Fernando J. Pereda’s blag

May 25, 2010

iostreams are VERY thread-unsafe on Mac OS X

Filed under: blag — Tags: , , , — Fernando J. Pereda @ 8:19 am

This was both surprising and disgusting. Our in-house data acquisition software needs to convert lots of floats to ascii so that they can ve viewed in real time. This is very easy to do with a stringify-like function like the one found in paludis/paludis/util/stringify.hh.

stringify looks trivial, there’s no shared state nor there is any static data, so I thought that I could call stringify from several threads at the same time. I don’t think there’s any guarantee about thread safety of ostringstream’s operator<< but it looked like a safe assumption. Well, incorrect.

operator<< calls vsnprintf, which should be thread-safe by itself. However, OSX's vsnprintf ultimately calls localeconv_l, which is not thread-safe. And, you end up with something like this:

#0 0x96f734a9 in malloc_error_break ()
#1 0x96f6e497 in szone_error ()
#2 0x96e98503 in szone_free ()
#3 0x96e9836d in free ()
#4 0x96e97f24 in localeconv_l ()
#5 0x96e93335 in __vfprintf ()
#6 0x96ecb9b5 in vsnprintf ()
#7 0x0144615e in std::__convert_from_v ()
#8 0x01437d2a in std::num_put<char, std::ostreambuf_iterator<char, std::char_traits > >::_M_insert_float ()
#9 0x0143803d in std::num_put<char, std::ostreambuf_iterator<char, std::char_traits > >::do_put ()
#10 0x0144ab6a in std::ostream::_M_insert ()
#11 0x0000fb25 in std::basic_ostringstream<char, std::char_traits, std::allocator >::str () at sstream:217
...

Which means there’s no way of converting stuff to human readable in a thread-safe manner. I’m not sure whether I’m missing something or not… but this looks very fishy. I ended up adding a stupid big mutex around my stringify calls, but that looks more like a workaround that a final solution to the problem. Plus, this doesn’t protect ANY other uses of iostream’s operator<< such as loggers.

Ok… looks like that wasn’t a very accurate diagnostic. This is going to be more annoying than I thought.

— ferdy

Advertisements

Leave a Comment »

No comments yet.

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Blog at WordPress.com.

%d bloggers like this: