I'm preparing a POSIX shared memory driver (PSHM) for ntp to address a
few issues that exist with the present SHM driver. In no particular
order, these are:
o POSIX (not SVID) shared memory
-- POSIX shared memory namespace rather than hexadecimal constant
-- avoids 0x4e545030 [...] Big units will give non-ascii
-- provides ample namespace size for ridiculously huge numbers of
units w/o obfuscation
o nanoseconds, not microseconds
-- resolution compatible with bulk of the ntp reference
implementation
-- using POSIX struct timespec
-- client compatibility with POSIX clock_gettime()
o per-unit configurable source type (a.k.a. class)
-- unlike present SHM driver, which treats all units as identical
and unconfigurable
-- currently UHF; wrong for everything else
-- was TELEPHONE; wrong for everything else
o per-unit PPS flag
-- permits shared memory PPS drivers
o POSIX-comforming code
-- no attempt to work around buggy (non-POSIX-conforming) systems!
o separate header file to simplify client code
-- shared memory structure clearly defined and well-documented
o source code/header version strings for use by what(1) or ident(1)
-- for facilitation of bug reporting, version verification, daemon-
client campatibility checks
o client/driver run-time implementation/compatibility tests
-- integer and pointer sizes
-- endianness
o no dead code
-- e.g. SHM driver "nsamples"-related cruft
o provision in shared memory to specify (variable part of) clockstats
output
-- client can control clockstats format and content (and frequency
of logging)
-- can be different for each unit
o POSIX mutex for synchronized access to shared memory for updates
-- obviates mode 0 / mode 1 / OLDWAY
Comments and suggestions are welcome.
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