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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 2786
Lew Pitcher <Le*********@td .com> writes:
August Karlstrom wrote:
ra*********@gma il.com wrote:
! 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"


Right, which is why Lew used the ";-)" operator.

--
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 #11
"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.

!! 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.
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.
I've seen/used !! in functions that allocate memory, and return 1 if the
memory was allocated ok, else 0.

A routine like this would look something like ...

void * p;

int allocStuff(size _t n)
{
p = malloc(n);

return !!p;
}


That's fine, since !!p is an int value and the function returns an int.

The thing to remember is that !!foo doesn't just normalize foo to 0 or
1; it yields a value of type int, regardless of the type of foo.

--
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 #12

<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?


There is no "!!" operator. What you are seeing is
two adjacent "!" (logical 'not') operators. In a
Boolean context, any zero value is considered 'false',
any nonzero value, 'true'. C's explicit values for
reporting (e.g. with 'if()') 'true' and 'false' are
zero (0) and one (1) respectively.
0 == 0
!0 == 1
!!0 == 0

1 == 1
!1 == 0
!!1 == 1

42 == 42
!42 == 0
!!42 == 1
!! is sometimes used to convert a true (but not == 1)
value to 1.

-Mike
Jan 18 '06 #13
Craig Taylor <ct******@gmail .com> writes:
Nick Keighley wrote:

[...]
no, not really. In C a non-zero value is taken to be true. "!" as you
state is the not operator and yields either 0 (false) or 1 (true). So
"!!"
yeilds a "normalised " boolean value that is it has the same logical
value but is guaranteed to be 0 or 1. Using a truth table:-
x !x !!x
0 1 0
1 0 1
2 0 1
where "2" represents an arbitary non-zero value


You make reference to it but your table doesn't reflect that C simply
defines logical true as non zero so you can't say that it's == 1 or 2
or anything - only that it's non-zero.

More precisely:
x !x !!x
0 1 0 ie: logical false
1 0 != 0 ie: logical true
2 0 != 0 logical true


No, Craig's table is correct and is more precise than yours. The "!"
operator yields a value of type int; that value is guaranteed to be
either 0 or 1.

All C operators that yield logical values:
&& || ! == != < <= > >=
always yield a value of either 0 or 1. If any of them yield a
non-zero value other than 1, your compiler is buggy.

It's probably bad style to rely on this unless you have a specific
reason to do so.

All contexts that require a condition:
if()
while()
for(...;conditi on;...)
do while()
condition ? ... : ...
condition && condition
condition || condition
! condition
treat zero as false, and any non-zero value as true.

A *function* that returns a logical value (as type int) can reasonably
return any non-zero value for true; for example, the is*() functions
in <ctype.h> are allowed to do this.

--
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 #14
Keith Thompson wrote:
Lew Pitcher <Le*********@td .com> writes:
August Karlstrom wrote:
ra*********@gma il.com wrote:

! 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"


Right, which is why Lew used the ";-)" operator.


;-) is an operator? I've never seen it before. Is this a C99 extension?

Does it modify the line of code that it's on to perform what you meant
it to do instead of how you coded it for? If so, I've been looking for
that for a while. (This may also explain why there's few C99 compilers
around... ). ;-)

- Craig Taylor
http://www.ctalkobt.net
Jan 18 '06 #15
"Keith Thompson" <ks***@mib.or g> wrote in message
news:ln******** ****@nuthaus.mi b.org...
Craig Taylor <ct******@gmail .com> writes:
Nick Keighley wrote:

[...]
no, not really. In C a non-zero value is taken to be true. "!" as you
state is the not operator and yields either 0 (false) or 1 (true). So
"!!"
yeilds a "normalised " boolean value that is it has the same logical
value but is guaranteed to be 0 or 1. Using a truth table:-
x !x !!x
0 1 0
1 0 1
2 0 1
where "2" represents an arbitary non-zero value


You make reference to it but your table doesn't reflect that C simply
defines logical true as non zero so you can't say that it's == 1 or 2
or anything - only that it's non-zero.

More precisely:
x !x !!x
0 1 0 ie: logical false
1 0 != 0 ie: logical true
2 0 != 0 logical true


No, Craig's table is correct and is more precise than yours.


Newsreader problem Keith?
Would have let it slide, but this is the 2nd post of yours I've
read in the last minute which improperly attributes quoted text.
Nick's table is the one you are claiming is more precise...
Craig's table is the one you are complaining about.

(note: the other attribution was further up thread...
August was the one who used the ";-)" operator, not Lew)
Jan 18 '06 #16
"Mark B" <so***@localbar .com> writes:
"Keith Thompson" <ks***@mib.or g> wrote in message
news:ln******** ****@nuthaus.mi b.org...
Craig Taylor <ct******@gmail .com> writes:
Nick Keighley wrote: [...]
no, not really. In C a non-zero value is taken to be true. "!" as you
state is the not operator and yields either 0 (false) or 1 (true). So
"!!"
yeilds a "normalised " boolean value that is it has the same logical
value but is guaranteed to be 0 or 1. Using a truth table:-
x !x !!x
0 1 0
1 0 1
2 0 1
where "2" represents an arbitary non-zero value

You make reference to it but your table doesn't reflect that C simply
defines logical true as non zero so you can't say that it's == 1 or 2
or anything - only that it's non-zero.

More precisely:
x !x !!x
0 1 0 ie: logical false
1 0 != 0 ie: logical true
2 0 != 0 logical true


No, Craig's table is correct and is more precise than yours.


Newsreader problem Keith?


Yes. Not the software, just the guy at the keyboard reading the news.
Would have let it slide, but this is the 2nd post of yours I've
read in the last minute which improperly attributes quoted text.
Nick's table is the one you are claiming is more precise...
Craig's table is the one you are complaining about.

(note: the other attribution was further up thread...
August was the one who used the ";-)" operator, not Lew)


My apologies, and thanks for catching my errors.

--
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 #17
In article <wl************ *@bignews5.bell south.net>
Craig Taylor <ct******@gmail .com> wrote:
You make reference to it but your table doesn't reflect that C simply
defines logical true as non zero so you can't say that it's == 1 or 2 or
anything - only that it's non-zero.

More precisely:
x !x !!x
0 1 0 ie: logical false
1 0 != 0 ie: logical true
2 0 != 0 logical true


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>.
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.
Jan 18 '06 #18
Craig Taylor wrote:
Keith Thompson wrote:
Lew Pitcher <Le*********@td .com> writes:
August Karlstrom wrote:

ra*********@gma il.com wrote:

> ! 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"

Right, which is why Lew used the ";-)" operator.


;-) is an operator? I've never seen it before. Is this a C99 extension?

Does it modify the line of code that it's on to perform what you meant
it to do instead of how you coded it for? If so, I've been looking for
that for a while. (This may also explain why there's few C99 compilers
around... ). ;-)


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.
August

--
I am the "ILOVEGNU" signature virus. Just copy me to your
signature. This email was infected under the terms of the GNU
General Public License.
Jan 18 '06 #19
August Karlstrom <fu********@com hem.se> writes:
Craig Taylor wrote:
Keith Thompson wrote:
Lew Pitcher <Le*********@td .com> writes:

August Karlstrom wrote:

> ra*********@gma il.com wrote:
>
>> ! 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"
Right, which is why Lew used the ";-)" operator.

August, not Lew. Sorry.
;-) is an operator? I've never seen it before. Is this a C99
extension?
Does it modify the line of code that it's on to perform what you
meant it to do instead of how you coded it for? If so, I've been
looking for that for a while. (This may also explain why there's few
C99 compilers around... ). ;-)


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.

--
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 #20

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