tryick use of sizeof again[Explaination Wanted]

**Peter Nilsson** · Sep 16 '07, 12:35 AM

Re: tryick use of sizeof again[Explaination Wanted]

Harald van D k <true...@gmail. comwrote:

Ian Collins wrote:

Harald van D k wrote:

Keith Thompson wrote:
Can anyone think of a case where the evaluation of the
argument of sizeof (when it's a VLA) has a visible
effect on the behavior of the program?
>
#include <stdio.h>
int f(void) {
puts("f is called!");
return 0;
}
int main(void) {
enum { constant_size = 1 };
const int nonconstant_siz e = 1;
>
char nonvla[constant_size];
char vla[nonconstant_siz e];
>
printf("sizeof (&nonvla)[f()] = %zu\n",
sizeof (&nonvla)[f()]);
printf("sizeof (&vla)[f()] = %zu\n",
sizeof (&vla)[f()]);
>
return 0;
}
>
This should print:
>
sizeof (&nonvla)[f()] = 1
f is called!
sizeof (&vla)[f()] = 1

Should it?

>
This is the behaviour mandated by the standard and
implemented by the compiler I'm using. If you believe
the standard would have been better if sizeof on VLAs
were specified differently, then I don't necessarily
disagree, but if you believe the standard requires (or
allows) any different output, then could you please
explain why?

6.5.3.4p2: "... If the type of the operand is a variable
length array type, the operand is evaluated; otherwise,
the operand is not evaluated and the result is an
integer constant."

Since you don't apply sizeof to an operand of variable
length array type, I don't see how or why f should be
called.

I believe the evaluation clause in 6.5.3.4p2 is more for
cases like...

sizeof (int [nonconstexpr])

Since pointer arithmetic is required to work on VLAs,
such pointers (including those created by using the name
of a VLA) necessarily carry information about the VLAs
size, either directly or indirectly.

Consider...

#include <stdio.h>
#include <stdlib.h>

int foo()
{
return 1 + rand() % 10;
}

size_t bah(char (*vlap)[*])
{
return sizeof *vlap;
}

int main()
{
int vla[foo()];
printf("%zu\n", bah(&vla));
return 0;
}

The function foo() is only called once. A compiler that
calls foo() twice would be broken.

--
Peter

**Richard Tobin** · Sep 16 '07, 12:55 AM

Re: tryick use of sizeof again[Explaination Wanted]

In article <lnd4wjzoxk.fsf @nuthaus.mib.or g>,
Keith Thompson <kst-u@mib.orgwrote:

>Can anyone think of a case where the evaluation of the argument of
>sizeof (when it's a VLA) has a visible effect on the behavior of the
>program?

Gcc with flags "-std=c99 -pedantic -Wall' thinks the following program
is legal:

#include <stdio.h>

int main(void)
{
int n = 42;
printf("%d\n", (int)sizeof(cha r [n++]));
printf("%d\n", (int)sizeof(cha r [n++]));
return 0;
}

and it prints:

42
43

-- Richard
--
"Considerat ion shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

**Keith Thompson** · Sep 16 '07, 07:45 AM

Re: tryick use of sizeof again[Explaination Wanted]

Harald van D©¦k <truedfx@gmail. comwrites:

On Sat, 15 Sep 2007 14:35:46 -0700, Keith Thompson wrote:

>"Harald van D)&k" <truedfx@gmail. comwrites:

>>On Sat, 15 Sep 2007 12:52:39 -0700, Keith Thompson wrote:

>[...]

>>>Can anyone think of a case where the evaluation of the argument of
>>>sizeof (when it's a VLA) has a visible effect on the behavior of the
>>>program?
>>>
>>#include <stdio.h>
>>int f(void) {
>> puts("f is called!");
>> return 0;
>>}
>>int main(void) {
>> enum { constant_size = 1 };
>> const int nonconstant_siz e = 1;
>>>
>> char nonvla[constant_size];
>> char vla[nonconstant_siz e];
>>>
>> printf("sizeof (&nonvla)[f()] = %zu\n", sizeof (&nonvla)[f()]);
>> printf("sizeof (&vla)[f()] = %zu\n", sizeof (&vla)[f()]);
>>>
>> return 0;
>>}
>>>

>The type of the operand in both of your sizeof expressions is char, not
>a VLA type.

