posix tai time converter
C functions for conversion between POSIX time and TAI time.
POSIX time is the system time returned by the functions time(), gettimeofday() or clock_gettime(CLOCK_REALTIME,...). It is the number of seconds elapsed since 1970-01-01T00:00:00 UTC subtracted by the number of leap seconds.
TAI time is the time value returned by clock_gettime(CLOCK_TAI,...). It is the number of seconds elapsed since 1970-01-01T00:00:00 UTC without leap seconds subtracted.
The following conversion functions can be used to convert time between POSIX and TAI time.
The conversion uses a table of leap seconds obtained from https://www.ietf.org/timezones/data/leap-seconds.list. This file is updated as new leap seconds are added.
In order to teest if the table is outdated use the following code
if (!is_valid_posix_time(time(NULL))) /* table is outdated */;
The table is currently hard code but should ideally be loaded from the referenced source when possible. It should also be reloaded at runtime when its validity limit is reached so that programs running for a very long duration get a properly updated leap second table. Dynamic loading and updating of the leap second table might be implemented in future version of this files.
Compilation
To compile and run use the following command
gcc -O3 main.c posix_tai.c && ./a.out
It will first output the C code for the hardcoded table and the run the unit tests.
If the hardcoded leap second table is outdated, the unit test will fail. You then should download the latest version of the leap-seconds.list file at
https://www.ietf.org/timezones/data/leap-seconds.list.
By running the test again, you will get as first output the new version of the code for the table. Replace the corresponding source code in posix_tai.c and run the test again. The unit test should then succeed.