1132451Sroberto<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2132451Sroberto 3132451Sroberto<html> 4132451Sroberto 5132451Sroberto <head> 6132451Sroberto <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> 7132451Sroberto <meta name="GENERATOR" content="Mozilla/4.01 [en] (Win95; I) [Netscape]"> 8290000Sglebius <title>Shared Memory Driver</title> 9182007Sroberto <link href="scripts/style.css" type="text/css" rel="stylesheet"> 10290000Sglebius <style type="text/css"> 11290000Sglebius table.dlstable { font-size:85%; } 12290000Sglebius td.ttf{ font-family:Courier; font-weight:bold; } 13290000Sglebius </style> 14132451Sroberto </head> 15132451Sroberto 16132451Sroberto <body> 17132451Sroberto <h3>Shared Memory Driver</h3> 18290000Sglebius<p>Last update: 19290000Sglebius <!-- #BeginDate format:En2m -->8-Aug-2014 19:17<!-- #EndDate --> 20290000Sglebius UTC</p> 21132451Sroberto <hr> 22132451Sroberto <h4>Synopsis</h4> 23132451Sroberto <p>Address: 127.127.28.<i>u</i><br> 24132451Sroberto Reference ID: <tt>SHM</tt><br> 25132451Sroberto Driver ID: <tt>SHM</tt></p> 26290000Sglebius 27132451Sroberto <h4>Description</h4> 28290000Sglebius <p>This driver receives its reference clock info from a shared 29290000Sglebius memory-segment. The shared memory-segment is created with owner-only 30290000Sglebius access by default, unless otherwise requested by the mode word for units 31290000Sglebius ≥2. Units 0 and 1 are always created with owner-only access for 32290000Sglebius backward compatibility. 33290000Sglebius </p> 34290000Sglebius 35290000Sglebius 36132451Sroberto <h4>Structure of shared memory-segment</h4> 37132451Sroberto <pre>struct shmTime { 38290000Sglebius int mode; /* 0 - if valid is set: 39290000Sglebius * use values, 40290000Sglebius * clear valid 41290000Sglebius * 1 - if valid is set: 42290000Sglebius * if count before and after read of data is equal: 43290000Sglebius * use values 44290000Sglebius * clear valid 45290000Sglebius */ 46290000Sglebius volatile int count; 47290000Sglebius time_t clockTimeStampSec; 48290000Sglebius int clockTimeStampUSec; 49290000Sglebius time_t receiveTimeStampSec; 50290000Sglebius int receiveTimeStampUSec; 51290000Sglebius int leap; 52290000Sglebius int precision; 53290000Sglebius int nsamples; 54290000Sglebius volatile int valid; 55290000Sglebius unsigned clockTimeStampNSec; /* Unsigned ns timestamps */ 56290000Sglebius unsigned receiveTimeStampNSec; /* Unsigned ns timestamps */ 57290000Sglebius int dummy[8]; 58132451Sroberto};</pre> 59290000Sglebius 60132451Sroberto <h4>Operation mode=0</h4> 61290000Sglebius <p>Each second, the value of <code>valid</code> of the shared memory-segment is checked:</p> 62290000Sglebius <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> 63290000Sglebius <p>If not set, <code>count</code> is bumped.</p> 64132451Sroberto <h4>Operation mode=1</h4> 65290000Sglebius <p>Each second, <code>valid</code> in the shared memory-segment is checked:</p> 66290000Sglebius <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> 67290000Sglebius <p>If not set, <code>count</code> is bumped</p> 68290000Sglebius 69290000Sglebius<h4>Mode-independent post-processing</h4> 70290000SglebiusAfter the time stamps have been successfully plucked from the SHM 71290000Sglebiussegment, some sanity checks take place: 72290000Sglebius<ul> 73290000Sglebius <li>The receive time stamp of the SHM data must be in the last 5 74290000Sglebius seconds before the time the data is processed. This helps in weeding 75290000Sglebius out stale data. 76290000Sglebius <li>If the absolute difference between remote and local clock 77290000Sglebius exceeds the limit (either <i>time2</i> or the default of 4hrs), then 78290000Sglebius the sample is discarded. This check is disabled when <i>flag1</i> is 79290000Sglebius set to 1. 80290000Sglebius</ul> 81290000Sglebius 82290000Sglebius<h4>GPSD</h4> 83290000Sglebius 84290000Sglebius<a href="http://gpsd.berlios.de/"><i>GPSD</i></a> 85290000Sglebiusknows how to talk to many GPS devices. 86290000SglebiusIt can work with <i>NTPD</i> through the SHM driver. 87290000Sglebius<P> 88290000SglebiusThe <i>GPSD</i> man page suggests setting minpoll and maxpoll to 4. 89290000SglebiusThat was an attempt to reduce jitter. 90290000SglebiusThe SHM driver was fixed (ntp-4.2.5p138) to collect data each second rather than 91290000Sglebiusonce per polling interval so that suggestion is no longer reasonable. 