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Date: Sun, 2 Aug 2020 21:56:41 +0300 (MSK)
From: Alexander Monakov <>
Subject: Re: Musl's FD_{SET,ISSET,CLR} macros from sys/select.h trigger
 gcc's -Wsign-conversion warnings

On Sun, 2 Aug 2020, Alexander Monakov wrote:

> On Sun, 2 Aug 2020, Petr Skocik wrote:
> >     #define FD_SET(d, s)   ((s)->fds_bits[(d)/(8*(int)sizeof(long))] |=
> > (1UL<<((d)%(8*(int)sizeof(long)))))
> >     #define FD_CLR(d, s)   ((s)->fds_bits[(d)/(8*(int)sizeof(long))] &=
> > ~(1UL<<((d)%(8*(int)sizeof(long)))))
> >     #define FD_ISSET(d, s) !!((s)->fds_bits[(d)/(8*(int)sizeof(long))] &
> > (1UL<<((d)%(8*(int)sizeof(long)))))
> > 
> > You might want to add them.
> (casting 'd' to size_t would have been more appropriate, as there's no need
> to perform signed division and modulus here)
> This is one of the cases where the warning should have been suppressed by
> GCC unless -Wsystem-headers is also given: the problem appears when expanding
> a macro defined in a system header, so the user can't cleanly avoid it.

I meant to also mention that as -Wsign-conversion is not part of -Wall -Wextra,
it's relatively rarely used, so lack of suppression is probably just an
oversight in the implementation, not a deliberate choice on GCC part.

> Would you care to open an issue in the GCC Bugzilla about it?
> Alexander

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