variable length array (was: question about dynamic memory allocation)

[Fernando Barsoba]

Hi,

I decided to start a new topic about my memory allocation question. One
of the answers was that variable length arrays are possible using C99. I
found this topic that also mentions it.

http://groups.google.com/group/comp.lang.c/browse_frm/thread/f7285cf1ffb172c3/f5335d22d6e6fb34?q=variable+length+array&rnum=2#f5335d22d6e6fb34

I did some test, but I get strange results:

int size_array = sizeof(struct ip) + cnf->msg_length;
unsigned char packet[size_array];

And then, when asking the size of the 'packet' variable I get 4. How is
that? It should be another number (31).. but I guess i'm getting the
size of type unsigned char. Right?


Thanks,

FBM

[Skarmander]

Re: variable length array (was: question about dynamic memory allocation)

Fernando Barsoba wrote:[color=blue]
> I decided to start a new topic about my memory allocation question. One
> of the answers was that variable length arrays are possible using C99. I
> found this topic that also mentions it.
>
> http://groups.google.com/group/comp....335d22d6e6fb34
>
>
> I did some test, but I get strange results:
>
> int size_array = sizeof(struct ip) + cnf->msg_length;
> unsigned char packet[size_array];
>
> And then, when asking the size of the 'packet' variable I get 4. How is
> that? It should be another number (31).. but I guess i'm getting the
> size of type unsigned char. Right?
>[/color]
Technically there's no way to tell, because we don't know what system
you're compiling this on.

That said, you're probably getting the size of an unsigned char*, to
which `packet' decays.

If sizeof packet != size_array after the declarations above, your
compiler does not conform to the C99 standard. gcc in particular does
not support VLAs; it supports an earlier gcc extension that is similar
but not identical to C99 VLAs.

S.

[Artie Gold]

Re: variable length array (was: question about dynamic memory allocation)

Fernando Barsoba wrote:[color=blue]
> Hi,
>
> I decided to start a new topic about my memory allocation question. One
> of the answers was that variable length arrays are possible using C99. I
> found this topic that also mentions it.
>
> http://groups.google.com/group/comp....335d22d6e6fb34
>
>
> I did some test, but I get strange results:
>
> int size_array = sizeof(struct ip) + cnf->msg_length;
> unsigned char packet[size_array];
>
> And then, when asking the size of the 'packet' variable I get 4. How is
> that? It should be another number (31).. but I guess i'm getting the
> size of type unsigned char. Right?
>[/color]

Show us the code you used to test.
[You passed `packet' to a function, didn't you...]

HTH,
--ag

--
Artie Gold -- Austin, Texas
http://goldsays.blogspot.com (new post 8/5)
http://www.cafepress.com/goldsays
"If you have nothing to hide, you're not trying!"

[Fernando Barsoba]

Re: variable length array (was: question about dynamic memory allocation)

Artie Gold wrote:[color=blue]
> Fernando Barsoba wrote:[color=green]
>> Hi,
>>
>> I decided to start a new topic about my memory allocation question. One
>> of the answers was that variable length arrays are possible using C99. I
>> found this topic that also mentions it.
>>
>> http://groups.google.com/group/comp....335d22d6e6fb34
>>
>>
>> I did some test, but I get strange results:
>>
>> int size_array = sizeof(struct ip) + cnf->msg_length;
>> unsigned char packet[size_array];
>>
>> And then, when asking the size of the 'packet' variable I get 4. How is
>> that? It should be another number (31).. but I guess i'm getting the
>> size of type unsigned char. Right?
>>[/color]
>
> Show us the code you used to test.
> [You passed `packet' to a function, didn't you...][/color]

I pass 'packet' to a function later on.. but not for this test. I
checked the size of 'packet' after[color=blue][color=green]
>>unsigned char packet[size_array];[/color][/color]
and there's where I get the value 4.

Regarding to the other question, I'm using Eclipse + CDT (and the
compiler is the one that comes with Cygwin which I am using too).

FBM

[Keith Thompson]

Re: variable length array (was: question about dynamic memoryallocatio n)

Fernando Barsoba <fbarsoba@veriz on.net> writes:
[snip][color=blue]
> I did some test, but I get strange results:
>
> int size_array = sizeof(struct ip) + cnf->msg_length;
> unsigned char packet[size_array];
>
> And then, when asking the size of the 'packet' variable I get 4. How is
> that? It should be another number (31).. but I guess i'm getting the
> size of type unsigned char. Right?[/color]

No, sizeof(unsigned char) is 1 by definition. It looks like it could
be sizeof(unsigned char*), but that's not

How exactly did you ask the size of the 'packet' variable? Show us
some code.

Here's an example that works properly with gcc 2.95.2, 3.4.4, and
4.0.2:

#include <stdio.h>
int main(void)
{
int len = 42;
char vla[len];
printf("sizeof vla = %d\n", (int)sizeof vla);
return 0;
}

The output is:

sizeof vla = 42

If you passed it as an argument to a function and applied sizeof to
the parameter, you'd get the size of the pointer, but that's because
array expressions decay to pointers in most contexts; if you apply
sizeof directly to the array object, it doesn't decay and you get the
size of the array.

--
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.

