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.
强调两个点：1）reentrant强调的是procedure被中断，并且同一个procedure又被调用；2）强调safely run，两个方面：a. 可以运行(run)，也就是不能被block(例如，一个procedure拿着锁lock被中断了，同一个procedure又被调用的时候就会因为拿不到锁lock被block，也就是不可重入)；b. 所谓safely是指每次procedure调用都应该产生正确的结果。
不强调concurrently，也就是说并不强调多个调用同时运行，只强调一个procedure被中断，并且同一个procedure又被调用。concurrently是在thread safe中强调的。
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.
强调的是，1）多个thread同时运行(concurrently)，2）并且结果正确。
Why are malloc() and printf() said as non-reentrant? See excellent answer in stackoverflow.