rtapi_get_time.3rtapi

CNC 的开放码,EMC2 V2.2.8版

.TH rtapi_get_time "3rtapi" "2006-10-12" "EMC Documentation" "HAL"
.SH NAME

rtapi_get_time \- get the current time

.SH SYNTAX
.HP
long long rtapi_get_time()
.HP
long long rtapi_get_clocks()

.SH DESCRIPTION
\fBrtapi_get_time\fR returns the current time in nanoseconds.  Depending on the
RTOS, this may be time since boot, or time since the clock period was set, or
some other time.  Its absolute value means nothing, but it is monotonically
increasing and can be used to schedule future events, or to time the duration
of some activity.  Returns a 64 bit value.  The resolution of the returned
value may be as good as one nano-second, or as poor as several microseconds.
May be called from init/cleanup code, and from within realtime tasks.  

Experience has shown that the implementation of this function in some
RTOS/Kernel combinations is horrible.  It can take up to several microseconds,
which is at least 100 times longer than it should, and perhaps a thousand times
longer.  Use it only if you MUST have results in seconds instead of clocks, and
use it sparingly.  In most cases, \fBrtapi_get_clocks\fR shold be used instead.

\fBrtapi_get_clocks\fR returns the current time in CPU clocks.  It is 
fast, since it just reads the TSC in the CPU instead of calling a
kernel or RTOS function.  Of course, times measured in CPU clocks
are not as convenient, but for relative measurements this works
fine.  Its absolute value means nothing, but it is monotonically
increasing and can be used to schedule future events, or to time
the duration of some activity.  (on SMP machines, the two TSC's
may get out of sync, so if a task reads the TSC, gets swapped to
the other CPU, and reads again, the value may decrease.  RTAPI
tries to force all RT tasks to run on one CPU.)
Returns a 64 bit value.  The resolution of the returned value is
one CPU clock, which is usually a few nanoseconds to a fraction of
a nanosecond. 
    
Note that \fIlong long\fR math may be poorly supported on some platforms,
especially in kernel space. Also note that rtapi_print() will NOT
print \fIlong long\fRs.  Most time measurements are relative, and should
be done like this:
.RS
deltat = (long int)(end_time - start_time);
.RE
where end_time and start_time are longlong values returned from rtapi_get_time,
and deltat is an ordinary long int (32 bits).  This will work for times up to a
second or so, depending on the CPU clock frequency.  It is best used for
millisecond and microsecond scale measurements though.

.SH RETURN VALUE
Returns the current time in nanoseconds or CPU clocks.
.SH NOTES
Certain versions of the Linux kernel provide a global variable \fBcpu_khz\fR.
Computing 
.RS
	deltat = (end_clocks - start_clocks) / cpu_khz:
.RE
gives the duration measured in milliseconds.  Computing
.RS
	deltat = (end_clocks - start_clocks) * 1000000 / cpu_khz:
.RE
gives the duration measured in nanoseconds for deltas less than about 9
trillion clocks (e.g., 3000 seconds at 3GHz).