No, this is confused. Reentrancy ("reentrant-safe", or the somewhat related POSIX definition of async-signal-safe) and thread safety are not the same thing. A

No, this is confused. Reentrancy ("reentrant-safe", or the somewhat related POSIX definition of async-signal-safe) and thread safety are not the same thing. A reentrant function is thread-safe, but a thread-safe function may or may not be reentrant. For instance, if a function uses mutual exclusion (say, posix_mutex_lock() and friends) to ensure thread-safety it won't be reentrant, because if the function is invoked via a signal handler it may deadlock. Which is why many common libc functions like malloc and stdio are not required to be async-signal-safe in POSIX, whereas they are required to be thread-safe.