threads/79887: [patch] freopen() isn't thread-safe

Dmitrij Tejblum tejblum at
Wed Dec 28 21:20:08 PST 2005

The following reply was made to PR threads/79887; it has been noted by GNATS.

From: Dmitrij Tejblum <tejblum at>
To: David Xu <davidxu at>
Cc: bug-followup at
Subject: Re: threads/79887: [patch] freopen() isn't thread-safe
Date: Thu, 29 Dec 2005 08:12:58 +0300

 David Xu wrote:
 > Indeed, this a bug, but the patch you provided breaks the samentic the
 > FILE structure was designed for, here you conditionally call
 > fp->_close(), this is incorrect, because the hook may be an external
 > function, it should always be called to notify external code.
 I only assume that
 1) _file and _close fields are internal to stdio, i.e. only stdio code 
 manipulate with them directly
 2) If _file != -1, then the FILE is associated with the file descriptor, 
 fp->_close == __sclose (because the only code that can set fp_close to 
 something different is funopen, and it set _file to -1) and __sclose 
 just close the _fp->_file
 If so, we know that dup2() will close the descriptor too, dup2() is 
 required to do it.
 > I think the right fix is to fix those code which is still using
 > STDERR_FILENO, or don't do following hack in freopen.c:
 >     if (wantfd >= 0 && f != wantfd) {
 >            if (_dup2(f, wantfd) >= 0) {
 >                      (void)_close(f);
 >                      f = wantfd;
 >            }
 >     }
 > This is not required by standard.
 Well, I tried to keep existing behaviour, and I think that the hack is 
 indeed a good idea even though it is not required.
 > David Xu

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