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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 #1
43 2741
! is not operator.
so !! is read as not of not which implies true.

Jan 18 '06 #2
On 18 Jan 2006 01:28:23 -0800, ma******@gmail. com wrote:
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

It is "not not", thta is, it checks its argument to see if it is not
zero.

Is is a way to make explicit that the argument is not a bool, and is
being tested for a non-zero value.

Zara
Jan 18 '06 #3
Zara wrote:
On 18 Jan 2006 01:28:23 -0800, ma******@gmail. com wrote:

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?


It is "not not", thta is, it checks its argument to see if it is not
zero.

Is is a way to make explicit that the argument is not a bool, and is
being tested for a non-zero value.


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
--
Nick Keighley

Jan 18 '06 #4
oops! my previous post should have been addressed to Raguath not Zara.
It then would have read something like this:-
Nick Keighley wrote:
Raguath wrote:
On 18 Jan 2006 01:28:23 -0800, ma******@gmail. com wrote:
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 not operator.
so !! is read as not of not which implies true.


oh, and leave some context in your posts

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

--
Nick Keighley

Jan 18 '06 #5
ma******@gmail. com wrote:
What does "!!" operator do


!! is two instances of one operator.

(!(a)) means ((a) == 0)
(!!(a)) means ((a) != 0)

--
pete
Jan 18 '06 #6
ra*********@gma il.com wrote:
! is not operator.


You're wrong, it is an operator ;-)
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 #7

<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?
MR


!! is a good way of turning a scalar value into 1 or 0.

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

However, this

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

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

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;
}

int main(void)
{
if(allocStuff(. ..))
{
}
else
{
}
}

Although the code above would work if allocStuff simply returned the actual
pointer value, it would stop someone from finding out that fact, and doing
something like this ...

int main(void)
{
void * p1;

if(p1 = allocStuff(...) )
{
// Aha, we've now got a pointer to the memory allocated!!!
}
else
{
}
}

Yes, in the code above [first], you'd also get the same thing by accessing
p, but I hope you get the gist of this simple example, and see how it could
be extended?
Jan 18 '06 #8
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

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"
- --

Lew Pitcher, IT Specialist, Enterprise Data Systems
Enterprise Technology Solutions, TD Bank Financial Group

(Opinions expressed here are my own, not my employer's)
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Version: GnuPG v1.2.4 (MingW32)

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=x7SK
-----END PGP SIGNATURE-----
Jan 18 '06 #9
Nick Keighley wrote:
Zara wrote:
On 18 Jan 2006 01:28:23 -0800, ma******@gmail. com wrote:

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?

It is "not not", thta is, it checks its argument to see if it is not
zero.

Is is a way to make explicit that the argument is not a bool, and is
being tested for a non-zero value.


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

- Craig Taylor
http://www.ctalkobt.net

Jan 18 '06 #10

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