>
I think you've overlooked the &. The operands to both sizeof operators
have array types.

You're right (and gcc gets this wrong).

--
Keith Thompson (The_Other_Keit h) kst-u@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."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**Flash Gordon** · Sep 16 '07, 09:05 AM

Re: tryick use of sizeof again[Explaination Wanted]

Harald van DÄ³k wrote, On 16/09/07 00:52:

On Sat, 15 Sep 2007 23:13:33 +0100, Flash Gordon wrote:

>Harald van DÄ³k wrote, On 15/09/07 22:13:

>>On Sat, 15 Sep 2007 21:05:53 +0100, Flash Gordon wrote:
>>>Keith Thompson wrote, On 15/09/07 20:52:
>>>>
>>><snip>
>>>>
>>>>Can anyone think of a case where the evaluation of the argument of
>>>>sizeof (when it's a VLA) has a visible effect on the behavior of the
>>>>program?
>>>size_t size=5
>>>int array[++size];
>>No, sizeof(array) will always return 6 in this case. The array size is
>>evaluated when the declaration is reached, but not re-evaluated
>>afterwards.

>I meant to say
>sizeof array[++size];
>Obviously I my brain was disconnected at the time.

>
array[++size] is an expression of type int, so ++size will not be
evaluated.

I've definitely been working too many hours this week. Normal service
will be restored once I've taken my brain out and washed all the rubbish
out of it.
--
Flash Gordon

**Charlie Gordon** · Sep 16 '07, 10:35 AM

Re: tryick use of sizeof again[Explaination Wanted]

"Richard Tobin" <richard@cogsci .ed.ac.uka écrit dans le message de news:
fchthe$23po$1@p c-news.cogsci.ed. ac.uk...

In article <lnd4wjzoxk.fsf @nuthaus.mib.or g>,
Keith Thompson <kst-u@mib.orgwrote:
>

>>Can anyone think of a case where the evaluation of the argument of
>>sizeof (when it's a VLA) has a visible effect on the behavior of the
>>program?

>
Gcc with flags "-std=c99 -pedantic -Wall' thinks the following program
is legal:
>
#include <stdio.h>
>
int main(void)
{
int n = 42;
printf("%d\n", (int)sizeof(cha r [n++]));
printf("%d\n", (int)sizeof(cha r [n++]));
return 0;
}
>
and it prints:
>
42
43

While it seems logic to do so, the language of the standard is not clear:

6.5.3.4p2: "... If the type of the operand is a variable
length array type, the operand is evaluated; otherwise,
the operand is not evaluated and the result is an
integer constant."

What if the type of char[n++] ?
How can we evaluate char[n++] ?

char [n++] *is* a type, it does not *have* a type.

I don't think the language of the standard correctly specifies what should
be the behaviour of such code.

A more compelling example would be

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char** argv)
{
if (argc 1) {
int n = strtol(argv[1], NULL, 0);
int matrix[n][n];
int i = 1;

printf("i=%d, n=%d\n", i, n);
printf ("sizeof(matrix ) = %zd\n", sizeof(matrix)) ;
printf ("sizeof(mat rix[i++]) = %zd\n", sizeof(matrix[i++]));
printf("i=%d\n" , i);

printf ("sizeof(cha r[i++]) = %zd\n", sizeof(char[i++]));
printf("i=%d\n" , i);
}
return 0;
}

with gcc upto version 4.1.3, I get what I interpret as non conforming
behaviour:

$ ./vla 2
i=1, n=2
sizeof(matrix) = 16
sizeof(matrix[i++]) = 8
i=1
sizeof(char[i++]) = 1
i=2

I think we have a defect here: the argument to sizeof should be evaluated is
it *is* a variable length array type, not if it is an expression whois type
can be determined from static analysis.

--
Chqrlie.

