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:0
ife == 0
; otherwise1
.-!!(e)
: Numerically negate the expression from step 2:0
if it was0
; 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.
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