thread safety level of fwide

[Alexandre Oliva]

On Nov 21, 2014, Rich Felker <dalias@libc.org> wrote:

> As a function that operates on a FILE, fwide is required by POSIX to
> behave as if it acquires the internal recursive mutex on the target
> file.

> http://pubs.opengroup.org/onlinepubs/9699919799/functions/flockfile.html

Interesting...  This indeed appears to rule out the optimization of
testing for safe values instead of taking a lock.  It's not like we can
just assume unordered access is acceptable: flockfile/funlockfile does
force a global total order, and (as you said) they bring memory acquire
and release with them.

This means we have a bug in at least fwide and fileno.

I wonder if addmntent is also covered by this requirement: with the
current implementation, it takes and releases the stream's lock twice.

-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer