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<h3>Shared Memory Driver</h3>
<p>Last update:
<!-- #BeginDate format:En2m -->8-Aug-2014 19:17<!-- #EndDate -->
UTC</p>
<hr>
<h4>Synopsis</h4>
<p>Address: 127.127.28.<i>u</i><br>
Reference ID: <tt>SHM</tt><br>
Driver ID: <tt>SHM</tt></p>
<h4>Description</h4>
<p>This driver receives its reference clock info from a shared
memory-segment. The shared memory-segment is created with owner-only
access by default, unless otherwise requested by the mode word for units
≥2. Units 0 and 1 are always created with owner-only access for
backward compatibility.
</p>
<h4>Structure of shared memory-segment</h4>
<pre>struct shmTime {
int mode; /* 0 - if valid is set:
* use values,
* clear valid
* 1 - if valid is set:
* if count before and after read of data is equal:
* use values
* clear valid
*/
volatile int count;
time_t clockTimeStampSec;
int clockTimeStampUSec;
time_t receiveTimeStampSec;
int receiveTimeStampUSec;
int leap;
int precision;
int nsamples;
volatile int valid;
unsigned clockTimeStampNSec; /* Unsigned ns timestamps */
unsigned receiveTimeStampNSec; /* Unsigned ns timestamps */
int dummy[8];
};</pre>
<h4>Operation mode=0</h4>
<p>Each second, the value of <code>valid</code> of the shared memory-segment is checked:</p>
<p>If set, the values in the record (clockTimeStampSec, clockTimeStampUSec, receiveTimeStampSec, receiveTimeStampUSec, leap, precision) are passed to <i>NTPD</i>, and <code>valid</code> is cleared and <code>count</code> is bumped.</p>
<p>If not set, <code>count</code> is bumped.</p>
<h4>Operation mode=1</h4>
<p>Each second, <code>valid</code> in the shared memory-segment is checked:</p>
<p>If set, the <code>count</code> field of the record is remembered, and the values in the record (clockTimeStampSec, clockTimeStampUSec, receiveTimeStampSec, receiveTimeStampUSec, leap, precision) are read. Then, the remembered <code>count</code> is compared to current value of <code>count</code> now in the record. If both are equal, the values read from the record are passed to <i>NTPD</i>. If they differ, another process has modified the record while it was read out (was not able to produce this case), and failure is reported to <i>NTPD</i>. The <code>valid</code> flag is cleared and <code>count</code> is bumped.</p>
<p>If not set, <code>count</code> is bumped</p>
<h4>Mode-independent post-processing</h4>
After the time stamps have been successfully plucked from the SHM
segment, some sanity checks take place:
<ul>
<li>The receive time stamp of the SHM data must be in the last 5
seconds before the time the data is processed. This helps in weeding
out stale data.
<li>If the absolute difference between remote and local clock
exceeds the limit (either <i>time2</i> or the default of 4hrs), then
the sample is discarded. This check is disabled when <i>flag1</i> is
set to 1.
</ul>
<h4>GPSD</h4>
<a href="http://gpsd.berlios.de/"><i>GPSD</i></a>
knows how to talk to many GPS devices.
It can work with <i>NTPD</i> through the SHM driver.
<P>
The <i>GPSD</i> man page suggests setting minpoll and maxpoll to 4.
That was an attempt to reduce jitter.
The SHM driver was fixed (ntp-4.2.5p138) to collect data each second rather than
once per polling interval so that suggestion is no longer reasonable.
<P>
<b>Note:</b> The <i>GPSD</i> client driver (type 46) uses the <i>GPSD</i>
client protocol to connect and talk to <i>GPSD</i>, but using the
SHM driver is the ancient way to have <i>GPSD</i> talk to <i>NTPD</i>. There
are some tricky points when using the SHM interface to interface
with <i>GPSD</i>, because <i>GPSD</i> will use two SHM clocks, one for the
serial data stream and one for the PPS information when
available. Receivers with a loose/sloppy timing between PPS and serial data
can easily cause trouble here because <i>NTPD</i> has no way to join the two
data streams and correlate the serial data with the PPS events.
</p>
<p>
<h4>Clockstats</h4>
If flag4 is set when the driver is polled, a clockstats record is written.
The first 3 fields are the normal date, time, and IP address common to all clockstats records.
<P>
The 4th field is the number of second ticks since the last poll.
The 5th field is the number of good data samples found. The last 64 will be used by <i>NTPD</i>.
The 6th field is the number of sample that didn't have valid data ready.
The 7th field is the number of bad samples.
The 8th field is the number of times the the mode 1 info was update while <i>NTPD</i> was trying to grab a sample.
