multiple threads writing to std::cout or std::cerr

Question

I have OpenMP threads that write to the console via cout and cerr. This of course is not safe, since output can be interleaved. I could do something like

#pragma omp critical(cerr)
{
   cerr << "my variable: " << variable << endl;
}

It would be nicer if could replace cerr with a thread-safe version, similar to the approach explained in the valgrind DRD manual (http://valgrind.org/docs/manual/drd-manual.html#drd-manual.effective-use) which involves deriving a class from std::ostreambuf. Ideally in the end I would just replace cerr with my own threaded cerr, e.g. simply:

tcerr << "my variable: " << variable << endl;

Such a class could print to the console as soon as it encounters an "endl". I do not mind if lines from different threads are interleaved, but each line should come only from one thread.

I do not really understand how all this streaming in C++ works, it is too complicated. Has anybody such a class or can show me how to create such a class for that purpose?

Answer 1

As others pointed out, in C++11, std::cout is thread-safe.

However if you use it like

std::cout << 1 << 2 << 3;

with different threads, the output can still be interleaved, since every << is a new function call which can be preceeded by any function call on another thread.

To avoid interleaving without a #pragma omp critical - which would lock everything - you can do the following:

std::stringstream stream; // #include <sstream> for this
stream << 1 << 2 << 3;
std::cout << stream.str();

The three calls writing 123 to the stream are happening in only one thread to a local, non-shared object, therefore aren't affected by any other threads. Then, there is only one call to the shared output stream std::cout, where the order of items 123 is already fixed, therefore won't get messed up.

Answer 2

You can use an approach similar to a string builder. Create a non-template class that:

• offers templated operator<< for insertion into this object
• internally builds into a std::ostringstream
• dumps the contents on destruction

Rough approach:

 class AtomicWriter {
    std::ostringstream st;
 public:
    template <typename T> 
    AtomicWriter& operator<<(T const& t) {
       st << t;
       return *this;
    }
    ~AtomicWriter() {
       std::string s = st.str();
       std::cerr << s;
       //fprintf(stderr,"%s", s.c_str());
       // write(2,s.c_str(),s.size());
    }
 };

Use as:

AtomicWriter() << "my variable: " << variable << "\n";

Or in more complex scenarios:

{
   AtomicWriter w;
   w << "my variables:";
   for (auto & v : vars) {
      w << ' ' << v;
   }
}  // now it dumps

You will need to add more overloads if you want manipulators, you can use write better than fprintf for the atomic write in the destructor, or std::cerr, you can generalize so that the destination is passed to the constructor (std::ostream/file descriptor/FILE*),

Answer 3

I don't have enough reputation to post a comment, but I wanted to post my addition to the AtomicWriter class to support std::endl and allow for other streams to be used besides std::cout. Here it is:

class AtomicWriter {
    std::ostringstream st;
    std::ostream &stream;
public:
    AtomicWriter(std::ostream &s=std::cout):stream(s) { }
    template <typename T>
    AtomicWriter& operator<<(T const& t) {
        st << t;
        return *this;
    }
    AtomicWriter& operator<<( std::ostream&(*f)(std::ostream&) ) {
        st << f;
        return *this;
    }
    ~AtomicWriter() { stream << st.str(); }
};

Answer 4

Put the following code in header file atomic_stream_macro.h

#ifndef atomic_stream_macro_h
#define atomic_stream_macro_h

#include <mutex>

/************************************************************************/
/************************************************************************/

extern std::mutex print_mutex;

#define PRINT_MSG(out,msg)                                           \
{                                                                    \
    std::unique_lock<std::mutex> lock (print_mutex);                 \
                                                                     \
    out << __FILE__ << "(" << __LINE__ << ")" << ": "                \
        << msg << std::endl;                                         \
}

/************************************************************************/
/************************************************************************/

#endif

Now the macro can be used from a file as follows.

#include <atomic_stream_macro.h>
#include <iostream>

int foo (void)
{
    PRINT_MSG (std::cout, "Some " << "Text " << "Here ");
}

Finally, in the main.cxx, declare the mutex.

#include <mutex>

std::mutex print_mutex;

int main (void)
{
    // launch threads from here
}

Answer 5

You could do it by inheriting std::basic_streambuf, and override the correct functions to make it threadsafe. Then use this class for your stream objects.