Re: rwlock(9) and mutex(9) definitions

On Tue, Oct 26, 2021 at 09:49:58PM -0700, Gleb Smirnoff wrote:
> On Wed, Oct 27, 2021 at 07:39:38AM +0300, Konstantin Belousov wrote:
> K> > K> Both rw_rlock and rw_wlock are in tail context.  You cannot _return_ void.
> K> > 
> K> > You actually can return void to hint compiler for a tail call optimization.
> K> > It is not a wrong syntax.
> K> > 
> K> > Other code that is working with true void functions (e.g. with WITNESS) and
> K> > doesn't work with "do {} while" is:
> K> > 
> K> > void
> K> > something(bool clue)
> K> > {
> K> > 	return (clue ? rw_rlock(lock) : rw_wlock(lock));
> K> > }
> K> > 
> K> > This is explicitly allowed in 6.5.15 of the C11 standard.
> K> > 
> K> > Of course all this code can be written in some other way, so constraint
> K> > of KPI not being true functions can be worked around, but I believe better
> K> > it be fixed.
> K> 
> K> Hum.  I know about ternary operator allowing void-typed expressions on both
> K> sides of ':'.  What I replied to is the following, from C17
> K> 
> K> 6.8.6.4 The return statement
> K>       Constraints
> K> 1 A return statement with an expression shall not appear in a function
> K> whose return type is void. A return statement without an expression
> K> shall only appear in a function whose return type is void.
> K> 
> K> So syntax above is explicitly prohibited by the standard.  I was quite
> K> surprised that both gcc and clang silently accept this, unless -pedantic
> K> is specified.  There is no mention of this extension in gcc manual.
> K> Compiler explorer demonstrates that compilers like msvc do warn about
> K> the construct, and some even error out (tendra):
> K> https://godbolt.org/z/xqcPssTcY
> K> 
> K> Another part of the confusion, perhaps, is that
> K> 	return <void expression>;
> K> is explicitly allowed by the C++ standard (I looked at 2020 version),
> K> unlike C.
> 
> Hmm, so I mistakenly transferred my knowledge about the tail call
> optimization hint from C++ to C.
> 
> Okay, let's put return aside. This would compile with true
> functions (e.g. WITNESS), otherwise not:
> 
> void
> something(bool clue)
> {
> 	clue ? rw_rlock(lock) : rw_wlock(lock);
> }
> 
> And this is correct code per 6.5.15.

So why cannot you write it as 
...
	if (clue)
		rw_rlock(lock);
	else
		rw_wlock(lock);