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using "!!" in "c"

Hello,
I saw in some open source projects a use of "!!" in "C" code;
for example:
in some header file
#define event_pending(v ) \
(!!(v)->vcpu_info->evtchn_upcall_ pending & \
!(v)->vcpu_info->evtchn_upcall_ mask)

whereas evtchn_upcall_p ending is of type unsigned char
(and also evtchn_upcall_m ask is of type unsigned char).

What does "!!" operator do in this case ? Any ideas?
MR

Jan 18 '06
43 2772
pemo wrote:
<ma******@gmail .com> wrote in message
news:11******** **************@ z14g2000cwz.goo glegroups.com.. .
Hello,
I saw in some open source projects a use of "!!" in "C" code;
for example:
in some header file
#define event_pending(v ) \
(!!(v)->vcpu_info->evtchn_upcall_ pending & \
!(v)->vcpu_info->evtchn_upcall_ mask)

whereas evtchn_upcall_p ending is of type unsigned char
(and also evtchn_upcall_m ask is of type unsigned char).

What does "!!" operator do in this case ? Any ideas?
!! is a good way of turning a scalar value into 1 or 0.


Yes.
For example, this

printf("%p\n", (void *)"boo");

will output some value that's not 0, and very unlikely to be 1, e.g.,
0040300
Fine.
However, this

printf("%p\n", (void *)!!"boo");

will output 000001, e.g., '1'


Or almost anything else since the conversion from integer to pointer is
implementation defined. If you want something like that do:
printf("%d\n", !!"boo");

<snip>
--
Flash Gordon
Living in interesting times.
Although my email address says spam, it is real and I read it.
Jan 18 '06 #21
Keith Thompson wrote:
August Karlstrom <fu********@com hem.se> writes:

.... snip ...

So you haven't heard about it? It's called the ironic
statement terminator. It's mostly used by expert C programmers
writing programs intended to be read by other experts. For
instance

x = 5;-)

adds some interesting uncertainty to the statement `x = 5;'.
These people find the preciseness of the latter statement
boring. The only way to be sure of what an ironic statement
really means is to get to know the author really well.


Just as "!!" is effectively a double-negative operator, yielding
the same logical result as a single-positive operator, ";-)" is
a double-positive operator (pronounced "Yeah, right!"), yielding
a positively ambiguous result. The behavior is not merely
undefined; it's both ill-defined and ill-mannered.


You are confusing things. There are the ambiguity operators :-)
and ;-), and the disgust operators ;-( and :-(, just to name a few
classifications . Disgust normally takes precedence.

Don't forget the inverses: (-: (-; )-: )-;
or the equivalents for squares: :-] ;-] :-[ ;-[

--
"If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson
More details at: <http://cfaj.freeshell. org/google/>
Jan 18 '06 #22
Chris Torek wrote:
.... snip ...
C distinguishes clearly between "input" and "output" logical
values for its logical operators (!, ||, &&): on input, any
nonzero value is true and zero is false, but on output, true is
exactly 1.

Note that not all "apparently logical" functions are necessarily
logical functions, e.g., isspace() and so on from <ctype.h> do
not necessarily produce only 0-or-1.

See also <http://www.c-faq.com/bool/bool2.html>.


Nor does strcmp(), another trap. The elusiveness and low
visibility of ! is a reason, to me, to #include <iso646.h> and use
the word "not".

--
"If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson
More details at: <http://cfaj.freeshell. org/google/>
Jan 18 '06 #23
ma******@gmail. com a écrit :
What does "!!" operator do <... ?


It's a combination of 2 unary not-operator. The type returned by the
not-operator is int and the value is 0 or 1.

It acts like the 'binary-converter-operator'. It converts 0 to a 0 and
non-0 to 1.

#include <stdio.h>

int main (void)
{
printf("%d\n", !!0); /* 0 */
printf("%d\n", !!123); /* 1 */

return 0;
}

--
A+

Emmanuel Delahaye
Jan 18 '06 #24
Lew Pitcher a écrit :
! is not operator.


