重入和可重入用C？

问题描述

我读了一本书叫的Linux系统编程。从这本书中引用：

But then it will follow:

I am confused, is write() reentrant, or not? Because I think it clashes with the statement:

解决方案

Just to add what Mr. @Joachim Pileborg already mentioned in his answer, as per the wiki entry for Reentrancy, the basic rules for a function being re-entrant are

1. Reentrant code may not hold any static (or global) non-constant data.
2. Reentrant code may not modify its own code.
3. Reentrant code may not call non-reentrant computer programs or routines.

To elaborate, the function, if reentrant, will not have any issue with its own implementation (inducing the internal data structures it uses for itself) whether being called from different context.

A parameter, (such as a file descriptor) which is supplied to the function does not affect it's reentrancy.

So, for write(), the function itself is Reentrant, but if called with same file descriptor from different thread, it will obviously produce erroneous result. Again, that does not mean, the Reentrancy of write() is gone. It is Reentrant, but not thread-safe, and these two are different aspects.

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