Re-entrant vs Thread safe

Both concepts of reentrancy and thread safety relate to the way functions handle resources. However, they are not the same. While the concept of reentrancy can affect the external interface of a function, thread safety only concerns the implementation of the function and not its external interface.

• In most cases, to make a non-reentrant function reentrant, its external interface must be modified such that all data is provided by the caller of the function.

• To make a thread-unsafe function thread-safe, only the implementation needs to be changed, usually by addingsynchronization blocks to protect shared resources from concurrent accesses by different threads.

Therefore, reentrancy is a more fundamental property than thread-safety and by definition, leads to thread-safety: Every reentrant function is thread-safe; however, not every thread-safe function is reentrant.

Reference from http://en.wikipedia.org/wiki/Reentrant_(subroutine)