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Diffstat (limited to 'manual')
| -rw-r--r-- | manual/time.texi | 165 |
1 files changed, 165 insertions, 0 deletions
diff --git a/manual/time.texi b/manual/time.texi index 7b6c4b3a13..7084e7f110 100644 --- a/manual/time.texi +++ b/manual/time.texi @@ -25,6 +25,8 @@ an Alarm}. @menu * Processor Time:: Measures processor time used by a program. * Calendar Time:: Manipulation of ``real'' dates and times. +* Precision Time:: Manipulation and monitoring of high accuracy + time. * Setting an Alarm:: Sending a signal after a specified time. * Sleeping:: Waiting for a period of time. * Resource Usage:: Measuring various resources used. @@ -1801,6 +1803,169 @@ The time is 01:02 PM. @end smallexample +@node Precision Time +@section Precision Time + +@cindex time, high precision +@pindex sys/timex.h +The @code{net_gettime} and @code{ntp_adjtime} functions provide an +interface to monitor and manipulate high precision time. These +functions are declared in @file{sys/timex.h}. + +@tindex struct ntptimeval +@deftp {Data Type} {struct ntptimeval} +This structure is used to monitor kernel time. It contains the +following members: +@table @code +@item struct timeval time +This is the current time. The @code{struct timeval} data type is +described in @ref{High-Resolution Calendar}. + +@item long int maxerror +This is the maximum error, measured in microseconds. Unless updated +via @code{ntp_adjtime} periodically, this value will reach some +platform-specific maximum value. + +@item long int esterror +This is the estimated error, measured in microseconds. This value can +be set by @code{ntp_adjtime} to indicate the estimated offset of the +local clock against the true time. +@end table +@end deftp + +@comment sys/timex,h +@comment GNU +@deftypefun int ntp_gettime (struct ntptimeval *@var{tptr}) +The @code{ntp_gettime} function sets the structure pointed to by +@var{tptr} to current values. The elements of the structure afterwards +contain the values the timer implementation in the kernel assumes. They +might or might not be correct. If they are not a @code{ntp_adjtime} +call is necessary. + +The return value is @code{0} on success and other values on failure. The +following @code{errno} error conditions are defined for this function: + +@table @code +@item TIME_ERROR +The precision clock model is not properly set up at the moment, thus the +clock must be considered unsynchronized, and the values should be +treated with care. +@end table +@end deftypefun + +@tindex struct timex +@deftp {Data Type} {struct timex} +This structure is used to control and monitor kernel time in a greater +level of detail. It contains the following members: +@table @code +@item unsigned int mode +This variable controls whether and which values are set. Several +symbolic constants have to be combined with @emph{binary or} to specify +the effective mode. These constants start with @code{MOD_}. + +@item long int offset +This value indicates the current offset of the local clock from the true +time. The value is given in microseconds. If bit @code{MOD_OFFSET} is +set in @code{mode}, the offset (and possibly other dependent values) can +be set. The offset's absolute value must not exceed @code{MAXPHASE}. + +@item long int frequency +This value indicates the difference in frequency between the true time +and the local clock. The value is expressed as scaled PPM (parts per +million, 0.0001%). The scaling is @code{1 << SHIFT_USEC}. The value +can be set with bit @code{MOD_FREQUENCY}, but the absolute value must +not exceed @code{MAXFREQ}. + +@item long int maxerror +This is the maximum error, measured in microseconds. A new value can be +set using bit @code{MOD_MAXERROR}. Unless updated via +@code{ntp_adjtime} periodically, this value will increase steadily +and reach some platform-specific maximum value. + +@item long int esterror +This is the estimated error, measured in microseconds. This value can +be set using bit @code{MOD_ESTERROR}. + +@item int status +This valiable reflects the various states of the clock machinery. There +are symbolic constants for the significant bits, starting with +@code{STA_}. Some of these flags can be updated using the +@code{MOD_STATUS} bit. + +@item long int constant +This value represents the bandwidth or stiffness of the PLL (phase +locked loop) implemented in the kernel. The value can be changed using +bit @code{MOD_TIMECONST}. + +@item long int precision +This value represents the accuracy or the maximum error when reading the +system clock. The value is expressed in microseconds and can't be changed. + +@item long int tolerance +This value represents the maximum frequency error of the system clock in +scaled PPM. This value is used to increase the @code{maxerror} every +second. + +@item long int ppsfreq +This is the first of a few optional variables that are present only if +the system clock can use a PPS (pulse per second) signal to discipline +the local clock. The value is expressed in scaled PPM and it denotes +the difference in frequency between the local clock and the PPS signal. + +@item long int jitter +This value expresses a median filtered average of the PPS signal's +dispersion in microseconds. + +@item int int shift +This value is a binary exponent for the duration of the PPS calibration +interval, ranging from @code{PPS_SHIFT} to @code{PPS_SHIFTMAX}. + +@item long int stabil +This value represents the median filtered dispersion of the PPS +frequency in scaled PPM. + +@item long int jitcnt +This counter represents the numer of pulses where the jitter exceeded +the allowed maximum @code{MAXTIME}. + +@item long int calcnt +This counter reflects the number of successful calibration intervals. + +@item long int errcnt +This counter represents the number of calibration errors (caused by +large offsets or jitter). + +@item long int stbcnt +This counter denotes the number of of calibrations where the stability +exceeded the threshold. +@end table +@end deftp + +@comment sys/timex.h +@comment GNU +@deftypefun int ntp_adjtime (int @var{mode}, struct timex *@var{tptr}) +The @code{ntp_adjtime} function sets the structure specified by +@var{tptr} to current values. In addition, values passed in @var{tptr} +can be used to replace existing settings. Therefore several magic +values can be passed in @var{mode}. Setting @var{mode} to zero only +reads the current state. + +The return value is @code{0} on success and other values on failure. The +following @code{errno} error conditions are defined for this function: + +@table @code +@item TIME_ERROR +The precision clock model is not properly set up at the moment, thus the +clock must be considered unsynchronized, and the values should be +treated with care. Another reason could be that the specified new values +are not allowed. +@end table + +For more details see RFC1305 (Network Time Protocol, Version 3) and +related documents. +@end deftypefun + + @node Setting an Alarm @section Setting an Alarm |