Subj : Re: Is std::cerr thread safe
To   : comp.programming.threads
From : Maciej Sobczak
Date : Wed Jul 20 2005 10:52 pm

Sean Kelly wrote:

> The STL is guaranteed to be thread-safe

No, it isn't (and cerr is not STL).

It is rather considered to be a Quality of Implementation issue (you 
will not find it in the standard) that the program at least does not 
crash when I/O is accessed in a MT way.

> std::ostringstream ostr;
> ostr << "error: " << errno << '\n';
> std::cerr << ostr.rdbuf();

Even if cerr has this Quality of Implementation guarantee, I would not 
bet my money that it is at the level which would be appropriate above.
Try with *very* long messages (to overflow any potential buffer on the 
road) to see where the synchronization is in your particular library 
implementation.


> The preferable alternative would be to wrap writes in a critical
> section

Definitely. Some tricks with temporary objects may help as well, at 
least with single-instruction output schemes this:

myerr << "some " << 123 << " lengthy " << abc
       << " error " << xyz << " message\n";


-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/

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