ flags.
Cannot be used to disable POLL_NONBLOCK.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WATCH_ENABLE</term>
<listitem>
<para>Disable the reporting modes specified by the other WATCH_ flags.
This is the default.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WATCH_JSON</term>
<listitem>
<para>Enable JSON reporting of data. If WATCH_ENABLE is set, and no
other WATCH flags are set, this is the default.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WATCH_NMEA</term>
<listitem>
<para>Enable generated pseudo-NMEA reporting on binary devices.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WATCH_RARE</term>
<listitem>
<para>Enable reporting of binary packets in encoded hex.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WATCH_RAW</term>
<listitem>
<para>Enable literal passthrough of binary packets.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WATCH_SCALED</term>
<listitem>
<para>When reporting AIS or Subframe data, scale integer quantities to 
floats if they have a divisor or rendering formula associated with them.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WATCH_NEWSTYLE</term>
<listitem>
<para>Force issuing a JSON initialization and getting new-style
responses. This is the default. </para>
</listitem>
</varlistentry>
<varlistentry>
<term>WATCH_OLDSTYLE</term>
<listitem>
<para>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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WATCH_DEVICE</term>
<listitem>
<para>Restrict watching to a specified device, patch given as second
argument.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>POLL_NONBLOCK</term>
<listitem>
<para>Normally <function>gps_read()</function> blocks until
either there is a read error or some data is received from the
daemon.  In this mode, <function>gps_read()</function> returns
immediately with a value of 0 if there is no input waiting.</para>
</listitem>
</varlistentry>
</variablelist>

<para><function>gps_errstr()</function> returns an ASCII string (in
English) describing the error indicated by a nonzero return value from
<function>gps_open()</function>.</para>

<para>Consult <filename>gps.h</filename> to learn more about the data
members and associated timestamps.  Note that information will
accumulate in the session structure over time, and the 'valid' field
is not automatically zeroed by each <function>gps_read()</function>.
It is up to the client to zero that field when appropriate and to keep
an eye on the fix and sentence timestamps.</para>

<para>The Python implementation supports the same facilities as the C
library. <function>gps_open()</function> is replaced by the
initialization of a gps session object; the other calls are methods of
that object, and have the same names as the corresponding C functions.
However, it is simpler just to use the sessiion object as an iterator,
as in the example given below.  Resources within the session object
will be properly released when it is garbage-collected. Note one
limitation: POLL_NONBLOCK is not yet supported in Python; use the
waiting() method instead.</para>
</refsect1>

<refsect1 id='example'><title>CODE EXAMPLE</title>

<para>The following is an excerpted and simplified version of the
libgps interface code from
<citerefentry><refentrytitle>cgps</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>

<programlisting>
    struct gps_data_t gps_data;

    ret = gps_open(hostName, hostPort, &amp;gps_data);

    (void) gps_stream(&amp;gps_data, WATCH_ENABLE | WATCH_JSON, NULL);

    /* Put this in a loop with a call to a high resolution sleep () in it. */
    if (gps_waiting (&amp;gps_data)) {
        errno = 0;
        if (gps_read (&amp;gps_data) == -1) {
            ...
        } else {
            /* Display data from the GPS receiver. */
            if (gps_data.set &amp; ...
        }
    }

    /* When you are done... */
    (void) gps_stream(&amp;gps_data, WATCH_DISABLE, NULL);
    (void) gps_close (&amp;gps_data);
</programlisting>

</refsect1>

<refsect1 id='limitations'><title>LIMITATIONS</title>

<para>In the C API, incautious use of <function>gps_send()</function>
may lead to subtle bugs. In order to not bloat <structname>struct
gps_data_t</structname> 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.</para>

<para>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 daemon's watcher mode is careful to avoid
dangerous sequences, but you should read and understand the layout of
<structname>struct gps_data_t</structname> before using
<function>gps_send()</function> to request any of these
responses.</para>

</refsect1>

<refsect1 id='compatibility'><title>COMPATIBILITY</title>

<para>The <function>gps_query()</function> 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 daemon after a
command is shipped to it will be the response to command.</para>

<para>If you must send commands to the daemon explicitly, use
<function>gps_send()</function> but beware that this ties your code to
the GPSD wire protocol. It is not recommended.</para>

<para>See the comment above the symbol GPSD_API_MAJOR_VERSION
in <filename>gps.h</filename> for recent changes.</para>
</refsect1>

<refsect1 id='see_also'><title>SEE ALSO</title>
<para>
<citerefentry><refentrytitle>gpsd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>gps</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>libgpsd</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
<citerefentry><refentrytitle>libgpsmm</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
</para>
</refsect1>

<refsect1 id='author'><title>AUTHOR</title>
<para>Eric S. Raymond &lt;esr@thyrsus.com&gt;, C sample code Charles Curley &lt;charlescurley@charlescurley.com&gt;</para>
</refsect1>
</refentry>