在ANSI C中是否由括号表达式包围的复合语句（块）？ [英] Are compound statements (blocks) surrounded by parens expressions in ANSI C?

问题描述

浏览Linux内核源码，我发现一些代码片段，括号中包围的语句块被视为表达式 a la lisp （或ML），即一个表达式，该值是最后一个值

例如：

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

我一直在看 ANSI C语法试图弄清楚这段代码如何适合解析树，但是没有成功。

所以，有人知道这个行为是由标准规定的还是只是GCC的特色？

更新：我已经尝试使用flag -pedantic，编译器现在给我一个警告：

 警告：ISO C禁止表达式内的支撑组
  

解决方案

它被称为表达式内的支撑组。

ANSI / ISO C或C ++不允许，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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