重入与线程安全

在stackoverflow上看的的关于可重入与线程安全的区别：These functions are not reentrant. They are thread-safe. There's a big difference. Functions that are reentrant are automatically thread-safe, but thread-safe functions can still deadlock (or worse) if called again from the same thread they're already running in (for example from a signal handler).

函数可重入很天然会以为该函数是线程安全的，既然能被其余流程中断并可调用自身也就能够被多个线程同时调用。可是线程安全是否是可重入有点模糊，上面说了一种状况，当重复调用一个线程安全的函数，而且函数中有锁就有可能发生死锁，代表线程安全不必定可重入。看来重入的要求比线程安全要更严格一点。

附带查了查关于Posix的标准io操做，Posix标准要求操做字符流的函数实现为可重入，可是各个实现为了性能方面的考虑都没有实现为可重入的，这些函数有潜在不安全。可是又说明：all standard I/O functions that reference character streams shall behave as if they use flockfile() and funlockfile() internally to obtain ownership of the character streams，代表它们应该是线程安全的。