On Wed, May 19, 2021 at 09:20:38AM -0500, Segher Boessenkool wrote: > On Wed, May 19, 2021 at 06:37:44AM -0700, Guenter Roeck wrote: > > On 5/19/21 5:03 AM, Segher Boessenkool wrote: > > >On Tue, May 18, 2021 at 07:45:14PM -0500, Segher Boessenkool wrote: > > >>And it actually explicitly is undefined behaviour in C90 already > > >>(3.6.6.4 in C90, 6.8.6.4 in C99 and later). > > > > > >... but there is a GCC extension that allows this by default: > > ><https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wreturn-type> > > > For C only, warn about a 'return' statement with an expression in a > > > function whose return type is 'void', unless the expression type is > > > also 'void'. As a GNU extension, the latter case is accepted > > > without a warning unless '-Wpedantic' is used. > > > > In C99: > > > > "6.8.6.4 The return statement > > Constraints > > > > A return statement with an expression shall not appear in a function whose > > return type > > is void. A return statement without an expression shall only appear in a > > function > > whose return type is void." > > > > Sounds like invalid to me, not just undefined behavior. > > I don't know what "invalid" would mean here other than UB, it isn't a > specific defined term, unlike the latter, which is precisely defined in > 3.4.3/1: > undefined behavior > behavior, upon use of a nonportable or erroneous program construct or > of erroneous data, for which this International Standard imposes no > requirements > > This is the strongest thing the standard can say, it is not Law, it does > not prohibit anyone from doing anything :-) > > "Shall" and "shall not" X means it is undefined behaviour if X (or its > inverse) is violated. See 4.2: > If a ''shall'' or ''shall not'' requirement that appears outside of a > constraint or runtime-constraint is violated, the behavior is > undefined. Undefined behavior is otherwise indicated in this > International Standard by the words ''undefined behavior'' or by the > omission of any explicit definition of behavior. There is no > difference in emphasis among these three; they all describe ''behavior > that is undefined''. > which also explains that what you call "invalid" has undefined behaviour > just as well, most likely. > I'd have assumed that "shall not" is syntactically wrong, but I stand corrected. Guenter
