If in C one codes a function which includes
return a++;
does 'a' get incremented? Would the behaviour be safe to rely upon? I
guess this might apply if 'a' were static or such as
return ptr->element++;
I didn't find this in the comp.lang.c faq (at least the first one that
came up from Google; there seemed to be many) but didn't find it. No
doubt someone will tell me where I should have looked.
(For the time being I'm coding: a++; return a - 1; )
--
James
51  3959 
James Harris wrote:
>
If in C one codes a function which includes
return a++;
does 'a' get incremented?
Yes.
Would the behaviour be safe to rely upon?
Yes.
I guess this might apply if 'a' were static
Yes.
--
pete
Richard Harter wrote:
I believe you're missing his point. Ada has a very thorough
validation suite and stringent restrictions on what may be called
an Ada compiler. This isn't the case with C compilers; anybody
can produce what they call a C compiler. It is even possible
that there is no such thing as a fully conforming C compiler.
His point is that in the real world this is the kind of thing
that you should check whether the compiler gets it right. Off
hand, I would think that it is the sort of thing that a compiler
could be expected to get right but I might well be wrong.
Do set of programs exist that attempt to validate a C implementation?
If so, are any of these freely available?
- Larry
In article <13************ *@corp.supernew s.com>, Larry__Weiss
<lf*@airmail.ne twrites
>Richard Harter wrote:
>I believe you're missing his point. Ada has a very thorough
validation suite and stringent restrictions on what may be called
an Ada compiler. This isn't the case with C compilers; anybody
can produce what they call a C compiler. It is even possible
that there is no such thing as a fully conforming C compiler.
His point is that in the real world this is the kind of thing
that you should check whether the compiler gets it right. Off
hand, I would think that it is the sort of thing that a compiler
could be expected to get right but I might well be wrong.
Do set of programs exist that attempt to validate a C implementation?
Yes
>If so, are any of these freely available?
No
--
\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
\/\/\/\/\ Chris Hills Staffs England /\/\/\/\/
/\/\/ ch***@phaedsys. org www.phaedsys.org \/\/\
\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/
In article <47***********@ mindspring.com> , pete <pf*****@mindsp ring.com>
writes
>James Harris wrote:
>>
If in C one codes a function which includes
return a++;
does 'a' get incremented?
Yes.
>Would the behaviour be safe to rely upon?
Yes.
>I guess this might apply if 'a' were static
Yes.
Oh dear.......
--
\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
\/\/\/\/\ Chris Hills Staffs England /\/\/\/\/
/\/\/ ch***@phaedsys. org www.phaedsys.org \/\/\
\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/
On Tue, 30 Oct 2007 15:34:32 GMT, in comp.lang.c , cr*@tiac.net
(Richard Harter) wrote:
>Ada has a very thorough
validation suite and stringent restrictions on what may be called
an Ada compiler. This isn't the case with C compilers; anybody
can produce what they call a C compiler.
But this is only true to the same extent that anyone can produce
something they call an Ada compiler. For instance AFAIK its not
actually illegal in the UK or US to call your firstborn child "An Ada
Compiler".
>It is even possible
that there is no such thing as a fully conforming C compiler.
*shrug*. And possibly there are no general purpose multiplatform Ada
compilers either.
I think your point is moot since Ada and C have different uses and
scopes. Any discussion is likely to be as sensible as debating whether
fish are more useful than footballs. The answer is "it depends".
--
Mark McIntyre
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan
Kenneth:
However, if "a" were a non-static non-volatile automatic variable,
can the compiler skip the increment? I believe so, because it will
go out of scope upon executing the return, and the result is "as
if" the increment were actually done.
If the local variable were neither volatile nor static, then the
increment is utterly redundant... and I wouldn't mind the compiler
issuing a warning something like:
WARNING: Redundant operation on local automatic variable
Only two reasons come to mind as to why you'd have a redundant
operation on a local automatic variable:
1) You intended to define it as either static or volatile.
2) You're stupid.
Martin
Chris Hills <ch***@phaedsys .orgwrites:
In article <47************ ****@news.sbtc. net>, Richard Harter
<cr*@tiac.netwr ites
>>On Tue, 30 Oct 2007 02:03:23 -0700, Keith Thompson
<ks***@mib.or gwrote:
>>>Chris Hills <ch***@phaedsys .orgwrites:
[...]
>>>Regardless of the standard(s) I think this is one you would need to
empiricall y test of the compiler(s) in question.
If this were Ada one could just refer to the standard but it's C and
nothing is guaranteed.
Huh?
The C standard defines how the return statement and the "++" operator
work. If ``return a++;'' doesn't increment ``a'' (other than in cases
covered by the as-if rule), then the compiler is broken.
The C and Ada standards are both written in English, and in fairly
similar styles. C leaves more things undefined than Ada does, but
it's reasonably clear about *what* it leaves undefined.
I believe you're missing his point. Ada has a very thorough
validation suite and stringent restrictions on what may be called
an Ada compiler. This isn't the case with C compilers; anybody
can produce what they call a C compiler. It is even possible
that there is no such thing as a fully conforming C compiler.
His point is that in the real world this is the kind of thing
that you should check whether the compiler gets it right.
Precisely
>Off
hand, I would think that it is the sort of thing that a compiler
could be expected to get right but I might well be wrong.
In other words you wouldn't bet your life on it....
All software (including compilers) has bugs. All software should be
thoroughly tested. But a compiler bug in the handling of ``return
a++;'' is quite low on the list of things I'd worry about -- higher
than ``1 + 1 == 2'', lower than, say, some obscure C99 feature, and
about the same as ``return (a = 42);''.
Consider how the return statement is defined: the expression is
evaluated, and the result is returned. Updating ``a'' is part of the
evaluation of the expression.
Is there some particular reason you're concerned that a compiler might
handle something this simple incorrectly?
--
Keith Thompson (The_Other_Keit h) ks***@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"
Chris Hills wrote:
Larry__Weiss <lf*@airmail.ne twrites
.... snip ...
>
>Do set of programs exist that attempt to validate a C
implementation ?
Yes
>If so, are any of these freely available?
No
Yes. The gcc test suite is freely available. However it is geared
to "gcc C" rather than ISO std C.
--
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
"Martin Wells" <wa****@eircom. neta écrit dans le message de news: 11************* *********@i13g2 00...legr oups.com...
Kenneth:
>However, if "a" were a non-static non-volatile automatic variable,
can the compiler skip the increment? I believe so, because it will
go out of scope upon executing the return, and the result is "as
if" the increment were actually done.
If the local variable were neither volatile nor static, then the
increment is utterly redundant... and I wouldn't mind the compiler
issuing a warning something like:
WARNING: Redundant operation on local automatic variable
Only two reasons come to mind as to why you'd have a redundant
operation on a local automatic variable:
1) You intended to define it as either static or volatile.
2) You're stupid.
Or you could be using macros with side effects:
#define GETBYTE(p) (*(p)++ & 0xFF)
int safegetbyte(con st char *s) {
if (!s)
return -1;
return GETBYTE(s);
}
--
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