what's the two !! usage?

[baumann.Pan]

hi all,

in the linux kernel code compiler.h
i read
#define likely(x) __builtin_expect(!!(x), 1)

#if __GNUC__ < 2 || (__GNUC__ == 2 && __GNUC_MINOR__ < 96)
#define __builtin_expect(exp,c) (exp)
#endif

what's the meaing of !!(x)?

thanks

baumann@pan

 

[Ben Pfaff]

what's the meaing of !!(x)?

It transforms any nonzero value to 1 and leaves zero unchanged.
It transforms null pointers to 0 and any nonnull pointer to 1.
It is equivalent to (x != 0).

 

[Peter MacMillan]

hi all,

in the linux kernel code compiler.h
i read
#define likely(x) __builtin_expect(!!(x), 1)

#if __GNUC__ < 2 || (__GNUC__ == 2 && __GNUC_MINOR__ < 96)
#define __builtin_expect(exp,c) (exp)
#endif

what's the meaing of !!(x)?

thanks

baumann@pan

!! is "not not"

That is, x is an integer. if x is 0 then !x is 1. If x is not 0 then !x
is 0.

so say x is 42. !x is 0 and !!x is 1.

--
Peter MacMillan
e-mail/msn: (e-mail address removed)
icq: 1-874-927

GCS/IT/L d-(-)>-pu s()-) a- C+++(++++)>$ UL>$ P++ L+ E-(-) W++(+++)>$
N o w++>$ O !M- V PS PE Y+ t++ 5 X R* tv- b++(+) DI D+(++)>$ G e++ h r--
y(--)

 

[Richard Bos]

!! is "not not"

That is, x is an integer.

A scalar, rather. It works for pointers and floating point numbers as
well as for integers.

Richard