<P>
Here is a sample showing the GPS reception fading out:
<pre>
54364 84927.157 127.127.28.0 66 65 1 0 0
54364 84990.161 127.127.28.0 63 63 0 0 0
54364 85053.160 127.127.28.0 63 63 0 0 0
54364 85116.159 127.127.28.0 63 62 1 0 0
54364 85180.158 127.127.28.0 64 63 1 0 0
54364 85246.161 127.127.28.0 66 66 0 0 0
54364 85312.157 127.127.28.0 66 50 16 0 0
54364 85375.160 127.127.28.0 63 41 22 0 0
54364 85439.155 127.127.28.0 64 64 0 0 0
54364 85505.158 127.127.28.0 66 36 30 0 0
54364 85569.157 127.127.28.0 64 0 64 0 0
54364 85635.157 127.127.28.0 66 0 66 0 0
54364 85700.160 127.127.28.0 65 0 65 0 0
</pre>
<h4>The 'mode' word</h4>
<p>
Some aspects of the driver behavior can be adjusted by setting bits of
the 'mode' word in the server configuration line:<br>
<tt>server 127.127.28.</tt><i>x</i><tt> mode </tt><i>Y</i>
</p>
<table border="1" width="100%">
<caption>mode word bits and bit groups</caption>
<tbody><tr>
<th align="center">Bit</th>
<th align="center">Dec</th>
<th align="center">Hex</th>
<th align="left">Meaning</th>
</tr>
<tr>
<td align="center">0</td>
<td align="center">1</td>
<td align="center">1</td>
<td>The SHM segment is private (mode 0600). This is the fixed
default for clock units 0 and 1; clock units >1 are mode
0666 unless this bit is set for the specific unit.</td>
</tr><tr>
<td align="center">1-31</td>
<td align="center">-</td>
<td align="center">-</td>
<td><i>reserved -- do not use</i></td>
</tr>
</tbody>
</table>
<h4>Fudge Factors</h4>
<dl>
<dt><tt>time1 <i>time</i></tt>
<dd>Specifies the time offset calibration factor, in seconds and fraction, with default 0.0.
<dt><tt>time2 <i>time</i></tt>
<dd>Maximum allowed difference between remote and local
clock, in seconds. Values <1.0 or >86400.0 are ignored, and the
default value of 4hrs (14400s) is used instead. See also flag 1.
<dt><tt>stratum <i>number</i></tt>
<dd>Specifies the driver stratum, in decimal from 0 to 15, with default 0.
<dt><tt>refid <i>string</i></tt>
<dd>Specifies the driver reference identifier, an ASCII string from one to four characters, with default <tt>SHM</tt>.
<dt><tt>flag1 0 | 1</tt>
<dd><i>Skip</i> the difference limit check if set. Useful
for systems where the RTC backup cannot keep the time over
long periods without power and the SHM clock must be able
to force long-distance initial jumps. <i>Check</i> the
difference limit if cleared (default).
<dt><tt>flag2 0 | 1</tt>
<dd>Not used by this driver.
<dt><tt>flag3 0 | 1</tt>
<dd>Not used by this driver.
<dt><tt>flag4 0 | 1</tt>
<dd>If flag4 is set, clockstats records will be written when the driver is polled.
</dl>
<h4>Public vs. Private SHM segments</h4>
<p>The driver attempts to create a shared memory segment with an
identifier depending on the unit number. This identifier (which can be
a numeric value or a string) clearly depends on the method used, which
in turn depends on the host operating system:</p>
<ul>
<li><p>
<tt>Windows</tt> uses a file mapping to the page file with the
name '<tt>Global\NTP</tt><i>u</i>' for public accessible
mappings, where <i>u</i> is the clock unit. Private /
non-public mappings are created as
'<tt>Local\NTP</tt><i>u</i>'.
</p><p>
Public access assigns a NULL DACL to the memory mapping, while
private access just uses the default DACL of the process creating
the mapping.
</p>
</li>
<li><p>
<tt>SYSV IPC</tt> creates a shared memory segment with a key value
of <tt>0x4E545030</tt> + <i>u</i>, where <i>u</i> is again
the clock unit. (This value could be hex-decoded as 'NTP0',
'NTP1',..., with funny characters for units > 9.)
</p><p>
Public access means a permission set of 0666, while private access
creates the mapping with a permission set of 0600.
</p>
</li>
</ul>
<p>There's no support for POSIX shared memory yet.</p>
<p><i>NTPD</i> is started as root on most POSIX-like operating systems
and uses the setuid/setgid system API to run under reduced rights once
the initial setup of the process is done. One consequence out of this
is that the allocation of SHM segments must be done early during the
clock setup. The actual polling of the clock is done as the run-time
user; deferring the creation of the SHM segment to this point will
create a SHM segment owned by the runtime-user account. The internal
structure of <i>NTPD</i> does not permit the use of a fudge flag if
this is to be avoided; this is the reason why a mode bit is used for
the configuration of a public segment.
</p>
<p>When running under Windows, the chosen user account must be able to
create a SHM segment in the global object name space for SHM clocks with
public access. Otherwise the session isolation used by Windows kernels
after WinXP will get into the way if the client program does not run in
the same session.
</p>
<h4>Additional Information</h4>
<p><a href="../refclock.html">Reference Clock Drivers</a></p>
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