You're wrong, it is an operator ;-)


IIUC, ragnauth.cr /meant/ to say

! is "not" operator.

as in
! is the operator for "logical not"


Yes, even I, had understood ! Noticed the smiley ?

--
A+

Emmanuel Delahaye
Jan 18 '06 #25
"Chuck F. " <cb********@yah oo.com> writes:
Chris Torek wrote:

... snip ...

C distinguishes clearly between "input" and "output" logical
values for its logical operators (!, ||, &&): on input, any
nonzero value is true and zero is false, but on output, true is
exactly 1.

Note that not all "apparently logical" functions are necessarily
logical functions, e.g., isspace() and so on from <ctype.h> do
not necessarily produce only 0-or-1.

See also <http://www.c-faq.com/bool/bool2.html>.


Nor does strcmp(), another trap. The elusiveness and low
visibility of ! is a reason, to me, to #include <iso646.h> and use
the word "not".


I don't find strcmp() to be much of a trap because I don't think of
its results as boolean. I know a lot of programmers write a string
equality operation as (!strcmp(s1, s2)); I prefer the more explicit
(strcmp(s1, s2) == 0).

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
Jan 18 '06 #26
Chuck F. a écrit :
You are confusing things. There are the ambiguity operators :-) and
;-), and the disgust operators ;-( and :-(, just to name a few
classifications . Disgust normally takes precedence.

Don't forget the inverses: (-: (-; )-: )-;
or the equivalents for squares: :-] ;-] :-[ ;-[


Thanks for having tested ThunderBird's smiley interpretor.

These ones are not interpreted:

;-( (-: (-; )-: )-; :-] ;-] ;-[
--
A+

Emmanuel Delahaye
Jan 18 '06 #27
Keith Thompson wrote:
I don't find strcmp() to be much of a trap because I don't think of
its results as boolean.
strcmp is more similar to a compar function,
than it is to a boolean.
I know a lot of programmers write a string
equality operation as (!strcmp(s1, s2)); I prefer the more explicit
(strcmp(s1, s2) == 0).


So would I.

For pseudoboolean expressions like isspace(c),
I prefer to write
if (isspace(c))
or
if (!isspace(c))

For most other conditional expressions,
I prefer to compare explicitly to zero.

--
pete
Jan 18 '06 #28
Keith Thompson said:
Just as "!!" is effectively a double-negative operator, yielding the
same logical result as a single-positive operator,


....but ! necessarily the same numerical result.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)
Jan 19 '06 #29

"Keith Thompson" <ks***@mib.or g> wrote in message
news:ln******** ****@nuthaus.mi b.org...
"pemo" <us***********@ gmail.com> writes:
[snip]
!! is a good way of turning a scalar value into 1 or 0.
So is (value, 0) (i.e., a comma operator). It turns a scalar value
into 1 or 0. It just happens to be 0.


What's the point here?
!! maps 0 to 0, and any non-zero value to 1, for any scalar operand.
For example, this

printf("%p\n", (void *)"boo");

will output some value that's not 0, and very unlikely to be 1, e.g.,
0040300

It will output some implementation-specific sequence of printing
characters. The sequence may or may not look like a number.


Can you explain this comment too? Is it an allusion to some obscure
minutiae in the std, like "In reality, an implementation is free to encode
an address such that it is displayed in egyptian hieroglyphics"?
However, this

printf("%p\n", (void *)!!"boo");

will output 000001, e.g., '1'


That invokes undefined behavior.

"boo" yields a char* value. The "!" operator yields 1 if its operand
compares equal to 0, 0 otherwise. In this case, it compares the char*
value to a null pointer value. Since "boo" cannot be a null pointer,
!"boo" is guaranteed to yield 0. Applying "!" again yields the int
value 1.

So the above is equivalent to

printf("%p\n", (void*)1);

The conversion of the int value 1 to void* may yield a trap
representation; passing this to printf() (or doing anything else with
it) invokes undefined behavior.

Interesting, so something like this is basically illegal/non-portable now?

void (* n)(void) = (void(*)(void)) 42;

n();

Years ago I used to use something like the above for invoking interrupt
routines.
Jan 19 '06 #30

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