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Date: Mon, 3 Aug 2020 12:45:52 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: Petr Skocik <pskocik AT gmail DOT com>
Subject: Re: FD_{SET,ISSET,CLR} macros from sys/select.h triggerring gcc's
 -Wsign-conversion warnings
Message-ID: <20200803104552.GJ460314@calimero.vinschen.de>
Mail-Followup-To: Petr Skocik <pskocik AT gmail DOT com>, cygwin AT cygwin DOT com
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Hi Petr,

On Aug  2 20:08, Petr Skocik via Cygwin wrote:
> Example:
> 
> #include <sys/select.h>
> void f(int X)
> {
>     fd_set set;
>     FD_ZERO(&set);
>     FD_SET(X,&set);
>     FD_CLR(X+1,&set);
>     (void)FD_ISSET(X+2,&set);
> }
> 
> causes
> 
> fds.c:7:2: warning: conversion to ‘long unsigned int’ from ‘int’ may
> change the sign of the result [-Wsign-conversion]
>   FD_SET(X,&set);
>   ^~~~~~
> [...]
> on gcc with -Wconversion -Wsign-conversion.
> 
> The problem is caused by the following macros:
> 
>     #  define    NFDBITS    (sizeof (fd_mask) * 8)    /* bits per mask */
>     #  define    FD_SET(n, p)    ((p)->fds_bits[(n)/NFDBITS] |= (1L <<
> ((n) % NFDBITS)))
>     #  define    FD_CLR(n, p)    ((p)->fds_bits[(n)/NFDBITS] &= ~(1L <<
> ((n) % NFDBITS)))
>     #  define    FD_ISSET(n, p)    ((p)->fds_bits[(n)/NFDBITS] & (1L <<
> ((n) % NFDBITS)))
> 
> int-casting the sizeof and using 1UL instead of 1L fixes the problem:

Thanks.  I pushed a patch which will show up in the next Cygwin release:
https://sourceware.org/git/?p=newlib-cygwin.git;a=commitdiff;h=5717262b8ecf

Funny enough, while the GLibc version of sys/select.h does not generate
these warnings, the same problem still exists in the upstream FreeBSD
code.

For testing I uploaded a new developer snapshot to
https://cygwin.com/snapshots/  You only need to fetch the new
sys/select.h file.


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer
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