- Why does glibc check /etc/localtime every time localtime is called? Wild guess: so that new values of /etc/localtime are picked at runtime without restarting programs.
- Corollary: why does glibc not check /etc/localtime every time localtime is called, when TZ is set to :/etc/localtime? Arguably the reason above should still apply when TZ is set to a file name, shouldn't it?
https://sourceware.org/git/?p=glibc.git;a=commit;h=68dbb3a69...
I'd argue it should cache the same when both old_tz and tz are NULL (but start with an old_tz that is not NULL).
I was about to file an upstream bug, but found https://sourceware.org/bugzilla/show_bug.cgi?id=5184 and https://sourceware.org/bugzilla/show_bug.cgi?id=5186
The latter actually implies the opposite should be happening: files given in TZ should be stat()ed just as much as /etc/localtime.
/etc/localtime is set by the administrator. It may change without notice to the user.
TZ is part of the user's environment and the user sets it. All applications run by the user should honor the user's wishes if the user's not falling back to system defaults.
If you're setting TZ for yourself, your libc can update things when you update the variable and restart any applications you're running under the old value. It can therefore save cycles. If you're falling back to the system default that's not under the same user's control, then it must be ready to deal with unexpected changes.
First:
> What’s going on here is that the first call to localtime in glibc opens and reads the contents of /etc/localtime. All subsequent calls to localtime internally call stat, but they do this to ensure that the timezone file has not changed.
and second: read the section titled "Preventing extraneous system calls" for the answer to your second question.