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In article < .com>
chandanlinster <> wrote:
>Consider the following program fragment:
> struct some_struct { } a;
> printf("%d", sizeof(a));
>On GCC 4.1.1 the output is 0(zero).

I do not actually have gcc 4.1.1 handy, but I do not believe this
is the case. Note that to get gcc to compile C code (as opposed to
some other language that looks a lot like C, but is not actually C)
you must give it the "-ansi -pedantic" options, and if you do so:

% strictcc t.c
t.c:1: error: struct has no members

(where "strictcc" is "cc -ansi -pedantic -Werror" with the stderr
stream massaged to change the string "warning" to "error":

% cat ~/scripts/strictcc
#! /bin/sh
(cc -ansi -pedantic -Werror ${1+"$@"} 2>&1) |
sed -e 1d -e s/warning:/error:/
# it might be reasonable to add 1>&2 but this suffices for now

If gcc ever claims full C99 conformance, you can use -std=c99
instead of -ansi or -std=c89, presumably still with -pedantic,
to get a strict C99 compiler.)

>On another compiler {sorry I don't know the compiler } the output
>is 1(one).

This would be correct for a C++ compiler -- are you sure you invoked
a C compiler? (Note that, as with gcc, you may have to supply
various command-line arguments to get compiler for the C language,
rather than one for some other language that looks a lot like C,
but is different.)
--
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Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

chandanlinster wrote:
> Consider the following program fragment:
>
> /************************/
> struct some_struct {
>
> }a;
>
> printf("%d", sizeof(a));
> /*************************/
>
> On GCC 4.1.1 the output is 0(zero).
> On another compiler {sorry I don't know the compiler } the output
> is 1(one).
>
> Is it compiler dependent?
> If so how can a variable like "a" have a zero size ( I mean how can a
> variable with 0(zero) byte size be stored in memory).

In your view, what should be the size of a structure with no members?

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---

In article <> Chris Torek <> writes:
> In article < .com>
> chandanlinster <> wrote:
> >Consider the following program fragment:
> > struct some_struct { } a;
> > printf("%d", sizeof(a));
> >On GCC 4.1.1 the output is 0(zero).
>
> I do not actually have gcc 4.1.1 handy, but I do not believe this
> is the case.

4.0.2 also does it. With -ansi (or other flags) it does indeed also give
the warning.
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/

Richard Bos wrote:
> "chandanlinster" <> wrote:
>
> > Consider the following program fragment:
> >
> > /************************/
> > struct some_struct {
> >
> > }a;
> >
> > printf("%d", sizeof(a));
> > /*************************/
> >
> > On GCC 4.1.1 the output is 0(zero).
> > On another compiler {sorry I don't know the compiler } the output
> > is 1(one).
>
> Neither is correct. An empty struct declaration is syntactically
> invalid. Even one which only has unnamed members (which would at least
> have a size) invokes undefined behaviour.
>
> > Is it compiler dependent?
>
> It shouldn't be;

Like all undefined behavior, it is of course compiler-dependent.

> it should be refused outright, rather than give a
> random answer.

Refused outright is a bit strong. There is no requirement that the
compiler reject anything except for an #error directive that survives
preprocessing. There is nothing wrong with GCC allowing it as an extension.

> > If so how can a variable like "a" have a zero size
>
> It can't. That is presumably one of the reasons why it's disallowed.
>
> Richard


--
Clark S. Cox III

chandanlinster wrote:
> Consider the following program fragment:
>
> /************************/
> struct some_struct {
>
> }a;
>
> printf("%d", sizeof(a));
> /*************************/
>
> On GCC 4.1.1 the output is 0(zero).
> On another compiler {sorry I don't know the compiler } the output
> is 1(one).

Your other compiler very well may have been a C++ compiler (where empty
structs are defined, and produce a valid value when used with the sizeof
operator).

>
> Is it compiler dependent?

Yes. It is undefined behavior, in C a struct must have at least one
named member; A C compiler is allowed to do anything or nothing at all.
To put it bluntly, don't do this

> If so how can a variable like "a" have a zero size ( I mean how can a
> variable with 0(zero) byte size be stored in memory).

It can't.


--
Clark S. Cox III

On 5 Sep 2006 18:29:37 GMT, Chris Torek <> wrote:

> % strictcc t.c
> t.c:1: error: struct has no members
>
> (where "strictcc" is "cc -ansi -pedantic -Werror" with the stderr
> stream massaged to change the string "warning" to "error":
>
> % cat ~/scripts/strictcc
> #! /bin/sh
> (cc -ansi -pedantic -Werror ${1+"$@"} 2>&1) |
> sed -e 1d -e s/warning:/error:/
> # it might be reasonable to add 1>&2 but this suffices for now
>
<IMPLSPECIFIC=GCC> -pedantic-errors makes them errors (fail
compilation) (but without putting the string "error:"). This is
consistent with what -Werror does for other warnings. </>


- David.Thompson1 at worldnet.att.net