[jcul]

You could be surprised.

For example, recently I wanted to call `gettid()` in a signal handler. Which I guessed was just a simple wrapper around the syscall.

However, it seems this can cache the thread ID in thread local storage (can't remember exact details).

I switched to making a syscall instead.

[SomeoneFromCA]

Well, this is possible for sure, but not for primitive functions, such as strcmp. It just does not happen in practice.

[jcul]

For functions like strcmp, I think they must be signal safe, to be POSIX compliant.

https://man7.org/linux/man-pages/man7/signal-safety.7.html

If it's on this list I generally trust it is safe.

I guess my point is, that if it's not, even a simple function may appear safe, but could do surprising things.