在Linux中使用C代码的Curly Brace用途(include / linux / list.h)?

我在Linux中遇到了以下代码(include / linux / list.h).我对第713行感到困惑.特别是,我不明白({n = pos-> member.next; 1;}).

花括号做什么?为什么这个陈述中有’1’?

如果有人能解释这一特定的行,我将不胜感激.注意,我不需要解释链接列表和#defines如何工作等.

704 /**
705  * hlist_for_each_entry_safe - iterate over list of given type safe against removal of list entry
706  * @pos:        the type * to use as a loop cursor.
707  * @n:          another &struct hlist_node to use as temporary storage
708  * @head:       the head for your list.
709  * @member:     the name of the hlist_node within the struct.
710  */
711 #define hlist_for_each_entry_safe(pos, n, head, member)                 \
712         for (pos = hlist_entry_safe((head)->first, typeof(*pos), member);\
713              pos && ({ n = pos->member.next; 1; });                     \
714              pos = hlist_entry_safe(n, typeof(*pos), member))
715 
最佳答案
这是一个声明表达式.它是gcc extension并且根据文档6.1 Statements and Declarations in Expressions

The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct.

在这种情况下,对于代码:

({ n = pos->member.next; 1; })

值为1.根据文档:

This feature is especially useful in making macro definitions “safe” (so that they evaluate each operand exactly once).

它给出了这个例子而不使用语句表达式:

#define max(a,b) ((a) > (b) ? (a) : (b))

与这个安全版本相比,您需要知道操作数的类型:

#define maxint(a,b) \
   ({int _a = (a), _b = (b); _a > _b ? _a : _b; })

这是众多gcc extensions used in the Linux kernel中的一个.

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