Re: rwlock(9) and mutex(9) definitions
- Reply: Konstantin Belousov : "Re: rwlock(9) and mutex(9) definitions"
- In reply to: Konstantin Belousov : "Re: rwlock(9) and mutex(9) definitions"
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Date: Wed, 27 Oct 2021 04:49:58 UTC
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. -- Gleb Smirnoff