Re: [PATCH] Cygwin: Have tmpfile(3) use O_TMPFILE

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

On Feb 10 01:07, Mark Geisert wrote:
> Hi Corinna,
> 
> Corinna Vinschen via Cygwin-patches wrote:
> > Hi Mark,
> > 
> > On Feb  9 02:50, Mark Geisert wrote:
> > > Per discussion on cygwin-developers, a Cygwin tmpfile(3) implementation
> > > has been added to syscalls.cc.  This overrides the one supplied by
> > > newlib.  Then the open(2) flag O_TMPFILE was added to the open call that
> > > tmpfile internally makes.
> > > ---
> > >   winsup/cygwin/release/3.2.0 |  4 ++++
> > >   winsup/cygwin/syscalls.cc   | 20 ++++++++++++++++++++
> > >   2 files changed, 24 insertions(+)
> > > 
> > > diff --git a/winsup/cygwin/release/3.2.0 b/winsup/cygwin/release/3.2.0
> > > index f748a9bc8..d02d16863 100644
> > > --- a/winsup/cygwin/release/3.2.0
> > > +++ b/winsup/cygwin/release/3.2.0
> > > @@ -19,6 +19,10 @@ What changed:
> > >   - A few FAQ updates.
> > > +- Have tmpfile(3) make use of Win32 FILE_ATTRIBUTE_TEMPORARY via open(2)
> > > +  flag O_TMPFILE.
> > > +  Addresses: https://cygwin.com/pipermail/cygwin/2021-January/247304.html
> > > +
> > >   Bug Fixes
> > >   ---------
> > > diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
> > > index 52a020f07..b79c1c7cd 100644
> > > --- a/winsup/cygwin/syscalls.cc
> > > +++ b/winsup/cygwin/syscalls.cc
> > > @@ -5225,3 +5225,23 @@ pipe2 (int filedes[2], int mode)
> > >     syscall_printf ("%R = pipe2([%d, %d], %y)", res, read, write, mode);
> > >     return res;
> > >   }
> > > +
> > > +extern "C" FILE *
> > > +tmpfile (void)
> > > +{
> > > +  char *dir = getenv ("TMPDIR");
> > 
> > This isn't what Linux tmpfile does.  Per the man page, it tries to
> > create the file in P_tmpdir first, and if that fails, it tries
> > "/tmp".
> 
> Oops, I was following newlib's code here.  I'll adjust this.

Oops, I didn't check with newlib, sorry.  Hmm, so we're stuck with
either Linux-compat or backward-compat but not both.  Decisions,
decisions...

Let's stick to backward-compat then, so no changes here.

> > > +  if (!dir)
> > > +    dir = P_tmpdir;
> > > +  int fd = open (dir, O_RDWR | O_CREAT | O_BINARY | O_TMPFILE,
> > 
> > You have to specify O_EXCL here.  The idea is that this file cannot be
> > made permanent, and missing the O_EXCL flag allows exactly that.  See
> > https://man7.org/linux/man-pages/man2/open.2.html, the lengthy
> > description in terms of O_TMPFILE.
> 
> I started out with O_EXCL as you suggested, but found syscalls.cc:1504
> reporting EEXIST.  Is there some clash there between fh->exists() and
> O_TMPFILE?  Hmm.

You specify O_CREAT as well.  O_TMPFILE replaces O_CREAT.  The
combination O_CREAT|O_EXCL still checks for existence of the file, in
that case, the dir.


Thanks,
Corinna