On 2007.05.26 01:53:59 +0530, Satyam Sharma wrote:
> Hello,
>
> Grepping through the sources I found 500+ occurrences of double
> exclamation marks before identifier names (such as !!x -- I took care
> to ignore occurrences of !! inside comment blocks, because there
> are plenty of that sort too).
>
> !! are to be found even in the definitions of common macros such as
> likely() and unlikely(), which hundreds of eyes must have seen over
> the last year or so ...
>
> Are all these occurrences merely the debris of
> s/something/!notsomething/g kind of patches or is there some
> dark, unknown C / gcc wizardry I have absolutely no clue of?
It avoids useless warnings. If you have a pointer and want to store if
it is NULL/non-NULL in an integer variables, you have to use !x or !!x.
Just using x will result in a warning that you convert a pointer to an
integer without a cast.
Given this example:
int main()
{
int ret;
void *foo;
ret = foo;
ret = !!foo;
return 0;
}
gcc will emit this warning:
ttt.c:6: warning: assignment makes integer from pointer without a cast
HTH
Björn
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