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?
Source: Tips4all
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.
ReplyDeleteThe 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): Declare an expression e.
!!(e): Negate it twice. This produces 0 if e was 0 originally, or a nonzero positive number
if it wasn't.
-!!(e): Multiply the value by -1. This results in 0 if step 2 was 0, or a negative number if it wasn't.
struct{int: -!!(0);} --> struct{int: 0;}: If it was zero, then we declare a struct with an 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 a 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.
ReplyDeleteIt'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 intended for use with a pointer input.
Some people seem to be confusing these macros with assert().
ReplyDeleteThese macros implement a compile-time test, while assert() is a run-time test.
It's creating a size 0 bitfield if the condition is false, but a size -1 (-!!1) bitfield if the condition is true/non-zero. In the former case, there is no error and the struct is initialized with an int member. In the latter case, there is a compile error (and no such thing as a size -1 bitfield is created, of course).
ReplyDelete