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2012-02-10T15:04:12.097
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2017-05-23T11:55:00.137
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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 <code>BUILD_BUG_OR_ZERO</code>, rather than <code>...ON_ZERO</code>. (There have been <a href="http://lkml.indiana.edu/hypermail/linux/kernel/0703.1/1546.html" rel="noreferrer">occasional discussions about whether this is a confusing name</a>.) You should read the expression like this: <pre><code>sizeof(struct { int: -!!(e); })) </code></pre> <ol> <li><code>(e)</code>: Compute expression <code>e</code>.</li> <li><code>!!(e)</code>: Logically negate twice: <code>0</code> if <code>e == 0</code>; otherwise <code>1</code>.</li> <li><code>-!!(e)</code>: Numerically negate the expression from step 2: <code>0</code> if it was <code>0</code>; otherwise <code>-1</code>.</li> <li><code>struct{int: -!!(0);} --> struct{int: 0;}</code>: 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.</li> <li><code>struct{int: -!!(1);} --> struct{int: -1;}</code>: 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.</li> </ol> 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 <code>sizeof</code> that field, so we get a <code>size_t</code> with the appropriate width (which will be zero in the case where <code>e</code> is zero). <hr/> Some people have asked: Why not just use an <code>assert</code>? <a href="https://stackoverflow.com/a/9230305/75170">keithmo's answer</a> here has a good response: <blockquote> These macros implement a compile-time test, while assert() is a run-time test. </blockquote> 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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1. POWhat is ":-!!" in C code?
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1. USJohn Feminella
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