stdio and short file descriptors revisited

[Konstantin Belousov]

On Fri, Sep 28, 2012 at 06:47:39PM -0400, John Baldwin wrote:
> Four years or so ago I cleaned up some of the stdio internals as fallout from 
> running into problems with stdio using a short instead of an int to hold file 
> descriptors.  Back then I got sidetracked with attempting to make FILE opaque 
> and ended up never getting around to bumping _file from a short to an int.  I 
> recently ran back into the SHRT_MAX limit at work again and came up with a 
> patch to fix this.
> 
> To preserve the ABI, it is necessary to leave the existing short _file in 
> place and add a new int _file to the end of the FILE structure.  Also, for old 
> applications, the old _file (_ofile in the patch) must still be valid.  The 
> approach I have taken is to bump the symbol version for routines that create 
> FILE objects with a non-fake _file (fopen, fdopen, and freopen).  The old 
> FBSD_1.0 variants still fail if an fd is greater than SHRT_MAX (and thus 
> cannot be safely stored in _ofile).  The new FBSD_1.3 variants assign to both 
> _file and _ofile if the fd is less than SHRT_MAX.  I also changed fileno()
> to no longer be an inline macro in <stdio.h> but to always be a function call 
> going forward.
> 
> If folks think this is ok, I'll hack up a modified version that hides _file
> from outside consumers (rename it to _nfile or some such) and send it for a
> ports-exp run before committing to make sure there aren't any 3rd party apps
> accessing _file directly.
> 
> http://www.FreeBSD.org/~jhb/patches/stdio_file.patch

The corner case left unhandled is the situation where we have a dso which
is linked against FBSD_1.0 version of libc, but which gets FILE * as an
API argument for some of its exported routines. If the implementation
uses fileno(3), it would fail. I have no idea how to fix this, most
likely, the issue is not fixable at all. Workaround seems to be to force
the __isthreaded to 1. Might be, as an ugly hack, some flag could be
added to the stdbuf(1), if anybody cares enough.

Otherwise, the patch looks good.

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