What does !! (bang bang) mean?

Question

Can't find a reference to this anywhere -- I have the following code I'm trying to work with and can anyone please tell me what the !! means in this context? Just to be clear it's "bang bang" !! not "pipe pipe" ||. Thanks in advance!

/* Branch prediction */
#ifdef __GNUC__
# define likely(p)     __builtin_expect(!!(p), 1)
# define unlikely(p)   __builtin_expect(!!(p), 0)
# define unreachable() __builtin_unreachable()
#else
# define likely(p)     (!!(p))
# define unlikely(p)   (!!(p))
# define unreachable() ((void)0)
#endif

`!!(expression)` turns expression (assuming non-void) into `0` or `1`: `!!3.14159 ==> 1`; `!!sqrt(0) ==> 0`; `!!true ==> 1`; `!!false ==> 0` — pmg, Apr 17 '20 at 14:30
Ask yourself what would `!(!(p))` mean - same thing. [Example](https://stackoverflow.com/a/10307447/2410359) — chux - Reinstate Monica, Apr 17 '20 at 14:30
Does this answer your question? [What is "!!" in C?](https://stackoverflow.com/questions/14751973/what-is-in-c) — Paul T., Apr 17 '20 at 14:32

abelenky · Answer 1 · 2020-04-17T14:45:01.740

0

! is a simple boolean "Not" operation.

You're probably familiar with it as in:

if (!x)  // If x is 0, then execute the if-statement.

!! is two of those chained together, which has the effect of taking any non-zero value and converting it to 1/true, and any zero value stays zero/false

It is used in certain macros where a 1-or-0 value is needed, or when you want to convert an arbitrary value (pointer, double, char, etc) to a pure 1-or-0 boolean value.

edited Apr 17 '20 at 14:45

answered Apr 17 '20 at 14:30

abelenky

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"any zero value stays zero/false" --> true, yet its type may change. E.g. `!!(-0.0)` becomes `(int) 0`. – chux - Reinstate Monica Apr 17 '20 at 14:32
THANK you to everyone who responded -- many thanks! – GR99 Apr 17 '20 at 15:39

score 0 · Accepted Answer · answered Apr 17 '20 at 14:33

The ! operator performs the logical NOT operation. If it's operand is 0 the result is 1, and if the operand is non-zero the result is 0.

So !! is the logical NOT operator applied twice. So what does this do? If the operand is 0, the inner ! converts it to 1, then the outer ! converts that to 0. If the operand is non-zero, the inner ! converts it to 0, then the outer ! converts that to 1.

So !! converts a value to its boolean equivalent. If the value is 0, it remains 0, otherwise it becomes 1.

What does !! (bang bang) mean?

2 Answers2

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