G3log : Asynchronous logger with Dynamic Sinks


Optional to use either streaming or printf-like syntax

LOG(INFO) << "streaming API is as easy as ABC or " << 123;

LOGF(WARNING, "Printf-style syntax is also %s", "available");

Conditional logging

int less = 1; int more = 2
LOG_IF(INFO, (less<more)) <<"If [true], then this text will be logged";

// or with printf-like syntax
LOGF_IF(INFO, (less<more), "if %d<%d then this text will be logged", less,more);


CHECK(false) will trigger a "fatal" message. It will be logged, and then the application will exit.

CHECK(less != more); // not FATAL
CHECK(less > more) << "CHECK(false) triggers a FATAL message";

What G3Log is:

  • G3log is the acting name for the third version of g2log and it stands for g2log with dynamic sinks
  • G3log is an asynchronous, "crash-safe" logger. You can read more about it here [g2log version]
  • You can choose to use the default log receiver which saves all LOG calls to file, or you can choose to use your own custom made log receiver(s), or both, or as many sinks as you need.

Benefits you get when using G3log

  1. Easy to use, clean syntax and a blazing fast logger.

  2. All the slow log I/O disk access is done in a background thread. This ensures that the LOG caller can immediately continue with other tasks and do not have to wait for the LOG call to finish.

  3. G3log provides logging, Design-by-Contract [#CHECK], and flush of log to file at shutdown. Buffered logs will be written to the sink before the application shuts down.

  4. It is thread safe, so using it from multiple threads is completely fine.

  5. It is CRASH SAFE. It will save the made logs to the sink before it shuts down. The logger will catch certain fatal signals, so if your application crashes due to, say a segmentation fault, SIGSEGV, or some other fatal signal it will log and save the crash and all previously buffered log entries before exiting.

  6. It is cross platform. Tested and used by me or by clients on OSX, Windows, Ubuntu, CentOS

  7. On Nix systems a caught fatal signal will generate a stack dump to the log. A Beta version exist on Windows and can be released on request.

  8. G2log is used world wide in commercial products as well as hobby projects since early 2011. The code is given for free as public domain. This gives the option to change, use, and do whatever with it, no strings attached.

  9. Three versions of g2log exist.

    • This version: g3log : which is made to facilitate easy adding of custom log receivers. Its tested on at least the following platforms with Linux(Clang/gcc), Windows (mingw, visual studio 2013)
    • g2log: The original. Simple, easy to modify and with the most OS support. Clients use g2log on environments such as OSX/Clang, Ubuntu, CentOS, Windows/mingw, Windows/Visual Studio.
    • g2log-dev*: Acting as feature try-out and playground.

G3log with sinks

Sinks are receivers of LOG calls. G3log comes with a default sink (the same as G2log uses) that can be used to save log to file. A sink can be of any class type without restrictions as long as it can either receive a LOG message as a std::string or as a g2::LogMessageMover.

The std::string comes pre-formatted. The g2::LogMessageMover is a wrapped struct that contains the raw data for custom handling in your own sink.

A sink is owned by the G3log and is added to the logger inside a std::unique_ptr. The sink can be called though its public API through a handler which will asynchronously forward the call to the receiving sink.

auto sinkHandle = logworker->addSink(std2::make_unique<CustomSink>(),

Code Examples

Example usage where a custom sink is added. A function is called though the sink handler to the actual sink object.

// main.cpp
#include <std2_make_unique.hpp>

#include "CustomSink.h"

int main(int argc, char**argv) {
   using namespace g2;
   std::unique_ptr<LogWorker> logworker{ LogWorker::createWithNoSink() };
   auto sinkHandle = logworker->addSink(std2::make_unique<CustomSink>(),

   // initialize the logger before it can receive LOG calls
   LOG(WARNING) << "This log call, may or may not happend before"
                << "the sinkHandle->call below";

   // You can call in a thread safe manner public functions on your sink
   // The call is asynchronously executed on your custom sink.
   std::future<void> received = sinkHandle->call(&CustomSink::Foo, 
                                                 param1, param2);

   // before exiting you can always call g2::ShutdownLogging to avoid
   // LOG calls from static entities that goes out of scope later.

// some_file.cpp : To show how easy it is to get the logger to work
// in other parts of your software

#include <g2log.hpp>

void SomeFunction() {
   LOG(INFO) << "Hello World";

Example usage where a the default file logger is used and a custom sink is added

// main.cpp
#include <std2_make_unique.hpp>

#include "CustomSink.h"

int main(int argc, char**argv) {
   using namespace g2;
   auto defaultHandler = LogWorker::createWithDefaultLogger(argv[0], 

   // logger is initialized

   LOG(DEBUG) << "Make log call, then add another sink";




The default is to build an example binary 'g2log-FATAL' I suggest you start with that, run it and view the created log also.

If you are interested in the performance or unit tests then you can enable the creation of them in the g2log/CMakeLists.txt file. See that file for more details

cd g2log
mkdir build
cd build

Building on Linux

cmake -DCMAKE_BUILD_TYPE=Release ..

Building on Windows Please use the Visual Studio 12 (2013) command prompt "Developer command prompt"

cmake -DCMAKE_BUILD_TYPE=Release -G "Visual Studio 12" ..
msbuild g2log_by_kjellkod.sln /p:Configuration=Release

Building on *nix with Clang:

cmake -DCMAKE_CXX_COMPILER=clang++ .. -DCMAKE_BUILD_TYPE=Release ..


If you like this logger (or not) it would be nice with some feedback. That way I can improve g3log and g2log and it is also nice to see if someone is using it.

If you have ANY questions or problems please do not hesitate in contacting me on my blog or at <Hedstrom at KjellKod dot cc>

Good luck :) Cheers Kjell (a.k.a. KjellKod)