gpsd/man/libgps.xml

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<refentry>
<refentryinfo><date>4 Feb 2019</date></refentryinfo>
<refmeta>
<refentrytitle>libgps</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>
    <paramdef>char *<parameter>message</parameter></paramdef>
    <paramdef>int <parameter>message_size</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>bool <function>gps_waiting</function></funcdef>
    <paramdef>const struct gps_data_t *<parameter>gpsdata</parameter></paramdef>
    <paramdef>int <parameter>timeout</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>char *<function>gps_data</function></funcdef>
    <paramdef>const struct gps_data_t *<parameter>gpsdata</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>gps_unpack</function></funcdef>
    <paramdef>char *<parameter>buf</parameter></paramdef>
    <paramdef>struct gps_data_t *<parameter>gpsdata</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <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>int <function>gps_mainloop</function></funcdef>
    <paramdef>struct gps_data_t *<parameter>gpsdata</parameter></paramdef>
    <paramdef>int <parameter>timeout</parameter></paramdef>
    <paramdef>void (*<parameter>hook</parameter>)(struct gps_data_t *gpsdata)</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=gps.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 sets up access to
<citerefentry><refentrytitle>gpsd</refentrytitle><manvolnum>1</manvolnum>
</citerefentry>
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.</para>

<para><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 socket or shared-memory interface; 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, an IPV6 address, or the special value
<constant>GPSD_SHARED_MEMORY</constant> referring to the
shared-memory export; the library will do the right thing for any of
these.</para>

<para><function>gps_close()</function> ends the session and should only be
called after a successful <function>gps_open()</function>.
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 <function>close()</function>. </para>

<para><function>gps_send()</function> writes a command to the 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
<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.  This function does
either a nonblocking read for data from the 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 daemon has closed or if the shared-memory segment was
unavailable, and 0 if no data is available.</para>

<para><function>gps_waiting()</function> can be used to check whether
there is new data from the daemon. The second argument is the maximum
amount of time to wait (in microseconds) 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
<citerefentry><refentrytitle>select</refentrytitle><manvolnum>2</manvolnum>
</citerefentry>
call, and zeros <varname>errno</varname> on entry; you can test
<varname>errno</varname> after exit to get more information about
error conditions.  Warning: under the shared-memory interface there is
a tiny race window between <function>gps_waiting()</function> and a
following <function>gps_read()</function>; in that context, because the
latter does not block, it is probably better to write a simple read
loop.</para>

<para><function>gps_mainloop()</function> 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.
</para>

<para><function>gps_unpack()</function> 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.</para>

<para><function>gps_data()</function> returns the contents of the
client data buffer (it returns NULL when using the shared-memory
export). Use with care; this may fail to be a NUL-terminated string if
WATCH_RAW is enabled.</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 (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
<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.</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, path given as second
argument.</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
socket-export calls in the C library; there is no shared-memory
interface. <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 session object as an iterator,
as in the example given below.  Resources within the session object
will be properly released when it is garbage-collected.</para>
</refsect1>

<refsect1 id='environment'><title>ENVIRONMENT VARIABLES</title>

<para>By setting the environment variable <envar>GPSD_SHM_KEY</envar>,
you can control the key value used to create shared-memory segment
used for communication with <application>gpsd</application>.  This
will be useful mainly when isolating test instances of
<application>gpsd</application> from production ones.</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, 500)) {
        errno = 0;
        if (gps_read(&amp;gps_data, NULL, 0) == -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>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.</para>

<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>In earlier versions of the API <function>gps_read()</function> was
a blocking call and there was a POLL_NONBLOCK option to make it nonblocking.
<function>gps_waiting()</function> was added to reduce the number of
wrong ways to code a polling loop.</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>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>