Re: double exclamation (!!) suckage in the kernel

[Bodo Eggert <7eggert@gmx.de>]

Jan Engelhardt <jengelh@linux01.gwdg.de> wrote:
> On May 25 2007 14:14, David Miller wrote:

>>"!!" is used in contexts where pointers might be being
>>tested as well as plain integers, the "!!" turns a pointer
>>into the equivalent integer boolean for testing.
>>
>>NULL pointers become 0
>>non-NULL pointers become 1
> 
> Though,
> if(!!ptr)
> is effectively the same as
> if(ptr)

Not exactly, if(foo) is the same as if( (int) foo), which is not
guaranteed to result in non-null values for non-null pointers.
ISO 9899/1999 says: "Any pointer may be converted to an integer type. [...]
         The result need not be in the range of values of any integer type."

"if(!!foo)" is the same as "if(0 == (0 == foo))", which is (I asume) the same
as "if(0 == ((type_of_foo)NULL == foo))", or if((type_of_foo)NULL != foo).
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