reentrancy

In UNIX systems we know malloc() is a non-reentrant function (system call). Why is that? Similarly, printf() also is said to be non-reentrant; why? I know the definition of re-entrancy, but I wanted to know why it applies to these functions. What prevents them being guaranteed reentrant? malloc and printf usually use global structures, and employ lock-based synchronization internally. That's why they're not reentrant. The malloc function could either be thread-safe or thread-unsafe. Both are not reentrant: Malloc operates on a global heap, and it's possible that two different invocations of

Recently, I asked a question, with title as "Is malloc thread safe?" , and inside that I asked, "Is malloc re-entrant?" I was under the impression that all re-entrant are thread-safe. Is this assumption wrong? Re-entrant functions do not rely on global variables that are exposed in the C library headers .. take strtok() vs strtok_r() for example in C. Some functions need a place to store a 'work in progress' , re-entrant functions allow you to specify this pointer within the thread's own storage, not in a global. Since this storage is exclusive to the calling function, it can be interrupted