Are *_r UNIX calls reentrant (async-signal safe), thread safe or both?

Question

重入和线程安全函数有所不同，我不知道以_r结尾的Linux函数是否是线程安全的，可重入的（我的意思是异步信号安全）或两者兼而有之，

Answer 1

They are thread-safe.

Stevens/Rago APUE teaches the distinction between thread-safe functions (reentrant with respect to being called by multiple threads), and async-signal-safe functions (reentrant with respect to signal handlers, so can be called safely from within a signal handler).

APUE ch 12.5 Reentrancy lists ~79 functions which are not thread-safe, then ~11 have equivalents which are reentrant, those are the *_r functions. That means those 11 can be called by multiple threads at the same time.

APUE ch 10.6 Reentrant Functions lists ~135 functions which are async-signal-safe. They block signal delivery when needed. So, you can use them in signal handler code. Note, async-signal-safeness only matters when functions are called inside a signal handler. That may motivate one to not write signal handler code, as further details are tricky.

Kerrisk TLPI ch 21 Signals: Signal Handlers has its own table of functions which are async-signal-safe. Interestingly it is not quite same as APUE.

None of the *._r are listed as async-signal-safe by either of these references.