离奇的使用在Linux中条件运算符 [英] Bizarre use of conditional operator in Linux

问题描述

在3.0.4 Linux内核，毫米/ filemap.c有这一行code的：

In the 3.0.4 Linux kernel, mm/filemap.c has this line of code:

retval = retval ?: desc.error;

我试过编译类似最小测试用例用gcc -Wall并没有得到任何警告;的行为似乎相同的：

I've tried compiling a similar minimal test case with gcc -Wall and don't get any warnings; the behavior seems identical to:

retval = retval ? retval : desc.error;

综观C99标准，我想不出有什么正式介绍了这种行为。这是为什么好？

Looking at the C99 standard, I can't figure out what formally describes this behavior. Why is this OK?

推荐答案

随着其他几个人都表示，这是一个GCC扩展，没有任何标准的一部分。如果您使用 -pedantic 开关，你会得到它的警告。

As several others have said, this is a GCC extension, not part of any standard. You'll get a warning for it if you use the -pedantic switch.

的点的的这个扩展是不是在这种情况下确实可见，但想象一下，如果相反，它是

The point of this extension is not really visible in this case, but imagine if instead it was

retval = foo() ?: desc.error;

通过扩展，富（）只调用一次。没有它，你必须引入一个临时变量，以避免调用富（）两次。

With the extension, foo() is called only once. Without it, you have to introduce a temporary variable to avoid calling foo() twice.

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