[LinuxPPS] multiple access to /dev/ppsN entries

[George Spelvin]

> I don't understand what 'offset' means in this context. Is it a time
> offset or a position offset into a bytestream provided by /dev/ppsX?

A time offset.  To be precise, the assert_off_tu and clear_off_tu
fields of pps_params_t, which map on to the similary named fields
of the pps_kparams structure.

RFC2783 says, in the last paragraph of section 3.2, that

# Since the offsets, if any, apply to all users of a given PPS source, the
# implementation SHOULD impose access controls on the use of this feature;
# for example, allowing only the super-user to set the offset values.
# The default value for both offsets is zero.

> I would like to have two separate processes use the same /dev/ppsX
> entry to react to the same PPS 'events' and I am imagining doing so by
> blocking reads in each application. It seems that if this offset is a
> bytestream position then my two processes will race to get the event
> information and inconsistent behaviour will result.

No; it's a time delta which is applied to the captured timestamps.
Each process has its own read pointer.  Each process will get all the
events.