[Barry Schwarz]

Re: variable length array (was: question about dynamic memory allocation)

On Thu, 10 Nov 2005 00:31:56 GMT, Fernando Barsoba
<fbarsoba@veriz on.net> wrote:
[color=blue]
>Hi,
>
>I decided to start a new topic about my memory allocation question. One
>of the answers was that variable length arrays are possible using C99. I
>found this topic that also mentions it.
>
>http://groups.google.com/group/comp....335d22d6e6fb34
>
>I did some test, but I get strange results:
>
> int size_array = sizeof(struct ip) + cnf->msg_length;
> unsigned char packet[size_array];
>
>And then, when asking the size of the 'packet' variable I get 4. How is
>that? It should be another number (31).. but I guess i'm getting the
>size of type unsigned char. Right?[/color]

Not likely. An unsigned char is guaranteed to have a size of one.


<<Remove the del for email>>

[Skarmander]

Re: variable length array (was: question about dynamic memory allocation)

Keith Thompson wrote:[color=blue]
> Fernando Barsoba <fbarsoba@veriz on.net> writes:
> [snip]
>[color=green]
>>I did some test, but I get strange results:
>>
>> int size_array = sizeof(struct ip) + cnf->msg_length;
>> unsigned char packet[size_array];
>>
>>And then, when asking the size of the 'packet' variable I get 4. How is
>>that? It should be another number (31).. but I guess i'm getting the
>>size of type unsigned char. Right?[/color]
>
>
> No, sizeof(unsigned char) is 1 by definition. It looks like it could
> be sizeof(unsigned char*), but that's not
>
> How exactly did you ask the size of the 'packet' variable? Show us
> some code.
>
> Here's an example that works properly with gcc 2.95.2, 3.4.4, and
> 4.0.2:
>[/color]
<snip>
And there's me badmouthing gcc about not supporting VLAs yet. At least
this part works. :-)

S.

[Keith Thompson]

Re: variable length array (was: question about dynamic memoryallocatio n)

Fernando Barsoba <fbarsoba@veriz on.net> writes:[color=blue]
> Artie Gold wrote:[color=green]
>> Fernando Barsoba wrote:[/color][/color]
[snip][color=blue][color=green][color=darkred]
>>> I did some test, but I get strange results:
>>>
>>> int size_array = sizeof(struct ip) + cnf->msg_length;
>>> unsigned char packet[size_array];
>>>
>>> And then, when asking the size of the 'packet' variable I get 4. How is
>>> that? It should be another number (31).. but I guess i'm getting
>>> the size of type unsigned char. Right?
>>>[/color]
>> Show us the code you used to test.
>> [You passed `packet' to a function, didn't you...][/color]
>
> I pass 'packet' to a function later on.. but not for this test. I
> checked the size of 'packet' after[color=green][color=darkred]
> >>unsigned char packet[size_array];[/color][/color]
> and there's where I get the value 4.[/color]

That shouldn't be happening.

Show us the code you used to test. That means posting a complete
self-contained program that we can compile and run ourselves, along
with the exact output that it gave you. Without that, we really can't
help.
[color=blue]
> Regarding to the other question, I'm using Eclipse + CDT (and the
> compiler is the one that comes with Cygwin which I am using too).[/color]

Cygwin comes with gcc (version 3.4.4 if you've updated it recently).

--
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.

[Fernando Barsoba]

Re: variable length array (was: question about dynamic memory allocation)

Keith Thompson wrote:[color=blue]
> Fernando Barsoba <fbarsoba@veriz on.net> writes:[color=green]
>> Artie Gold wrote:[color=darkred]
>>> Fernando Barsoba wrote:[/color][/color]
> [snip][color=green][color=darkred]
>>>>
>>>>
>>> Show us the code you used to test.
>>> [You passed `packet' to a function, didn't you...][/color]
>> I pass 'packet' to a function later on.. but not for this test. I
>> checked the size of 'packet' after[color=darkred]
>> >>unsigned char packet[size_array];[/color]
>> and there's where I get the value 4.[/color]
>
> That shouldn't be happening.
>
> Show us the code you used to test. That means posting a complete
> self-contained program that we can compile and run ourselves, along
> with the exact output that it gave you. Without that, we really can't
> help.[/color]

Ok, I tested the following code:

#include <stdio.h>
#include <stdlib.h>
#include <netinet/ip.h>


int main(int argc, char **argv) {

int size_array = sizeof(struct ip) + 11;
unsigned char packet[size_array];
// unsigned char ipheader_cpy[sizeof(struct ip)];

printf("Result for sizeof(packet) is: %d", sizeof(packet)) ;
exit(0);

}

And I got as a result:
$ ./Test.exe
Result for sizeof(packet) is: 31

So, I think I have to assume that it's working correctly. The problem is
that I'm using Eclipse/CDT and the debugger seems to have problems with
this kind of code (with VL Arrays). It showed me the '4' that I was
referring to, in the debugging perspective/window from Eclipse... I
thought the problem was with the code, but all you made me focus on the
test.. I guess I relied too much on the debugger...sigh ..

Thanks,

FBM
[color=blue][color=green]
>> Regarding to the other question, I'm using Eclipse + CDT (and the
>> compiler is the one that comes with Cygwin which I am using too).[/color]
>
> Cygwin comes with gcc (version 3.4.4 if you've updated it recently).
>[/color]