Extending domain of O_TMPFILE?

Corinna Vinschen corinna-cygwin@cygwin.com
Mon Feb 8 10:34:02 GMT 2021

On Feb  5 23:56, Mark Geisert wrote:
> On Fri, 5 Feb 2021, Corinna Vinschen via Cygwin-developers wrote:
> > On Feb  5 02:31, Mark Geisert wrote:
> > > Hi folks,
> > > I've been following up on a response I made to a Cygwin user in
> > > https://cygwin.com/pipermail/cygwin/2021-January/247306.html .
> > > I've figured out that Cygwin's implementation of the open() flag O_TMPFILE
> > > follows Linux in that one can't specify the name of a file when using this
> > > flag.  User supplies only the path, and Cygwin chooses an obscure file name
> > > for you.
> > > 
> > > That means the OP's suggested improvement of applying O_TMPFILE semantics to
> > > files created by tmpfile() won't work.
> > 
> > I don't understand the problem.  tmpfile(3) does not take filenames, it
> > creates its own filenames.  Thus, just adding O_TMPFILE in _tmpfile_r's
> > and _tmpfile64's calls to open() on systems supporting this flag and not
> > calling _remove_r subsequently would already do the trick.
> That's what I thought too.  But the open() fails and strace reveals ENOENT
> is being generated at syscalls.cc:1516.  The unix_path arg to open() needs
> to indicate a directory, but _tmpfile_r is currently passing in a filename
> path generated by _tmpnam_r.
> So that's what led me to contemplate extending the domain of O_TMPFILE such
> that one could proactively name the temporary file.  But it's probably more
> sane to just have _tmpfile_r skip the generation of a file name and instead
> pass in a directory name to open(), either from environment variable TMPDIR
> or the libc #define P_tmpdir.
> Does this sound OK?

Yes, the matching patch should be in _tmpfile_r / _tmpfile64_r.

Alternatively, and probably much simpler, you could define our
own tmpfile / tmpfile64 in Cygwin's syscalls.cc, kind of like this:

  extern "C" FILE *
  tmpfile (void)

  EXPORT_ALIAS (tmpfile, tmpfile64)


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