重入與線程安全

在stackoverflow上看的的關於可重入與線程安全的區別：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).

函數可重入很天然會以爲該函數是線程安全的，既然能被其餘流程中斷並可調用自身也就能夠被多個線程同時調用。可是線程安全是否是可重入有點模糊，上面說了一種狀況，當重複調用一個線程安全的函數，而且函數中有鎖就有可能發生死鎖，代表線程安全不必定可重入。看來重入的要求比線程安全要更嚴格一點。

附帶查了查關於Posix的標準io操做，Posix標準要求操做字符流的函數實現爲可重入，可是各個實現爲了性能方面的考慮都沒有實現爲可重入的，這些函數有潛在不安全。可是又說明：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，代表它們應該是線程安全的。