[ x-posting removed ]
peter.kourzanov@xxxxxxxxx wrote:
> On Wed, Apr 09, 2008 at 01:22:15PM +0100, Andrew Haley wrote:
>> peter.kourzanov@xxxxxxxxx wrote:
>>> Dear gcc users and developers,
>>>
>>> This might be a stupid question, nevertheless...
>>>
>>> I've been wondering for a long time, why the behaviour of
>>> variable-length arrays w.r.t. the sizeof operator is different
>>> for local/auto variables and for function arguments (in C99):
>>>
>>> #include <stdio.h>
>>> void foo(int s, int a[s]) {
>>> printf("%u\n",sizeof a);
>>> }
>>> int main()
>>> {
>>> int s=10,a[s];
>>> printf("%u\n",sizeof a);
>>> foo(sizeof a/sizeof a[0],a);
>>> }
>>>
>>> The printf's produce very different results: the first one
>>> returns "40" the other one returns 4, implying that the compiler
>>> forgets that the size of the array is actually "s", not the size
>>> of the pointer argument. Is it so difficult to make "sizeof a"
>>> return "s" in both cases?
>> That's C for you, I'm afraid: arrays always decay to pointers to the
>> first element when passed as arguments. The size of a VLA is not passed.
>
>
> Well, it sort of is: it is actually passed as a parameter to the
> function - look at the C99 defining a syntax for VLA parameters.
Well, OK, you may either do
int foo(int n, int p[])
or
int foo(int n, int p[])
but sizeof doesn't distinguish these two cases. In any case, how would
you do it? Would an assignment to n change the result of sizeof, or not?
If not, you'd have to create a "shadow" variable.
> The fact the the compiler forgets about it could be compensated by
> this...
>
> What I would like to know, is why the standard (ISO/IEC 9899) does
> explicitly require sizeof to return size of the pointer rather than
> more obvious response? Is it backwards compatibility?
> Is there any use in this behaviour besides making the life of
> the compiler development team easy?
The rationale doesn't say one way or another.
> Surely, such an extension should be not too difficult to implement,
> provided that the compiler can account for (potential) modifications
> to the size parameter and require sizeof the return the value that
> was actually passed to the function, which BTW, it already does,
> since this behaviour is easily verified to work well for variable
> VLAs.
But that would break compliant code. Why would you want to do that?
The fact that arrays don't have a size beyond the scope of their
declaration is an old wart in the design of C that everyone is used
to living with.
Andrew.
