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const and 'constant expression'

Hi C experts

I've a bunch of questions.

Consider this simplified piece of code.

const int a = 10;

int main () {
static int b = a;
return b;
}

~>gcc test.c
test.c: In function `main':
test.c:4: error: initializer element is not constant
~>

Question1
I understand that const is 'read-only' but I could not convince my
co-worker that its value can change.

6.7.8 Initialization

[...]

[#4] All the expressions in an initializer for an object
that has static storage duration shall be constant
expressions or string literals.

Can someone point me to text that says that 'const' is not 'constant
expression' and can in fact change?

He claims that the compiler may choose to substitute the value at
compile time.
I tried to explain how the value can change (but I had to use pointers
to const int) which he shot down saying the comparison of examples is
not valid (it's not initializer and it involves pointers).

Question2
Moreover, if we turn on optimization, (and assume that the real code
does not have any dead code like above), the compiler does not give any
error. Is the compiler correct after optimisation?
I think it is a bug in compiler but I need text in standard to prove
that.

Thanks in advance for your comments
-Dev

Apr 19 '06
13 2583
Kenneth Brody wrote:
Marc Boyer wrote:
Le 19-04-2006, de**********@ya hoo.com <de**********@y ahoo.com> a écrit :
Thanks Marc, point noted.

Now getting back to my example,

const int a = 10;

In this case, can value of 'a' change?

Not to my knowledge.


Isn't this legal?

==========

#include <stdio.h>

const int a = 10;

int main(int argc,char *argv[])
{
int *pt = (int *)&a;

printf("%d\n",a );

*pt = 20;

printf("%d\n",a );
}

==========

It fails here, at runtime, but only because "a" has been placed into
read-only memory on this platform. But it's perfectly "legal" C, as
far as I know. It compiles without even a warning about "pt" and "a",
even with maximum warnings turned on.


It's legal C syntax and no diagnostic is required, but the attempt to
modify a const qualified object invokes undefined behaviour. The reason
for this is, I believe, specifically so that implementations *can* put
const qualified objects in to read only memory, so you implementation is
quite within its rights to do this. To quote from section 6.7.3 of
n1124.pdf:
| Semantics
| ...
| 5 If an attempt is made to modify an object defined with a
| const-qualified type through use of an lvalue with
| non-const-qualified type, the behavior is undefined...
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc
Apr 19 '06 #11
Marc Boyer wrote:
Le 19-04-2006, de**********@ya hoo.com <de**********@y ahoo.com> a écrit :
Thanks Marc, point noted.

Now getting back to my example,

const int a = 10;

In this case, can value of 'a' change?


Not to my knowledge.
If so how?
If not, why can not variable 'a' be treated a compile time constant.


It 'can' in this case. But, just try to explain in wich case
it can and when it can not....

My bad. I did not include complete text.
Let me include enough context and repost.

~> cat test.c
const int a = 10;
int main () {
static int b = a;
return b;
}

If the compiler can treat this 'a' as a compile time constant then this
should also compile.

~> cat test2.c
const int size = 5;
int array[size];
int main() {
return 0;
}

But seems like it does not compile even with optimization turned on.
This is causing confusion. In one case, it compiles test.c with
optimisation.
In another case, it does not compile test2.c

Appreciate your patience,
-Dev
PS I'm aware of volatile const and VLA but that's a different topic.
I'm not using both here.

Apr 19 '06 #12
On 19 Apr 2006 07:22:03 -0700, de**********@ya hoo.com wrote in
comp.lang.c:
Hi C experts

I've a bunch of questions.

Consider this simplified piece of code.

const int a = 10;

int main () {
static int b = a;
return b;
}

~>gcc test.c
test.c: In function `main':
test.c:4: error: initializer element is not constant
~>

Question1
I understand that const is 'read-only' but I could not convince my
co-worker that its value can change.
No strictly conforming program can change its value. Whether or not
it can change makes no difference.
6.7.8 Initialization

[...]

[#4] All the expressions in an initializer for an object
that has static storage duration shall be constant
expressions or string literals.

Can someone point me to text that says that 'const' is not 'constant
expression' and can in fact change?
Nothing in the standard states that a const object may change. But
there is a precise definition of what constitutes a constant
expression, in 6.6 Constant expressions. Nowhere in that section does
it include the value of an object, regardless of const qualification,
as a constant expression.
He claims that the compiler may choose to substitute the value at
compile time.
I tried to explain how the value can change (but I had to use pointers
to const int) which he shot down saying the comparison of examples is
not valid (it's not initializer and it involves pointers).
It is no longer a C program if you attempt to change an object defined
with the const qualifier.
Question2
Moreover, if we turn on optimization, (and assume that the real code
does not have any dead code like above), the compiler does not give any
error. Is the compiler correct after optimisation?
The compiler is neither correct nor incorrect, see below.
I think it is a bug in compiler but I need text in standard to prove
that.


No, it's not a bug in the compiler. But it is not portable behavior
either. Paragraph 10 of 6.6 consists of one sentence, which is "An
implementation may accept other forms of constant expressions."

So if your implementation happens to define the value of an object
defined with the const qualifier and an initializer, in scope, as a
constant expression, apparently the standard allows it to.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html
Apr 20 '06 #13
Le 19-04-2006, de**********@ya hoo.com <de**********@y ahoo.com> a écrit*:
Marc Boyer wrote:
Le 19-04-2006, de**********@ya hoo.com <de**********@y ahoo.com> a écrit :
> Thanks Marc, point noted.
>
> Now getting back to my example,
>
> const int a = 10;
>
> In this case, can value of 'a' change?


Not to my knowledge.
> If so how?
> If not, why can not variable 'a' be treated a compile time constant.


It 'can' in this case. But, just try to explain in wich case
it can and when it can not....

My bad. I did not include complete text.
Let me include enough context and repost.

~> cat test.c
const int a = 10;
int main () {
static int b = a;
return b;
}

If the compiler can treat this 'a' as a compile time constant then this
should also compile.


It depend what you mean by 'the compiler can'. I think that, if
someone is able to write a C compiler, he would also be able to
write a augmented C compiler that, *in this special case*,
see that a is constant at compile time.

You should notice also that this is a valid C++ code,
but C++ does not require that static variable are initialized
by constant.

But in C, there is a standard that defines what must be accepted
by a C compiler. In this case, it may be detected because the value
of your const variable can be known at compile time By, in this case,
it should also accept
int main(){
int a=1;
static int b=a;
a--;
return a;
}

In this case, a is not any more a constant, but b is always
initialized with value 1.

And what about
const int a= random()?1:1;
or
const int a= random()?1:10/10;

What I mean is that, in the general case, it is hard (perhaps
undecidable in fact) to know when a const variable is a compile time
constant, and whan is is not. But, what is sure is that a constant
expression is a compile time constant. Then, the people who wrote
the standard have chosen to allow only initialisation of static
variable with constant expression.

This choice has the benefit of simplicity.

Marc Boyer
Apr 20 '06 #14

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