Reckless logging. Low-latency, high-throughput, asynchronous logging library for C++.
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mattiasflodin Fix build error with glibc 2.26 and above
SIGUNUSED has been removed and causes compilation errors. Based on
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Latest commit c1d0932 Dec 31, 2018

Performance chart for a quad-core CPU


Reckless is an extremely low-latency, high-throughput logging library. It was created because I needed to perform extensive diagnostic logging without worrying about performance. Other logging libraries boast the ability to throw log messages away very quickly. Reckless boasts the ability to keep them all, without worrying about the performance impact. Filtering can and should wait until you want to read the log, or need to clean up disk space.

How it works

By low latency I mean that the time from calling the library and returning to the caller is as short as I could make it. The code generated at the call site consists of

  1. Pushing the arguments on a thread-local queue. This has the same cost as pushing the arguments on the stack for a normal function call.
  2. Call to Boost.Lockless (NB: this is bundled with the library, not an external dependency) to register the write request on a shared lockless queue.

The actual message formatting and writing is performed asynchronously by a separate thread. This removes or hides several costs:

  • No transition to the kernel at the call site. The kernel is an easily overlooked but important cost, not only because the transition costs time, but because it pollutes the CPU cache. In other words, avoiding this makes your non-logging code run faster than if you were using a library that has to enter the kernel to perform its work.
  • No locks need to be taken for synchronization between threads (unless the queue fills up; see the performance article for more information about the implications of this).
  • It doesn't have to wait for the actual I/O operation to complete.
  • If there are bursts of log calls, multiple items on the queue can be batched into a single I/O operation, improving throughput without sacrificing write latency.

For a more detailed performance discussion and statistics, see the performance article.

What's the catch?

As all string formatting and I/O is done asynchronously and in a single thread, there are a few caveats you need to be aware of:

  • If you choose to pass log arguments by reference or pointer, then you must ensure that the referenced data remains valid at least until the log has been flushed or closed (unless you're only interested in logging the value of the pointer itself). The best option for dynamically allocated data is typically std::string, std::shared_ptr or std::unique_ptr.
  • You must take special care to handle crashes if you want to make sure that all log data prior to the crash is saved. This is not unique to asynchronous logging—for example fprintf will buffer data until you flush it—but asynchronous logging arguably makes the issue worse. The library provides convenience functions to aid with this.
  • As all string formatting is done in a single thread, it could theoretically limit the scalability of your application if the formatting is very expensive.
  • Performance becomes somewhat less predictable and harder to measure. Rather than putting the cost of the logging on the thread that calls the logging library, the OS may suspend some other thread to make room for the logging thread to run.

Basic use

#include <reckless/severity_log.hpp>
#include <reckless/file_writer.hpp>

// It is possible to build custom loggers for various ways of formatting the
// log. The severity log is a stock policy-based logger that allows you to
// configure fields that should be put on each line, including a severity
// marker for debug/info/warning/error.
using log_t = reckless::severity_log<
    reckless::indent<4>,       // 4 spaces of indent
    ' ',                       // Field separator
    reckless::severity_field,  // Show severity marker (D/I/W/E) first
    reckless::timestamp_field  // Then timestamp field

reckless::file_writer writer("log.txt");
log_t g_log(&writer);

int main()
    std::string s("Hello World!");

    // You can use ordinary printf-style syntax, but unlike stdio this
    // is type-safe and extensible.
    g_log.debug("Pointer: %p", s.c_str());"Info line: %s", s);

    for(int i=0; i!=4; ++i) {
        reckless::scoped_indent indent;  // The indent object causes the lines
        g_log.warn("Warning: %d", i);    // within this scope to be indented.

    g_log.error("Error: %f", 3.14);

    return 0;

This would give the following output:

D 2015-03-29 13:23:35.288  Pointer: 0x1e18218
I 2015-03-29 13:23:35.288  Info line: Hello World!
W 2015-03-29 13:23:35.288      Warning: 0
W 2015-03-29 13:23:35.288      Warning: 1
W 2015-03-29 13:23:35.288      Warning: 2
W 2015-03-29 13:23:35.288      Warning: 3
E 2015-03-29 13:23:35.288  Error: 3.140000


The library currently works only on Linux. Windows and BSD are on the roadmap. I don't own any Apple computers, so OS X won't happen unless someone sends me a patch or buys me hardware.


Alternative 1: using Make

To build the library using GNU Make, clone the git repository and run make.

To build a program against the library, given the variable RECKLESS pointing to the reckless root directory, use:

g++ -std=c++11 myprogram.cpp -I$(RECKLESS)/boost -I$(RECKLESS)/reckless/include -L$(RECKLESS)/reckless/lib -lreckless -lpthread

Alternative 2: using CMake

To build the library using CMake, clone the git repository and run the following commands:

mkdir build; cd build
cmake ..

To build a program against this library using CMake, add the following line to your program's CMakeLists.txt:


Subsequently, to link this library to a program (e.g. your_executable), add the following to your program's CMakeLists.txt:

target_link_libraries(your_executable reckless pthread)

More information

For more details, see the manual.


Two other logging libraries with a similar, asynchronous design are spdlog and g2log.