How setting the TZ environment variable avoids thousands of system calls

I see one reference to Apache below, but not whether it actually made a measurable difference.

The way you'd do this is to open an inotify (or platform equivalent) file descriptor on the first call to localtime(), and on future calls, only bother statting /etc/localtime again if that file descriptor reports something has changed. But checking if an FD has data requires a system call; you'd do a non-blocking read on the FD (or a non-blocking select or poll, or an ioctl(FIONREAD)). It's possible that system call is faster than stat, but it's still a system call.

You could do it with a thread that does a blocking read, but that's a mistake under the current UNIX architecture: lots of stuff (signal delivery and masks, for instance) gets weird as soon as you have threads, so unconditionally sticking a thread into every process using libc is a bad plan.

You could do it with fcntl(F_SETSIG), which would send you a signal (SIGIO by default) when the inotify file descriptor is readable, but you couldn't actually use SIGIO, since the user's process might have a handler. You'd need to steal one of the real-time signals and set SIGRTMIN = old SIGRTMIN + 1. (See https://github.com/geofft/enviable for a totally-non-production-suitable example of this approach.) This would probably mostly work, except changing SIGRTMIN is technically an ABI break (since programs communicate with each other with numerical signal values). You could maybe use one of the real-time signals that pthread already steals. Also, using signals is sort of a questionable idea in general; user programs now risk getting EINTR when /etc/localtime changes, which they're likely not to be prepared for. A single-threaded glibc program that sets no signal handlers never gets any EINTRs, which is nice.

In an ideal world, every program would have a standard event loop / mechanism for waiting on FDs, and libc could just register the file descriptor with that event loop. Then you'll get notified of /etc/localtime changing after the next run of the event loop, which is good enough, and it would take zero additional system calls. But unfortunately, UNIX wasn't designed that way, so something at libc's level can't assume the existence of any message loop. A higher-level library like GLib or Qt or libuv could probably do this, though.