Wiki says, In computing, a computer program or subroutine is called reentrant if multiple invocations can safely run concurrently. The concept applies even on a single processor system, where a reentrant procedure can be interrupted in the middle of its execution and then safely be called again ("re-entered") before its previous invocations complete execution.
強調兩個點：1）reentrant強調的是procedure被中斷，並且同一個procedure又被調用；2）強調safely run，兩個方面：a. 可以運行(run)，也就是不能被block(例如，一個procedure拿着鎖lock被中斷了，同一個procedure又被調用的時候就會因爲拿不到鎖lock被block，也就是不可重入)；b. 所謂safely是指每次procedure調用都應該產生正確的結果。
不強調concurrently，也就是說並不強調多個調用同時運行，只強調一個procedure被中斷，並且同一個procedure又被調用。concurrently是在thread safe中強調的。
Wiki says, Thread safety is a computer programming concept applicable to multi-threaded code. Thread-safe code only manipulates shared data structures in a manner that ensures that all threads behave properly and fulfill their design specifications without unintended interaction.
強調的是，1）多個thread同時運行(concurrently)，2）並且結果正確。
Why are malloc() and printf() said as non-reentrant? See excellent answer in stackoverflow.