On Fri, 5 Nov 2010 08:41:24 -0700 (PDT), Luca Forlizzi
<> wrote:
>Hello,
>
> while studying incomplete types, I wrote the below example
>containing, I think,
> several undef. behaviors. Trying it with two different compilers I
>find out
> that expressions that, in my opinion, are all undef. b. for the same
>reason,
> produce non uniform translations (I used -std=c99 -pedantic-errors -
>Wall for gcc).
>
> I have the following questions:
> 1)Am I right that all the 8 expressions casted to void are undef.b.
>per 6.3.2.1 p.2
There is a slight difference between constraint violations and
undefined behavior. The former leads to the latter but there are
other causes of undefined behavior.
> 2) gcc prints "error: dereferencing pointer to incomplete type"
>while trying
> to translate
> (void) *ps;
> Does the standard forbids to dereference a pointer to an
>incomplete type?
There is footnote 36 which says an incomplete type is not an object.
6.5.3.2-4 defines the result of * operator only if it points to a
function or to an object. Your code seems to be a constraint
violation.
> I was unable to find such a prohibition, moreover gcc silently
>accepts
> dereferencing a pointer to an incomplete array type. In my
>opinion the
Show the code. Even if gcc is wrong, it doesn't change the language
spec.
> error message is misleading regading the nature of the error.
Based on 6.5.3.2-4, it seems almost letter perfect, if a little terse.
>
> /*------------------ EXAMPLE --------------------*/
>
> /* declaration of an external variabile of incomplete type; later
> type is completed and variable defined */
>struct S external_s;
>
>/* tentative definition of a variable with incomplete type;
> later type is completed and the tentative def becomes a definition
> (the variable is implicitly initialized to {0}) */
>struct S tentative_s;
>
>int main(void) {
>
> /* pointers to the incomplete structure type */
> struct S *ps, *qs;
>
> /* declaration of functions returning an incomplete structure type */
> struct S hht(void);
> struct S hhe(void);
6.9.1-3 says the return type must be void or an object type. Footnote
36 says incomplete types are not objects. This is a constraint
violation.
>
>
> ps = &tentative_s;
> qs = &external_s;
>
> /* all following statements are (discarded) computation of an
>expression
> having an incomplete structure type.
> Are they all undef.b. per 6.3.2.1 p.2 ?
> */
>
> /* gcc emits diagnostics, vbcc silently accepts them */
> (void) hht();
> (void) hhe();
>
> /* both gcc and vbcc emit diagnostics */
> (void) *ps;
> (void) *qs;
>
> /* gcc silently accepts them, vbcc emits diagnostics */
> (void) * &tentative_s;
> (void) * &external_s;
>
> /* both gcc and vbcc silently accept them */
> (void) tentative_s;
> (void) external_s;
>
> return 0;
>}
>
>/* declaration that completes the structure type and defines
>external_s */
>struct S { float a; int b, c; } external_s = { 1.1, 2, 3, };
>
>/* functions returning the here-complete structure type */
>struct S hht(void) {
>
> return tentative_s;
>}
>
>struct S hhe(void) {
>
> return external_s;
>}
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