[Python-Dev] Thread-safe file objects, the return

Antoine Pitrou solipsis at pitrou.net
Wed Apr 2 12:17:48 CEST 2008

Guido van Rossum <guido <at> python.org> writes:
> Your solution (a counter) seems fine except I think perhaps the
> close() call should not raise IOError -- instead, it should set a flag
> so that the thread that makes the counter go to zero can close the
> thread (after all the file got closed while it was being used).

I agree with Gregory: we should be explicit about what happens. I wonder
what we would gain from that approach - apart from encouraging dangerous
coding practices :)
It also depends how far we want to go: I am merely proposing to fix the 
crashes, do we want to provide a "smarter" close() variation that does what
you suggest for people that want (or need) to take the risk?

> There are of course other concurrency issues besides close -- what if
> two threads both try to do I/O on the file? What will the C stdio
> library do in that case? Are stdio files thread-safe at the C level?

According to the glibc documentation, at
http://www.gnu.org/software/libc/manual/html_node/Streams-and-Threads.html :

« The POSIX standard requires that by default the stream operations are
atomic. I.e., issuing two stream operations for the same stream in two
threads at the same time will cause the operations to be executed as if
they were issued sequentially. The buffer operations performed while
reading or writing are protected from other uses of the same stream. To do
this each stream has an internal lock object which has to be (implicitly)
acquired before any work can be done. »

So according to POSIX rules it should be perfectly safe.
In any case, someone would have to try my patch under Windows and OS X and 
see if test_file.py passes without crashing.



More information about the Python-Dev mailing list