Date: Wed, 21 Aug 2019 10:34:49 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Michael Paquier <michael@paquier.xyz>,
	Andres Freund <andres@anarazel.de>,
	Dilip Kumar <dilipbalaut@gmail.com>, jungleboogie0@gmail.com,
	pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #15964: vacuumdb.c:187:10: error: use of undeclared
 identifier 'FD_SETSIZE'
Message-ID: <20190821143449.GA27852@alvherre.pgsql>
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On 2019-Aug-21, Tom Lane wrote:

> Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> > I guess the other possibility is that there exists an operating system
> > that returns file descriptors higher than its FD_SETSIZE, when some fds
> > below FD_SETSIZE are still available.  Doesn't seem realistic.
> 
> I think POSIX requires open() to select the lowest unused FD.  Otherwise
> tricks like closing and reopening stdout wouldn't work reliably.

Ah, yes, that's right -- my system's open(3p) manpage says:

       The  open()  function  shall return a file descriptor for the named file
       that is the lowest file descriptor not currently open for that process.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services