What is “:-!!” in C code?

Posted 2020-08-24 PhoenixMY

 

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); }))

1. (e): Compute expression e.

2. !!(e): Logically negate twice: 0 if e == 0; otherwise 1.

3. -!!(e): Numerically negate the expression from step 2: 0 if it was 0; otherwise -1.

4. 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.

5. 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).