是复合语句（块）由ANSI C括号前pressions包围？ [英] Are compound statements (blocks) surrounded by parens expressions in ANSI C?

问题描述

浏览Linux内核源，我发现一些片$的C $ç在这里用括号包围的语句块被视为前pression的一拉口齿不清的（或ML），也就是说，一个前pression其价值是最后一条语句的值。

例如：

  int类型的=（{
    INT I;
    INT T = 1;
    为（ⅰ= 2; I＆小于5;我++）{
        T * = I;
    }
    吨;
}）;
 

我一直在看 ANSI C语法的试图找出如何这块code会适合在解析树，但我没有成功。

所以，没有任何人知道，如果这种行为是由标准规定的，或仅仅是一个GCC的特点？

更新：我试过旗-pedantic和编译器现在给我一个警告：

 警告：ISO C禁止前pressions内支撑基
 

解决方案

这就是所谓的EX pression内支撑组。

这不是由ANSI / ISO C允许，也不C ++，但GCC支持它。

Browsing the Linux kernel sources I found some piece of code where a block of statements surrounded by parenthesis is treated as a expression a la lisp (or ML), that is, an expression which value is the value of the last statement.

For example:

int a = ({
    int i;
    int t = 1;
    for (i = 2; i<5; i++) {
        t*=i;
    }
    t;
});

I've been looking at the ANSI C grammar trying to figure out how this piece of code would fit in the parse tree, but I haven't been successful.

So, does anybody know if this behaviour is mandated by the standard or is just a peculiarity of GCC?

Update: I've tried with the flag -pedantic and the compiler now gives me a warning:

warning: ISO C forbids braced-groups within expressions

解决方案

It's called "braced-group within expression".

It's not allowed by ANSI/ISO C nor C++ but gcc supports it.

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