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<sect1 id="packet.c"><title><filename>packet.c</filename></title>
<informaltable frame='all' pgwide='1'>
<tgroup cols='2'>
<colspec colname='c1'></colspec>
<colspec colname='c2'></colspec>
<spanspec spanname='s1' namest='c1' nameend='c2'></spanspec>
<thead>
<row>
<entry>Functions:-</entry><entry>The packet-sniffing engine for reading data packets from GPS devices.</entry>
</row>
</thead>
<tfoot>
<row>
<entry spanname='s1' align='left'>Notes based on code as of Tue Apr 6 10:17:55 2010 -0400.</entry>
</row>
</tfoot>
<tbody>
<row>
<entry><function>static void nextstate(struct gps_packet_t *lexer, unsigned char c)</function></entry>
<entry><para>This is the main packet-sniffer loop. It scans the character
against the definitions of all the packet structures known to
<application>gpsd</application> and, if possible, sets a new packet
state.</para><para>If the state is <quote>xxx_RECOGNIZED</quote>,
the packet_parse routine will despatch the packet to the appropriate
driver.</para></entry>
</row>
<row>
<entry><function>static void packet_accept(struct gps_packet_t *lexer, int packet_type)</function></entry>
<entry><para>This shifts a packet that has been recognized into the
output buffer, provided it is not bigger than the
buffer.</para></entry>
</row>
<row>
<entry><function>static void packet_discard(struct gps_packet_t *lexer)</function></entry>
<entry><para>This clears a handled packet out of the input
buffer.</para></entry>
</row>
<row>
<entry><function>static void character_discard(struct gps_packet_t *lexer)</function></entry>
<entry><para>This is called if the <function>nextstate()</function>
function returns <quote>GROUND_STATE</quote>.</para><para>In this
case the character does not match any pattern, so to discard it, the
input buffer is shifted back by one character to overwrite the
<quote>bad</quote> character.</para></entry>
</row>
<row>
<entry><function>ssize_t packet_parse(struct gps_packet_t *lexer, size_t fix)</function></entry>
<entry><para>Call the <function>nextstate()</function> function to
process the available data and set the recognition state
correctly.</para><para>When a packet is matched to a driver, call
<function>packet_accept()</function> and
<function>packet_discard()</function> to handle the packet. If it is
not matched, call <function>packet_discard()</function> and set the
state to <quote>GROUND_STATE</quote></para><para>Return the number
of characters handled.</para></entry>
</row>
<row>
<entry><function>ssize_t packet_get(int fd, struct gps_packet_t *lexer)</function></entry>
<entry><para>Reads raw data from the input port.</para><para>Returns
the number of characters read (0 or more) or BAD_PACKET if there was
an error in reading.</para><para>Errors
<errortype>EAGAIN</errortype> and <errortype>EINTR</errortype> are
not classed as failures and cause a return of 0.</para><para>In case
of a good read of more than 0 characters, the return value is the
output from a call to
<function>packet_parse()</function>.</para></entry>
</row>
<row>
<entry><function>void packet_reset(struct gps_packet_t *lexer)</function></entry>
<entry><para>This simply resets the entire packet state machine to
the ground state.</para></entry>
</row>
<row>
<entry><function>void packet_nit(struct gps_packet_t *lexer)</function></entry>
<entry><para>Zeros some counters, then resets the entire packet
state machine to the ground state.</para></entry>
</row>
<row>
<entry><function>void packet_pushback(struct gps_packet_t *lexer)</function></entry>
<entry><para>This pushes back the last packet from the output buffer
to the input buffer, provided doing so would not overflow the input
buffer.</para></entry>
</row>
</tbody>
</tgroup>
</informaltable>
</sect1>
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