Re: Cleaning up FILE in stdio..

On Wednesday 27 February 2008 01:59:25 am Peter Jeremy wrote:
On Tue, Feb 26, 2008 at 11:55:16PM -0500, John Baldwin wrote:
Yes, I discovered the macros today while working on my fd as short problem.

Macros and __inline functions mean that a significant proportion of
software compiled on FreeBSD has the existing definition of FILE
compiled into it.

However, I can't fix the fact that our stdio can't handle fd's > SHRT_MAX
(again, glibc handles this just fine) w/o making a royal mess.

I don't think a versioned FILE is practical so we are stuck with a
16-bit _file for the immediate future.

What I've gone with instead to fix the SHRT_MAX problem is to change
fopen/fdopen/freopen to fail to use fd's > SHRT_MAX with an error.

You could change _file from 'short' to 'unsigned short' without breaking
the ABI - this would allow either 65535 or 65536 file descriptors (I'm
not sure whether _file == -1 is special or not). This would postpone
the problem for some time.

-1 is used a lot in the stdio code for file's not backed by an fd. My problem
though is that this doesn't help with existing binaries that are already
compiled (which is what I have to deal with). Had fileno() not been inlined
I would have been ok, but that's pretty much done for me as far as my current
problem on 6.x. Had I just been able to change FILE * and not had inlines,
then a new fopen would have worked fine in my case.

My suggestion would be:
Now:
a) change _file to 'unsigned short' and add checks as proposed
b) merge __sFILEX into FILE
c) Remove the macros and inlines that poke around inside FILE
d) Note that directly accessing FILE innards is deprecated and
move the definition of struct __sFILE into libc/stdio/local.h

Yes, but also d2) tag all the fields that were previously exported and so they
are not changed in the future.

Once RELENG_8 is branched:
e) Don asbestos underwear and re-arrange struct __sFILE to grow _file etc.

We can't do e) because thanks to symbol versioning, 8.x and 9.x will have
libc.so.7, so a 7.0 binary will still use the brand new libc, so it has to
preserve the ABI of the currently exported fields pretty much forever. I do
think we can get away with renaming '_file' to '_ofile' and adding a new 'int
_file' at the bottom of the struct and making sure '_ofile' is always in sync
(when possible, truncated when _file is too bug).

Also, I think we can do the new _file in HEAD for 8.0 w/o any worries. I
don't think waiting until 9.0 buys anything there. Given that, I think I'd
rather just patch the current stable branches to handle the edge case better
and work on making _file an int in HEAD (with the ABI compat _ofile).

--
John Baldwin
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