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

[Mark Geisert]

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.

>> +  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.

..mark