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Date: Wed, 16 Jan 2019 20:57:15 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: Minor C99 conformance issue: FILE is an incomplete type
* Rich Felker <dalias@...c.org> [2019-01-16 14:33:02 -0500]:
> On Wed, Jan 16, 2019 at 01:25:57PM -0600, A. Wilcox wrote:
> > On 01/16/19 13:06, Keith Thompson wrote:
> > > The musl 1.0.0 manual says that it's intended to conform to C99.
> > >
> > > Both the C99 and C11 standards require FILE to be an object type.
> > >
> > > In C99, incomplete types are not object types. In C11, the definition
> > > of "object type" was changed, so now incomplete types are object types.
> > > (See section C99 and C11 6.2.5, C99 7.19.1, C11 7.21.1.)
> > >
> > > So, in C99 FILE is not permitted to be an incomplete type, but in C11
> > > it is. (The section describing type FILE did not change. I don't
> > > know whether the authors of the standard actually intended to change
> > > the requirement.)
> > >
> > > Using musl-gcc, FILE is an incomplete type, so it conforms to C11
> > > but not to C99. <stdio.h> could be modified so that, for example,
> > > it pays attention to the value of __STDC_VERSION__ to decide how to
> > > define FILE (whether to make struct _IO_FILE visible).
>
> The real struct _IO_FILE cannot be made visible because its size and
> representation are not public or preserved across versions. A fake one
> could be; the reason it hasn't been is that it requires extra work to
> suppress the fake definition inside libc. But it could be done,
> especially now with the internal wrapper headers (commit
> 13d1afa46f8098df290008c681816c9eb89ffbdb and related work).
i assume the only point of a fake FILE is to satisfy some
conformance test that uses sizeof(FILE) and not useful in
any other way.. which means it's not really relevant in
practice.
>
> > > I do not suggest that this minor non-conformance to C99 is a practical
> > > problem, or even that it should be fixed, merely that it should
> > > be noted.
> >
> > This was noted by our POSIX conformance testing. There was a discussion
> > in the #musl IRC about possibly making a fake _IO_FILE to satisfy not
> > just POSIX but some miscreant glibc apps. However, I believe it was
> > decided not to do that.
>
> I'm not opposed to it, but it also hasn't seemed like a priority to
> work on, and catching programs that are wrongly (from a
> future-proofness standpoint, and from a semantic nonsense standpoint)
> depending on it seems to have been helpful in getting them fixed.
>
> Rich
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