92290000Sglebius<P> 93290000Sglebius <b>Note:</b> The <i>GPSD</i> client driver (type 46) uses the <i>GPSD</i> 94290000Sglebius client protocol to connect and talk to <i>GPSD</i>, but using the 95290000Sglebius SHM driver is the ancient way to have <i>GPSD</i> talk to <i>NTPD</i>. There 96290000Sglebius are some tricky points when using the SHM interface to interface 97290000Sglebius with <i>GPSD</i>, because <i>GPSD</i> will use two SHM clocks, one for the 98290000Sglebius serial data stream and one for the PPS information when 99290000Sglebius available. Receivers with a loose/sloppy timing between PPS and serial data 100290000Sglebius can easily cause trouble here because <i>NTPD</i> has no way to join the two 101290000Sglebius data streams and correlate the serial data with the PPS events. 102290000Sglebius</p> 103290000Sglebius<p> 104290000Sglebius 105290000Sglebius<h4>Clockstats</h4> 106290000SglebiusIf flag4 is set when the driver is polled, a clockstats record is written. 107290000SglebiusThe first 3 fields are the normal date, time, and IP address common to all clockstats records. 108290000Sglebius<P> 109290000SglebiusThe 4th field is the number of second ticks since the last poll. 110290000SglebiusThe 5th field is the number of good data samples found. The last 64 will be used by <i>NTPD</i>. 111290000SglebiusThe 6th field is the number of sample that didn't have valid data ready. 112290000SglebiusThe 7th field is the number of bad samples. 113290000SglebiusThe 8th field is the number of times the the mode 1 info was update while <i>NTPD</i> was trying to grab a sample. 114290000Sglebius<P> 115290000Sglebius 116290000SglebiusHere is a sample showing the GPS reception fading out: 117290000Sglebius<pre> 118290000Sglebius54364 84927.157 127.127.28.0 66 65 1 0 0 119290000Sglebius54364 84990.161 127.127.28.0 63 63 0 0 0 120290000Sglebius54364 85053.160 127.127.28.0 63 63 0 0 0 121290000Sglebius54364 85116.159 127.127.28.0 63 62 1 0 0 122290000Sglebius54364 85180.158 127.127.28.0 64 63 1 0 0 123290000Sglebius54364 85246.161 127.127.28.0 66 66 0 0 0 124290000Sglebius54364 85312.157 127.127.28.0 66 50 16 0 0 125290000Sglebius54364 85375.160 127.127.28.0 63 41 22 0 0 126290000Sglebius54364 85439.155 127.127.28.0 64 64 0 0 0 127290000Sglebius54364 85505.158 127.127.28.0 66 36 30 0 0 128290000Sglebius54364 85569.157 127.127.28.0 64 0 64 0 0 129290000Sglebius54364 85635.157 127.127.28.0 66 0 66 0 0 130290000Sglebius54364 85700.160 127.127.28.0 65 0 65 0 0 131290000Sglebius</pre> 132290000Sglebius 133290000Sglebius <h4>The 'mode' word</h4> 134290000Sglebius 135290000Sglebius <p> 136290000Sglebius Some aspects of the driver behavior can be adjusted by setting bits of 137290000Sglebius the 'mode' word in the server configuration line:<br> 138290000Sglebius <tt>server 127.127.28.</tt><i>x</i><tt> mode </tt><i>Y</i> 139290000Sglebius </p> 140290000Sglebius 141290000Sglebius <table border="1" width="100%"> 142290000Sglebius <caption>mode word bits and bit groups</caption> 143290000Sglebius <tbody><tr> 144290000Sglebius <th align="center">Bit</th> 145290000Sglebius <th align="center">Dec</th> 146290000Sglebius <th align="center">Hex</th> 147290000Sglebius <th align="left">Meaning</th> 148290000Sglebius </tr> 149290000Sglebius 150290000Sglebius <tr> 151290000Sglebius <td align="center">0</td> 152290000Sglebius <td align="center">1</td> 153290000Sglebius <td align="center">1</td> 154290000Sglebius <td>The SHM segment is private (mode 0600). This is the fixed 155290000Sglebius default for clock units 0 and 1; clock units >1 are mode 156290000Sglebius 0666 unless this bit is set for the specific unit.</td> 157290000Sglebius </tr><tr> 158290000Sglebius <td align="center">1-31</td> 159290000Sglebius <td align="center">-</td> 160290000Sglebius <td align="center">-</td> 161290000Sglebius <td><i>reserved -- do not use</i></td> 162290000Sglebius </tr> 163290000Sglebius </tbody> 164290000Sglebius </table> 165290000Sglebius 166290000Sglebius <h4>Fudge Factors</h4> 167132451Sroberto <dl> 168132451Sroberto <dt><tt>time1 <i>time</i></tt> 169132451Sroberto <dd>Specifies the time offset calibration factor, in seconds and fraction, with default 0.0. 170132451Sroberto <dt><tt>time2 <i>time</i></tt> 171290000Sglebius <dd>Maximum allowed difference between remote and local 172290000Sglebius clock, in seconds. Values <1.0 or >86400.0 are ignored, and the 173290000Sglebius default value of 4hrs (14400s) is used instead. See also flag 1. 174132451Sroberto <dt><tt>stratum <i>number</i></tt> 175132451Sroberto <dd>Specifies the driver stratum, in decimal from 0 to 15, with default 0. 176132451Sroberto <dt><tt>refid <i>string</i></tt> 177132451Sroberto <dd>Specifies the driver reference identifier, an ASCII string from one to four characters, with default <tt>SHM</tt>. 178132451Sroberto <dt><tt>flag1 0 | 1</tt> 179290000Sglebius <dd><i>Skip</i> the difference limit check if set. Useful 180290000Sglebius for systems where the RTC backup cannot keep the time over 181290000Sglebius long periods without power and the SHM clock must be able 182290000Sglebius to force long-distance initial jumps. <i>Check</i> the 183290000Sglebius difference limit if cleared (default). 184132451Sroberto <dt><tt>flag2 0 | 1</tt> 185132451Sroberto <dd>Not used by this driver. 186132451Sroberto <dt><tt>flag3 0 | 1</tt> 187132451Sroberto <dd>Not used by this driver. 188132451Sroberto <dt><tt>flag4 0 | 1</tt> 189290000Sglebius <dd>If flag4 is set, clockstats records will be written when the driver is polled. 190132451Sroberto </dl> 191290000Sglebius 192290000Sglebius <h4>Public vs. Private SHM segments</h4> 193290000Sglebius 194290000Sglebius <p>The driver attempts to create a shared memory segment with an 195290000Sglebius identifier depending on the unit number. This identifier (which can be 196290000Sglebius a numeric value or a string) clearly depends on the method used, which 197290000Sglebius in turn depends on the host operating system:</p> 198290000Sglebius 199290000Sglebius <ul> 200290000Sglebius <li><p> 201290000Sglebius <tt>Windows</tt> uses a file mapping to the page file with the 202290000Sglebius name '<tt>Global\NTP</tt><i>u</i>' for public accessible 203290000Sglebius mappings, where <i>u</i> is the clock unit. Private / 204290000Sglebius non-public mappings are created as 205290000Sglebius '<tt>Local\NTP</tt><i>u</i>'. 206290000Sglebius </p><p> 207290000Sglebius Public access assigns a NULL DACL to the memory mapping, while 208290000Sglebius private access just uses the default DACL of the process creating 209290000Sglebius the mapping. 210290000Sglebius </p> 211290000Sglebius </li> 212290000Sglebius <li><p> 213290000Sglebius <tt>SYSV IPC</tt> creates a shared memory segment with a key value 214290000Sglebius of <tt>0x4E545030</tt> + <i>u</i>, where <i>u</i> is again 215290000Sglebius the clock unit. (This value could be hex-decoded as 'NTP0', 216290000Sglebius 'NTP1',..., with funny characters for units > 9.) 217290000Sglebius </p><p> 218290000Sglebius Public access means a permission set of 0666, while private access 219290000Sglebius creates the mapping with a permission set of 0600. 220290000Sglebius </p> 221290000Sglebius </li> 222290000Sglebius </ul> 223290000Sglebius 224290000Sglebius <p>There's no support for POSIX shared memory yet.</p> 225290000Sglebius 226290000Sglebius <p><i>NTPD</i> is started as root on most POSIX-like operating systems 227290000Sglebius and uses the setuid/setgid system API to run under reduced rights once 228290000Sglebius the initial setup of the process is done. One consequence out of this 229290000Sglebius is that the allocation of SHM segments must be done early during the 230290000Sglebius clock setup. The actual polling of the clock is done as the run-time 231290000Sglebius user; deferring the creation of the SHM segment to this point will 232290000Sglebius create a SHM segment owned by the runtime-user account. The internal 233290000Sglebius structure of <i>NTPD</i> does not permit the use of a fudge flag if 234290000Sglebius this is to be avoided; this is the reason why a mode bit is used for 235290000Sglebius the configuration of a public segment. 236290000Sglebius </p> 237290000Sglebius 238290000Sglebius <p>When running under Windows, the chosen user account must be able to 239290000Sglebius create a SHM segment in the global object name space for SHM clocks with 240290000Sglebius public access. Otherwise the session isolation used by Windows kernels 241290000Sglebius after WinXP will get into the way if the client program does not run in 242290000Sglebius the same session. 243290000Sglebius </p> 244290000Sglebius 245290000Sglebius <h4>Additional Information</h4> 246290000Sglebius <p><a href="../refclock.html">Reference Clock Drivers</a></p> 247290000Sglebius 248132451Sroberto <hr> 249182007Sroberto <script type="text/javascript" language="javascript" src="scripts/footer.txt"></script> 250132451Sroberto </body> 251132451Sroberto 252290000Sglebius</html> 253290000Sglebius 254