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); }))What does :-!! do?
Answer:
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.
The macro is somewhat misnamed; it should be something more like BUILD_BUG_OR_ZERO, rather than...ON_ZERO. (There have been occasional discussions about whether this is a confusing name.)
You should read the expression like this:
sizeof(struct { int: -!!(e); }))(e): Compute expressione.!!(e): Logically negate twice:0ife == 0; otherwise1.-!!(e): Numerically negate:0ife == 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_t with the appropriate width (which will be zero in the case where e is zero).
Some people have asked: Why not just use an assert?
keithmo’s answer here has a good response:
These macros implement a compile-time test, while assert() is a run-time test.
Exactly right. You don’t want to detect problems in your kernel at runtime that could have been caught earlier! It’s a critical piece of the operating system. To whatever extent problems can be detected at compile time, so much the better.
