Mail Archives: cygwin/2022/07/05/17:52:15

Ken Brown <kbrown AT cornell DOT edu> · Tue, 5 Jul 2022 17:51:27 -0400

On 7/5/2022 10:13 AM, Lavrentiev, Anton (NIH/NLM/NCBI) [C] via Cygwin wrote:
> Hi,
> 
> There's some inconsistency between <sys/select.h> and <sys/param.h>:
> 
> sys/select.h has this:
> -----------------------
> /*
>   * Select uses bit masks of file descriptors in longs.
>   * These macros manipulate such bit fields (the filesystem macros use chars).
>   * FD_SETSIZE may be defined by the user, but the default here
>   * should be >= NOFILE (param.h).
>   */
> #ifndef FD_SETSIZE
> #define FD_SETSIZE      64
> #endif
> ----------------------

I think Cygwin's FD macros are based on FreeBSD.  The most recent <sys/select.h> 
on FreeBSD says:

---------------------------------------------------------------------
/*
  * Select uses bit masks of file descriptors in longs.  These macros
  * manipulate such bit fields (the filesystem macros use chars).
  * FD_SETSIZE may be defined by the user, but the default here should
  * be enough for most uses.
  */
#ifndef	FD_SETSIZE
#define	FD_SETSIZE	1024
#endif
---------------------------------------------------------------------

NOFILE isn't mentioned.  Maybe Cygwin (or really newlib) should also remove the 
reference to NOFILE and, perhaps, change the default FD_SETSIZE to 1024.  But 
Cygwin isn't the only newlib target, so there might be good reasons to keep it 
at 64.

Corinna, WDYT?  Or should the discussion be moved to the newlib list?

> Now, this is the relevant part of sys/param.h looks like this:
> ----------------------
> /* Max number of open files.  The Posix version is OPEN_MAX.  */
> /* Number of fds is virtually unlimited in cygwin, but we must provide
>     some reasonable value for Posix conformance */
> #define NOFILE          8192
> ----------------------
> 
> So it's either "<= NOFILE" that was actually meant to be there in the comment (or,
> an equivalent "should NOT be > NOFILE"), or FD_SETSIZE should have been defined as 8192,
> if the comment is actually correct.  Or maybe I'm missing something here :-)

I suspect that the comment was actually meant as written.  On FreeBSD, NOFILE is 
defined to be OPEN_MAX, which is 64.  On Cygwin, however, it wouldn't make sense 
to follow that comment since, as noted in your quote from sys/param.h, there is 
effectively no limit on open files on Cygwin.  (Cygwin maintains a dynamically 
growing table of file descriptors.)

Ken

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Date:	Tue, 5 Jul 2022 17:51:27 -0400
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Subject:	Re: Typo in <sys/select.h>?
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