标签：and .com assert init struct 手记 oid tac ret
stackoverflow上看到的这个问题，觉得挺有趣，顺手记下来。
楼主提问：
I bumped into this strange macro code in /usr/include/linux/kernel.h:
/* Force a compilation error if condition is true, but also produce a
result (of value 0 and type size_t), so the expression can be used
e.g. in a structure initializer (or where-ever else comma expressions
aren‘t permitted). */
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
#define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
回答是：
This is, in effect, a way to check whether the expression e can be evaluated to be 0, and if not, to fail the build.
用于检测表达式是否为零或者为空，如果是，编译阶段直接报error。注意，是编译阶段，而传统的assert只能解决运行阶段的问题。
解析如下：
You should read the expression like this:
sizeof(struct { int: -!!(e); }))(e): Compute expression e.
!!(e): Logically negate twice: 0 if e == 0; otherwise 1.
-!!(e): Numerically negate the expression from step 2: 0 if it was 0; otherwise -1.
struct{int: -!!(0);} --> struct{int: 0;}: If it was zero, then we declare a struct with an anonymous integer bitfield that has width zero. Everything is fine and we proceed as normal.
struct{int: -!!(1);} --> struct{int: -1;}: On the other hand, if it isn‘t zero, then it will be some negative number. Declaring any bitfield with negative width is a compilation error.
So we‘ll either wind up with a bitfield that has width 0 in a struct, which is fine, or a bitfield with negative width, which is a compilation error. Then we take sizeof that field, so we get a size_twith the appropriate width (which will be zero in the case where e is zero).
标签：and .com assert init struct 手记 oid tac ret
原文地址：http://www.cnblogs.com/PhoenixMY/p/6208627.html
