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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: sizeof on an uninitialized pointer to a VLA |
| Date | 2015-11-20 21:08 -0800 |
| Organization | None to speak of |
| Message-ID | <lnoaeo7xe1.fsf@kst-u.example.com> (permalink) |
| References | (3 earlier) <87mvu8rbor.fsf@bsb.me.uk> <lnsi40axv9.fsf@kst-u.example.com> <dba734FjpqnU6@mid.individual.net> <lnd1v49dz5.fsf@kst-u.example.com> <dbaappFjpqnU7@mid.individual.net> |
Ian Collins <ian-news@hotmail.com> writes:
> Keith Thompson wrote:
>> Ian Collins <ian-news@hotmail.com> writes:
>>> Keith Thompson wrote:
>> [...]
>>>> Interesting point. Here's an example:
>>>>
>>>> #include <stdio.h>
>>>>
>>>> int zero(void) {
>>>> puts("zero()");
>>>> return 0;
>>>> }
>>>>
>>>> int main(void) {
>>>> int n = 10;
>>>> char vla[n][n];
>>>> printf("sizeof vla[zero()] = %zu\n", sizeof vla[zero()]);
>>>> }
>>>>
>>>> The output using every version of gcc and clang I've tried is:
>>>>
>>>> zero()
>>>> sizeof vla[zero()] = 10
>>>>
>>>> The sizeof computation is correct, but it *could* have been determined
>>>> without calling the function. As far as I can tell, the value returned
>>>> by the call to zero() can have no effect on the result of the sizeof
>>>> computation.
>>>
>>> Is this a case of the observer effect?
>>>
>>> I tired you code with Sun c99 and the whether the puts() line was there
>>> or not, the immediate value 10 was pushed.
>>
>> I'm not sure what you mean by "pushed". Are you looking at the
>> generated code?
>
> Yes.
>
>> I tried the example with a rather old Sun C compiler. The output was:
>>
>> sizeof vla[zero()] = 10
>>
>> (Which IMHO *should* be correct, but the call to zero() generated by gcc
>> and clang follows what the standard says.)
>
> If wee look at the gcc output (with -01), with puts():
>
> main:
[snip]
> call zero
[snip]
>
> Without:
>
> main:
> leal 4(%esp), %ecx
> andl $-16, %esp
> pushl -4(%ecx)
> pushl %ebp
> movl %esp, %ebp
> pushl %ecx
> subl $12, %esp
> pushl $10
> pushl $.LC0
> call printf
> addl $16, %esp
> movl -4(%ebp), %ecx
> leave
> leal -4(%ecx), %esp
> ret
>
> So without puts(), zero() isn't called. Even with it, the return value
> isn't used.
Without puts(), the call to zero() is simply being implicitly inlined.
Logically, it's still being called; the call is just optimized to a
constant value of 0. The same thing happens for a call that's not part
of the operand of sizeof.
The side effect of the puts() call is needed to demonstrate that the
operand of sizeof is or is not being evaluated.
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
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sizeof on an uninitialized pointer to a VLA John Bode <jfbode1029@gmail.com> - 2015-11-19 11:39 -0800
Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 12:09 -0800
Re: sizeof on an uninitialized pointer to a VLA "A. Bolmarcich" <aggedor@earl-grey.cloud9.net> - 2015-11-19 21:40 +0000
Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 13:50 -0800
Re: sizeof on an uninitialized pointer to a VLA James Kuyper <jameskuyper@verizon.net> - 2015-11-19 17:50 -0500
Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 15:31 -0800
Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-20 01:09 +0000
Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-19 19:10 -0800
Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 03:52 -0800
Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-20 14:24 +0000
Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-20 10:25 -0800
Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 18:29 -0800
Re: sizeof on an uninitialized pointer to a VLA Ian Collins <ian-news@hotmail.com> - 2015-11-21 16:35 +1300
Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 20:24 -0800
Re: sizeof on an uninitialized pointer to a VLA Ian Collins <ian-news@hotmail.com> - 2015-11-21 17:38 +1300
Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-20 21:08 -0800
Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 10:46 +0000
Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-21 03:26 -0800
Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 12:46 +0000
Re: sizeof on an uninitialized pointer to a VLA Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 14:01 +0000
Re: sizeof on an uninitialized pointer to a VLA supercat@casperkitty.com - 2015-11-21 09:02 -0800
Re: sizeof on an uninitialized pointer to a VLA Keith Thompson <kst-u@mib.org> - 2015-11-21 12:09 -0800
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