Extending domain of O_TMPFILE?

Mark Geisert mark@maxrnd.com
Sat Feb 6 07:56:57 GMT 2021

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?


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