What is ":-!!" in C code?

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?

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); }))

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_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.

The : is a bitfield. As for !!, that is logical double negation and so returns 0 for false or 1 for true. And the - is a minus sign, i.e. arithmetic negation.

It's all just a trick to get the compiler to barf on invalid inputs.

Consider BUILD_BUG_ON_ZERO. When -!!(e) evaluates to a negative value, that produces a compile error. Otherwise -!!(e) evaluates to 0, and a 0 width bitfield has size of 0. And hence the macro evaluates to a size_t with value 0.

The name is weak in my view because the build in fact fails when the input is not zero.

BUILD_BUG_ON_NULL is very similar, but yields a pointer rather than an int.

Some people seem to be confusing these macros with assert().

These macros implement a compile-time test, while assert() is a runtime test.