<sect1 id="ntpshm.c"><title><filename>ntpshm.c</filename></title>
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  <entry>Functions:-</entry><entry>This puts time information into a shared memory segment for the ntp daemon to use.</entry>
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  <entry spanname='s1' align='left'>Notes based on code as of Tue Apr 6 10:17:55 2010 -0400.</entry>
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  <entry><function>static struct shmTime *getShmTime(int unit)</function></entry>
  <entry><para>This requests a block of shared memory to communicate
  to the ntp daemon. It returns a pointer to the memory on success or
  NULL on failure.</para></entry>
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  <entry><function>void ntpshm_init(struct gps_context_t *context, bool enablepps)</function></entry>
  <entry><para>Attaches all ntp shared memory segments, flagging the
  availability of the NMEA and 1pps capabilities as
  appropriate.</para></entry>
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  <entry><function>int ntpshm_alloc(struct gps_context_t *context)</function></entry>
  <entry><para>This tries to allocate a free ntp shared memory
  segment.</para><para>If one is available, initialise it for use and
  flag it as in use.</para><para>Returns the segment number on success
  or -1 on failure.</para></entry>
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  <entry><function>bool ntpshm_free(struct gps_context_t *context, int segment)</function></entry>
  <entry><para>This tries to release a previously allocated ntp shared
  memory segment. Indicates the outcome by returning true (success) or
  false.</para></entry>
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  <entry><function>int ntpshm_put(struct gps_device_t *session, double fixtime)</function></entry>
  <entry><para>If the shared memory index allocated to the session is
  valid and the <function>shmTime</function> structure of the session
  is valid, store the time of the fix.</para><para>Success is
  indicated by a return value of 1 and errors are indicated by a
  return value of 0.</para></entry>
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  <entry><function>int ntpshm_pps(struct gps_device_t *session, struct timeval *tv)</function></entry>
  <entry><para>This code is only available if the 1PPS function is
  compiled in.</para><para>The shared memory structures are checked
  for validity and if not valid, a 0 is returned.</para><para>The time
  received is then checked to be within 100 milliseconds of the PC's
  internal time and if not, a -1 is
  returned.</para><para></para><para>The time received is then checked
  to be within 500 milliseconds of the second boundary and if not, the
  shared memory structure is advised that lock is lost and a -1 is
  returned.</para><para>If good, the time is stored in the shared
  memory and 1 is returned.</para></entry>
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