Re: Document further requirement on mixing streams / file descriptors

[Joseph Myers <josmyers@redhat.com>]

On Thu, 26 Sep 2024, Florian Weimer wrote:

> I do not think this rules accurate.  The standard streams are linked
> channels, typically with descriptors for the file description in the
> parent process.  They are streams.  A freshly started program does not
> know if another program seeked any of the descriptors before.  Does this
> mean programs need to add fseek calls for the standard streams?  What if
> those streams are not seekable?

This seems like it might be an omission in the POSIX specification.

What I'd expect is: after execve, the standard streams are set up from the 
relevant file descriptors.  If the previous process seeked on a handle for 
that open file description, then it (possibly in the child after fork) 
must make the file descriptor active, including seeking on it to get a 
defined offset, but then after execve nothing more is needed regarding 
seeking on the stream (assuming that other processes aren't using the 
same open file description at the same time).

I can't however find anything in POSIX that says that this is what happens 
with handles for file descriptors 0, 1, 2 on execve (and, in particular, 
that the requirement to seek on the stream does not apply).  If we think 
this is what the semantics should be for glibc, we could still document it 
as such.

> I think we have a step missing in the cleaning process: the new channel
> may indeed need seeking.  The current manual suggests that cleaning is
> only needed on the old channel, but I don't think this is accurate, for
> both linked and independent channels.  For example, an input stream may
> have old file contents buffered.

For an input stream with old contents buffered (that is the new, linked 
handle), I think it would have been the active handle earlier, and so have 
needed to be cleaned when it ceased to be the active handle.  (In the case 
of independent channels, the manual already says "You should clean an 
input stream before reading data that may have been modified using an 
independent channel.  Otherwise, you might read obsolete data that had 
been in the stream's buffer.".)

-- 
Joseph S. Myers
josmyers@redhat.com