**Richard Bos** · Sep 17 '07, 06:45 AM

Re: tryick use of sizeof again[Explaination Wanted]

CBFalconer <cbfalconer@yah oo.comwrote:

"christian. bau" wrote:

"Charlie Gordon" <n...@chqrlie.o rgwrote:

Why be unnecessarily obtuse when the issue is not C++ specific.

How do you know it is not C++ specific? For example, after

char c - '0';
c += sizeof ('0');

what is the value stored in c? There are subtle differences
between the sizeof operator in C and C++, and someone knowing
absolutely everything about C but nothing about C++ would most
likely not know the right answer to the problem I wrote.

>
Why did you snip away the expression that triggered the query?

Because it's immaterial to his point, which is that _all_ such
expressions _may_ be different in C++, and only comp.lang.c++ can tell
you for certain.

I.e:
>

> int v = sizeof ++ch;

>
which has the same answer in C and C++ (I think),

Precisely. _You think_. comp.lang.c++ would _know_.

Richard

**Keith Thompson** · Sep 17 '07, 07:55 AM

Re: tryick use of sizeof again[Explaination Wanted]

rlb@hoekstra-uitgeverij.nl (Richard Bos) writes:

CBFalconer <cbfalconer@yah oo.comwrote:

[...]

>I.e:
>>

>> int v = sizeof ++ch;

>>
>which has the same answer in C and C++ (I think),

>
Precisely. _You think_. comp.lang.c++ would _know_.

You're assuming that the folks in comp.lang.c++ know enough about C to
be sure about such things. (That may be an accurate assumption.)

--
Keith Thompson (The_Other_Keit h) kst-u@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."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**CBFalconer** · Sep 17 '07, 09:55 AM

Re: tryick use of sizeof again[Explaination Wanted]

Richard Bos wrote:

CBFalconer <cbfalconer@yah oo.comwrote:
>

.... snip ...

>>

>> int v = sizeof ++ch;

>>
>which has the same answer in C and C++ (I think),

>
Precisely. _You think_. comp.lang.c++ would _know_.

No, they wouldn't. In general their knowledge of C is probably
similar to my knowledge of C++ - i.e. fair, but not comprehensive.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

**Flash Gordon** · Sep 17 '07, 07:05 PM

Re: tryick use of sizeof again[Explaination Wanted]

CBFalconer wrote, On 17/09/07 08:29:

Richard Bos wrote:

>CBFalconer <cbfalconer@yah oo.comwrote:
>>

... snip ...

>>>>> int v = sizeof ++ch;
>>which has the same answer in C and C++ (I think),

>Precisely. _You think_. comp.lang.c++ would _know_.

>
No, they wouldn't. In general their knowledge of C is probably
similar to my knowledge of C++ - i.e. fair, but not comprehensive.

I think the point is they would _know_ the behaviour in C++. After all,
the original code _was_ C++ because it used C++ specific features on
another line.
--
Flash Gordon

**Kelsey Bjarnason** · Sep 17 '07, 08:05 PM

Re: tryick use of sizeof again[Explaination Wanted]

[snips]

On Sat, 15 Sep 2007 00:31:53 +0200, Charlie Gordon wrote:

Yeah right sizeof('a') == 1 in C++ and sizeof('a') == sizeof(int) in C (how
stupid and counter-intuitive!)

There were legitimate reasons for this, however.

someone knowing absolutely everything about C but nothing about C++ would
fit my definition of obtuse perfectly.

Actually, that'd fit my definition of "C programmer who hasn't learned C++."

**CBFalconer** · Sep 17 '07, 11:05 PM

Re: tryick use of sizeof again[Explaination Wanted]

Kelsey Bjarnason wrote:

On Sat, 15 Sep 2007 00:31:53 +0200, Charlie Gordon wrote:
>
[snips]
>

>Yeah right sizeof('a') == 1 in C++ and sizeof('a') == sizeof(int)
>in C (how stupid and counter-intuitive!)

>
There were legitimate reasons for this, however.
>

>someone knowing absolutely everything about C but nothing about
>C++ would fit my definition of obtuse perfectly.

>
Actually, that'd fit my definition of "C programmer who hasn't
learned C++."

