Re: rwlock(9) and mutex(9) definitions
- Reply: Gleb Smirnoff : "Re: rwlock(9) and mutex(9) definitions"
- In reply to: Gleb Smirnoff : "Re: rwlock(9) and mutex(9) definitions"
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Date: Wed, 27 Oct 2021 05:11:13 UTC
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);