A line-based thread-safe std::cerr for C++

Question

What is the easiest way to create my own std::cerr so that it is line-by-line thread-safe.

I am preferably looking for the code to do it.

What I need is so that a line of output (terminated with std::endl ) generated by one thread stays as a line of output when I actually see it on my console (and is not mixed with some other thread's output).

---

Solution : std::cerr is much slower than cstdio. I prefer using fprintf(stderr, "The message") inside of a CriticalSectionLocker class whose constructor acquires a thread-safe lock and the destructor releases it.

Answer 1

If available, osyncstream (C++20) solves this problem:

#include <syncstream> // C++20

std::osyncstream tout(std::cout);
std::osyncstream terr(std::cerr);

---

If the above feature is not available, here is a drop-in header file containing two macros for thread-safe writing to std::cout and std::cerr (which must share a mutex in order to avoid interleaving of output). These are based on two other answers , but I have made some changes to make it easy to drop into an existing code base. This works with C++11 and forward.

I've tested this with 4 threads on a 4-core processor, with each thread writing 25,000 lines per second to tout and occasional output to terr , and it solves the output interleaving problem. Unlike a struct-based solution, there was no measurable performance hit for my application when dropping in this header file. The only drawback I can think of is that since this relies on macros, they can't be placed into a namespace.

threadstream.h

#ifndef THREADSTREAM
#define THREADSTREAM

#include <iostream>
#include <sstream>
#include <mutex>

#define terr ThreadStream(std::cerr)
#define tout ThreadStream(std::cout)

/**
 * Thread-safe std::ostream class.
 *
 * Usage:
 *    tout << "Hello world!" << std::endl;
 *    terr << "Hello world!" << std::endl;
 */
class ThreadStream : public std::ostringstream
{
    public:
        ThreadStream(std::ostream& os) : os_(os)
        {
            // copyfmt causes odd problems with lost output
            // probably some specific flag
//            copyfmt(os);
            // copy whatever properties are relevant
            imbue(os.getloc());
            precision(os.precision());
            width(os.width());
            setf(std::ios::fixed, std::ios::floatfield);
        }

        ~ThreadStream()
        {
            std::lock_guard<std::mutex> guard(_mutex_threadstream);
            os_ << this->str();
        }

    private:
        static std::mutex _mutex_threadstream;
        std::ostream& os_;
};

std::mutex ThreadStream::_mutex_threadstream{};

#endif

---

test.cc

#include <thread>
#include <vector>
#include <iomanip>
#include "threadstream.h"

void test(const unsigned int threadNumber)
{
    tout << "Thread " << threadNumber << ": launched" << std::endl;
}

int main()
{
    std::locale mylocale(""); // get global locale
    std::cerr.imbue(mylocale); // imbue global locale
    std::ios_base::sync_with_stdio(false); // disable synch with stdio (enables input buffering)

    std::cout << std::fixed << std::setw(4) << std::setprecision(5);
    std::cerr << std::fixed << std::setw(2) << std::setprecision(2);

    std::vector<std::thread> threads;

    for (unsigned int threadNumber = 0; threadNumber < 16; threadNumber++)
    {
        std::thread t(test, threadNumber);
        threads.push_back(std::move(t));
    }

    for (std::thread& t : threads)
    {
        if (t.joinable())
        {
            t.join();
        }
    }

    terr << std::endl << "Main: " << "Test completed." << std::endl;

    return 0;
}

---

compiling

g++ -g -O2 -Wall -c -o test.o test.cc
g++ -o test test.o -pthread

---

output

./test
Thread 0: launched
Thread 4: launched
Thread 3: launched
Thread 1: launched
Thread 2: launched
Thread 6: launched
Thread 5: launched
Thread 7: launched
Thread 8: launched
Thread 9: launched
Thread 10: launched
Thread 11: launched
Thread 12: launched
Thread 13: launched
Thread 14: launched
Thread 15: launched

Main: Test completed.

Answer 2

Here's a thread safe line based logging solution I cooked up at some point. It uses boost mutex for thread safety. It is slightly more complicated than necessary because you can plug in output policies (should it go to a file, stderr, or somewhere else?):

logger.h:

#ifndef LOGGER_20080723_H_
#define LOGGER_20080723_H_

#include <boost/thread/mutex.hpp>
#include <iostream>
#include <cassert>
#include <sstream>
#include <ctime>
#include <ostream>

namespace logger {
    namespace detail {

        template<class Ch, class Tr, class A>
        class no_output {
        private:
            struct null_buffer {
                template<class T>
                null_buffer &operator<<(const T &) {
                    return *this;
                }
            };
        public:
            typedef null_buffer stream_buffer;

        public:
            void operator()(const stream_buffer &) {
            }
        };

        template<class Ch, class Tr, class A>
        class output_to_clog {
        public:
            typedef std::basic_ostringstream<Ch, Tr, A> stream_buffer;
        public:
            void operator()(const stream_buffer &s) {
                static boost::mutex mutex;
                boost::mutex::scoped_lock lock(mutex);
                std::clog << now() << ": " << s.str() << std::endl;
            }

        private:
            static std::string now() {
                char buf[64];
                const time_t tm = time(0);  
                strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S", localtime(&tm));
                return buf;
            }

        };

        template<template <class Ch, class Tr, class A> class OutputPolicy, class Ch = char, class Tr = std::char_traits<Ch>, class A = std::allocator<Ch> >
        class logger {
            typedef OutputPolicy<Ch, Tr, A> output_policy;
        public:
            ~logger() {
                output_policy()(m_SS);
            }
        public:
            template<class T>
            logger &operator<<(const T &x) {
                m_SS << x;
                return *this;
            }
        private:
            typename output_policy::stream_buffer m_SS;
        };
    }

    class log : public detail::logger<detail::output_to_clog> {
    };
}

#endif

Usage looks like this:

logger::log() << "this is a test" << 1234 << "testing";

note the lack of a '\\n' and std::endl since it's implicit. The contents are buffered and then atomically output using the template specified policy. This implementation also prepends the line with a timestamp since it is for logging purposes. The no_output policy is stricly optional, it's what I use when I want to disable logging.

Answer 3

Why not just create a locking class and use it where ever you want to do thread-safe IO?

class LockIO
{
    static pthread_mutex_t *mutex;  
public:
    LockIO() { pthread_mutex_lock( mutex ); }
    ~LockIO() { pthread_mutex_unlock( mutex ); }
};

static pthread_mutex_t* getMutex()
{
    pthread_mutex_t *mutex = new pthread_mutex_t;
    pthread_mutex_init( mutex, NULL );
    return mutex;
}
pthread_mutex_t* LockIO::mutex = getMutex();

Then you put any IO you want in a block:

std::cout <<"X is " <<x <<std::endl;

becomes:

{
    LockIO lock;
    std::cout <<"X is " <<x <<std::endl;
}

Answer 4

This:

#define myerr(e) {CiriticalSectionLocker crit; std::cerr << e << std::endl;}

works on most compilers for the common case of myerr("ERR: " << message << number) .

Answer 5

An improvement (that doesn't really fit in a comment) on the approach in unixman's comment.

#define LOCKED_ERR \
    if(ErrCriticalSectionLocker crit = ErrCriticalSectionLocker()); \
    else std::cerr

Which can be used like

LOCKED_ERR << "ERR: " << message << endl;

if ErrCriticalSectionLocker is implemented carefully.

But, I would personally prefer Ken's suggestion.