[Boost-users] thread-safe iostreams - ideas for implementation ?

From: marc (marc_at_[hidden])
Date: 2005-10-12 16:30:40

thread-safe iostreams - ideas for implementation ?

I am currently experimenting with the boosth thread/mutex libraries, and 
did notice that the standard io streams are not thread safe.

In the boost examples, this is handles by some global mutexes which are 
locked before writing to cout, cerr.

I would like to have some classes which

- behave exactly as cout, cerr and named file out streams
- are thread safe mutex locked

Are there any ideas on how to do this ?? I have the following 
possibilities in mind:

1) make a wrapper class around ostreams which have assigned a  specific 
file or ostream as a member, make each input to operator<<() locked by a 
  lock to a static mutex.

advantage: not that hard to implement
disadvantage: when outputting multiple things, like in

mutex-cout << 'A' << 'B' << std::endl;

another thread can come in between 'A' and 'B'. Not really thread safe, 
as I would like to have the entire line protected.

2) Make some trickery with manipulators and use the std:: stremas, 
something like

cout << lock << 'A' << 'B' << std::endl << unlock ;

advantage: pretty ok syntax (albeit one can forget to unlock!)

disadvantage: I really have no idea how to implement in a niece and easy 
way ..

3) Make some trickery with out-put buffering
something like having a class modeled like a stream_buff, which is large 
enough to hold what's written to the associated streams, and which does 
write to the cout or file streams in a mutex-locked fashion whenever 
it's manually flushed.

??? silly or possible ??

4) finaly, a class having a boring print(std::string) function, which 
sets a lock on the static mutex

advantage: easy-peasy
disadvantage: C-print like, needs a new class type for each out-file, as 
  a new static mutex has to be assigned to each out-file
No nice C++ straming possible
whatever I want to write needs to be converted into a std::string first.
implementatio: can be templated by a string parameter which is the name 
of the file, or 'cout'/'cerr'

Probably my ideas are not very bright, so I would appreciate some input 
from boost::thread users, which undoubtely have met the same problem before.

Cheers, Marc

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