Last update: 8-Aug-2014 19:17 UTC
Address: 127.127.28.u
Reference ID: SHM
Driver ID: SHM
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.
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];
};
Each second, the value of valid of the shared memory-segment is checked:
If set, the values in the record (clockTimeStampSec, clockTimeStampUSec, receiveTimeStampSec, receiveTimeStampUSec, leap, precision) are passed to NTPD, and valid is cleared and count is bumped.
If not set, count is bumped.
Each second, valid in the shared memory-segment is checked:
If set, the count field of the record is remembered, and the values in the record (clockTimeStampSec, clockTimeStampUSec, receiveTimeStampSec, receiveTimeStampUSec, leap, precision) are read. Then, the remembered count is compared to current value of count now in the record. If both are equal, the values read from the record are passed to NTPD. 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 NTPD. The valid flag is cleared and count is bumped.
If not set, count is bumped
The GPSD 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.
Note: The GPSD client driver (type 46) uses the GPSD client protocol to connect and talk to GPSD, but using the SHM driver is the ancient way to have GPSD talk to NTPD. There are some tricky points when using the SHM interface to interface with GPSD, because GPSD 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 NTPD has no way to join the two data streams and correlate the serial data with the PPS events.
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 NTPD. 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 NTPD was trying to grab a sample.
Here is a sample showing the GPS reception fading out:
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
Some aspects of the driver behavior can be adjusted by setting bits of
the 'mode' word in the server configuration line:
server 127.127.28.x mode Y
| Bit | Dec | Hex | Meaning |
|---|---|---|---|
| 0 | 1 | 1 | 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. |
| 1-31 | - | - | reserved -- do not use |
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:
Windows uses a file mapping to the page file with the name 'Global\NTPu' for public accessible mappings, where u is the clock unit. Private / non-public mappings are created as 'Local\NTPu'.
Public access assigns a NULL DACL to the memory mapping, while private access just uses the default DACL of the process creating the mapping.
SYSV IPC creates a shared memory segment with a key value of 0x4E545030 + u, where u is again the clock unit. (This value could be hex-decoded as 'NTP0', 'NTP1',..., with funny characters for units > 9.)
Public access means a permission set of 0666, while private access creates the mapping with a permission set of 0600.
There's no support for POSIX shared memory yet.
NTPD 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 NTPD 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.
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.