复合语句是C中的左值(或右值)吗? [英] Are compound statements lvalue (or rvalue) in C?

问题描述

当我检查Linux内核中container_of宏的定义时， 我看到了复合语句作为宏定义，

When I examined the definition of the container_of macro in Linux Kernel, I saw a compound statement as a macro definition,

#define container_of(ptr, type, member) ({                      \
    const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
    (type *)( (char *)__mptr - offsetof(type,member) );})

但是，在我看来，不清楚的问题是哪个语句作为右值被考虑在内. 显然最后一条语句的结果用作右值，为什么?

however the unclear issue in my mind is which statement gets taken into account as rvalue. Apparently result of the last statement is used as rvalue, but why?

(type *)( (char *)__mptr - offsetof(type,member) );

例如，下面的代码示例在C语言中是否合法?

For instance, is the code example below legal in C?

int g_count = 0xFF;

#define GLOBAL_COUNT do {g_count;} while(0)

int main(int argc, char *argv[])
{
    int local;
    local = GLOBAL_COUNT;
    local = 0;
    GLOBAL_COUNT = local;
    return 0;
}

复合语句中变量的分配规则是什么?

What is the assignment rule for variables in compound statements?

推荐答案

({})是对C的GNU扩展，称为语句表达式:

({}) is a GNU extension to C called a statement expression:

http://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

在C中，复合语句是一个语句，不能在表达式中使用该语句.

In C a compound statement is a statement and a statement cannot be used in an expression.

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