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
Oct 29 '07
51  3968 
Chris Hills wrote:
[...]
Regardless of the standard(s) I think this is one you would need to
empirically 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.
Well, some C compilers has been validated: http://www.peren.com/pages/cvsa_set.htm http://www.plumhall.com/stec.html
and at this point, the C90 test cases should detect such an compiler
bug. However, in safety-critical SW, I wouldn't advocate using construct
like
return a++;
anyway.
--
Tor <bw****@wvtqvm. vw | tr i-za-h a-z>
On 30 Oct, 20:10, Keith Thompson <ks...@mib.orgw rote:
....
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?
In any context other than a return statement
mov eax, a
{use eax}
inc eax
So in a return statement
mov eax, a
ret
inc eax
which, of course, is wrong so the compiler should handle two special
cases:
1. return a++; where a is automatic
mov eax, a
ret
/* ignore the ++ */
2. return a++; where a persists
mov eax, a
mov ebx, eax
inc ebx
mov a, ebx
ret
In article <ln************ @nuthaus.mib.or gKeith Thompson <ks***@mib.orgw rites:
....
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);''.
You would be surprised at the bugs I have seen in compilers and
assemblers. It ranges from blatant errors in something akin to
include files (wrong values all over the place for floating point
attributes in the DG Ada compiler) to issuing the wrong opcode
for a particular inxtruction (a Gould assembler).
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/
Tor Rustad wrote:
Chris Hills wrote:
[...]
>Regardless of the standard(s) I think this is one you would need to
empirically 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.
Well, some C compilers has been validated:
http://www.peren.com/pages/cvsa_set.htm
http://www.plumhall.com/stec.html
and at this point, the C90 test cases should detect such an compiler
bug. However, in safety-critical SW, I wouldn't advocate using construct
like
return a++;
I'm sorry - but if you have a compiler you can't trust to handle 'return
a++;' correctly, why are you using it, even for software that isn't
safety critical (and especially for code that is)? If there's any
significant likelihood that it might mishandle something that simple, I
most certainly wouldn't want to write anything more complicated in it.
On Oct 31, 4:22 am, James Harris <james.harri... @googlemail.com >
wrote:
On 30 Oct, 20:10, Keith Thompson <ks...@mib.orgw rote:
...
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.
The above answers the query :):)
So, return statement has defined that the expression is evaluated and
the
result is returned. :):)
>
Is there some particular reason you're concerned that a compiler might
handle something this simple incorrectly?
In any context other than a return statement
mov eax, a
{use eax}
inc eax
So in a return statement
mov eax, a
ret
inc eax
which, of course, is wrong so the compiler should handle two special
cases:
1. return a++; where a is automatic
mov eax, a
ret
/* ignore the ++ */
2. return a++; where a persists
mov eax, a
mov ebx, eax
inc ebx
mov a, ebx
ret
Interesting :):)
Karthik Balaguru
[... Talking about "return a++;" ...]
Martin Wells wrote:
>
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
"Redundant" , or "useless"?
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.
3) You're wondering whether your compiler will optimize it away.
--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer .h|
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:Th***** ********@gmail. com>
Tor Rustad wrote On 10/30/07 19:06,:
[...]
However, in safety-critical SW, I wouldn't advocate using construct
like
return a++;
What's your opinion of `return getchar();'? Keep
in mind that in many implementations , getchar() is a
macro that expands to an expression with side-effects.
On an implementation I happen to have handy at the
moment, `return getchar();' produces (reformatted for
clarity)
return ( --(( &__iob[0]))->_cnt < 0
? __filbuf( (&__iob[0]) )
: (int) *(( &__iob[0]))->_ptr++ );
So, what's your verdict? Should `return getchar();'
be avoided in safety-critical software?
-- Er*********@sun .com
On Oct 30, 11:35 am, Chris Hills <ch...@phaedsys .orgwrote:
In article <47274b17.15255 6...@news.sbtc. net>, Richard Harter
<c...@tiac.netw rites
[...]
>
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....
In that case (betting my life), Ada has no advantage over C.
"Trust, but verify."
Regards,
-=Dave
Kenneth Brody wrote:
[in addition, my response applies to Kuyper and Sosman]
Tor Rustad wrote:
[...]
>However, in safety-critical SW, I wouldn't advocate using construct
like
return a++;
anyway.
Why not? If the compiler doesn't handle it right, then I wouldn't
trust it for the rest of the "safety-critical" program either.
First, I didn't agree with Chris Hills, because
1. I didn't find such a C compiler bug likely
2. If such a compiler bug existed, the unit test should detect it
My comment on not advocating returning a++, is more of a stylish matter,
since post conditions in functions, usually follow after the last
expression, and before the return statement.
Returning expressions, obfuscate debugging too.
--
Tor <bw****@wvtqvm. vw | tr i-za-h a-z>
On Wed, 31 Oct 2007 02:50:41 GMT, in comp.lang.c , James Kuyper
<ja*********@ve rizon.netwrote:
>Tor Rustad wrote:
>However, in safety-critical SW, I wouldn't advocate using construct
like
return a++;
I'm sorry - but if you have a compiler you can't trust to handle 'return
a++;'
AFAICS, its not a question of whether the /compiler/ can handle it,
its a question of whether the programmers understand what it does. See
for instance this entire thread.
--
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 This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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