LOL. Accurate.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

**CBFalconer** · Sep 17 '07, 11:05 PM

Re: tryick use of sizeof again[Explaination Wanted]

Flash Gordon wrote:

CBFalconer wrote, On 17/09/07 08:29:

>Richard Bos wrote:

>>CBFalconer <cbfalconer@yah oo.comwrote:
>>>

>... snip ...
>>

>>>>>> int v = sizeof ++ch;
>>>>
>>>which has the same answer in C and C++ (I think),
>>>
>>Precisely. _You think_. comp.lang.c++ would _know_.

>>
>No, they wouldn't. In general their knowledge of C is probably
>similar to my knowledge of C++ - i.e. fair, but not comprehensive.

>
I think the point is they would _know_ the behaviour in C++. After
all, the original code _was_ C++ because it used C++ specific
features on another line.

Which (I think) is just what I said. An answer requires knowledge
of both systems.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

**Kelsey Bjarnason** · Sep 17 '07, 11:45 PM

Re: tryick use of sizeof again[Explaination Wanted]

On Mon, 17 Sep 2007 17:45:22 -0400, CBFalconer wrote:

Kelsey Bjarnason wrote:

>On Sat, 15 Sep 2007 00:31:53 +0200, Charlie Gordon wrote:
>>
>[snips]
>>

>>Yeah right sizeof('a') == 1 in C++ and sizeof('a') == sizeof(int)
>>in C (how stupid and counter-intuitive!)

>>
>There were legitimate reasons for this, however.
>>

>>someone knowing absolutely everything about C but nothing about
>>C++ would fit my definition of obtuse perfectly.

>>
>Actually, that'd fit my definition of "C programmer who hasn't
>learned C++."

>
LOL. Accurate.

He do seem to think that learning C magically imparts a deep knowledge of
C++, don't he?

Weird stuff.

**Richard Bos** · Sep 18 '07, 06:15 AM

Re: tryick use of sizeof again[Explaination Wanted]

CBFalconer <cbfalconer@yah oo.comwrote:

Flash Gordon wrote:

CBFalconer wrote, On 17/09/07 08:29:

Richard Bos wrote:
>CBFalconer <cbfalconer@yah oo.comwrote:
>>
... snip ...
>
>>>>> int v = sizeof ++ch;
>>>
>>which has the same answer in C and C++ (I think),
>>
>Precisely. _You think_. comp.lang.c++ would _know_.
>
No, they wouldn't. In general their knowledge of C is probably
similar to my knowledge of C++ - i.e. fair, but not comprehensive.

I think the point is they would _know_ the behaviour in C++. After
all, the original code _was_ C++ because it used C++ specific
features on another line.

>
Which (I think) is just what I said. An answer requires knowledge
of both systems.

No, it requires knowledge of C++. It was a question about the behaviour
of the posted code, which was C++ code; it was not a question about the
potential difference in behaviour between that code and similar C code.

Richard

**Flash Gordon** · Sep 18 '07, 08:05 AM

Re: tryick use of sizeof again[Explaination Wanted]

Richard Bos wrote, On 18/09/07 07:13:

CBFalconer <cbfalconer@yah oo.comwrote:
>

>Flash Gordon wrote:

>>CBFalconer wrote, On 17/09/07 08:29:
>>>Richard Bos wrote:
>>>>CBFalcone r <cbfalconer@yah oo.comwrote:
>>>>>
>>>... snip ...
>>>>
>>>>>>>> int v = sizeof ++ch;
>>>>>which has the same answer in C and C++ (I think),
>>>>Precisely . _You think_. comp.lang.c++ would _know_.
>>>No, they wouldn't. In general their knowledge of C is probably
>>>similar to my knowledge of C++ - i.e. fair, but not comprehensive.
>>I think the point is they would _know_ the behaviour in C++. After
>>all, the original code _was_ C++ because it used C++ specific
>>features on another line.

>Which (I think) is just what I said. An answer requires knowledge
>of both systems.

>
No, it requires knowledge of C++. It was a question about the behaviour
of the posted code, which was C++ code; it was not a question about the
potential difference in behaviour between that code and similar C code.

Yes, I meant what Richard has just posted.
--
Flash Gordon

tryick use of sizeof again[Explaination Wanted]

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