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Very fast, header-only/compiled, C++ logging library. ci  Build status Release


Header-only version

Copy the include folder to your build tree and use a C++11 compiler.

Compiled version (recommended - much faster compile times)

$ git clone
$ cd spdlog && mkdir build && cd build
$ cmake .. && make -j

see example CMakeLists.txt on how to use.


  • Linux, FreeBSD, OpenBSD, Solaris, AIX
  • Windows (msvc 2013+, cygwin)
  • macOS (clang 3.5+)
  • Android

Package managers:

  • Debian: sudo apt install libspdlog-dev
  • Homebrew: brew install spdlog
  • MacPorts: sudo port install spdlog
  • FreeBSD: pkg install spdlog
  • Fedora: dnf install spdlog
  • Gentoo: emerge dev-libs/spdlog
  • Arch Linux: pacman -S spdlog
  • openSUSE: sudo zypper in spdlog-devel
  • vcpkg: vcpkg install spdlog
  • conan: spdlog/[>=1.4.1]
  • conda: conda install -c conda-forge spdlog
  • build2: depends: spdlog ^1.8.2


  • Very fast (see benchmarks below).
  • Headers only or compiled
  • Feature-rich formatting, using the excellent fmt library.
  • Asynchronous mode (optional)
  • Custom formatting.
  • Multi/Single threaded loggers.
  • Various log targets:
    • Rotating log files.
    • Daily log files.
    • Console logging (colors supported).
    • syslog.
    • Windows event log.
    • Windows debugger (OutputDebugString(..)).
    • Log to Qt widgets (example).
    • Easily extendable with custom log targets.
  • Log filtering - log levels can be modified at runtime as well as compile time.
  • Support for loading log levels from argv or environment var.
  • Backtrace support - store debug messages in a ring buffer and display them later on demand.

Usage samples

Basic usage

#include "spdlog/spdlog.h"

int main() 
    spdlog::info("Welcome to spdlog!");
    spdlog::error("Some error message with arg: {}", 1);
    spdlog::warn("Easy padding in numbers like {:08d}", 12);
    spdlog::critical("Support for int: {0:d};  hex: {0:x};  oct: {0:o}; bin: {0:b}", 42);
    spdlog::info("Support for floats {:03.2f}", 1.23456);
    spdlog::info("Positional args are {1} {0}..", "too", "supported");
    spdlog::info("{:<30}", "left aligned");
    spdlog::set_level(spdlog::level::debug); // Set global log level to debug
    spdlog::debug("This message should be displayed..");    
    // change log pattern
    spdlog::set_pattern("[%H:%M:%S %z] [%n] [%^---%L---%$] [thread %t] %v");
    // Compile time log levels
    // Note that this does not change the current log level, it will only
    // remove (depending on SPDLOG_ACTIVE_LEVEL) the call on the release code.
    SPDLOG_TRACE("Some trace message with param {}", 42);
    SPDLOG_DEBUG("Some debug message");

Create stdout/stderr logger object

#include "spdlog/spdlog.h"
#include "spdlog/sinks/stdout_color_sinks.h"
void stdout_example()
    // create a color multi-threaded logger
    auto console = spdlog::stdout_color_mt("console");    
    auto err_logger = spdlog::stderr_color_mt("stderr");    
    spdlog::get("console")->info("loggers can be retrieved from a global registry using the spdlog::get(logger_name)");

Basic file logger

#include "spdlog/sinks/basic_file_sink.h"
void basic_logfile_example()
        auto logger = spdlog::basic_logger_mt("basic_logger", "logs/basic-log.txt");
    catch (const spdlog::spdlog_ex &ex)
        std::cout << "Log init failed: " << ex.what() << std::endl;

Rotating files

#include "spdlog/sinks/rotating_file_sink.h"
void rotating_example()
    // Create a file rotating logger with 5 MB size max and 3 rotated files
    auto max_size = 1048576 * 5;
    auto max_files = 3;
    auto logger = spdlog::rotating_logger_mt("some_logger_name", "logs/rotating.txt", max_size, max_files);

Daily files

#include "spdlog/sinks/daily_file_sink.h"
void daily_example()
    // Create a daily logger - a new file is created every day at 2:30 am
    auto logger = spdlog::daily_logger_mt("daily_logger", "logs/daily.txt", 2, 30);

Backtrace support

// Debug messages can be stored in a ring buffer instead of being logged immediately.
// This is useful to display debug logs only when needed (e.g. when an error happens).
// When needed, call dump_backtrace() to dump them to your log.

spdlog::enable_backtrace(32); // Store the latest 32 messages in a buffer. 
// or my_logger->enable_backtrace(32)..
for(int i = 0; i < 100; i++)
  spdlog::debug("Backtrace message {}", i); // not logged yet..
// e.g. if some error happened:
spdlog::dump_backtrace(); // log them now! show the last 32 messages
// or my_logger->dump_backtrace(32)..

Periodic flush

// periodically flush all *registered* loggers every 3 seconds:
// warning: only use if all your loggers are thread-safe ("_mt" loggers)


// Stopwatch support for spdlog
#include "spdlog/stopwatch.h"
void stopwatch_example()
    spdlog::stopwatch sw;    
    spdlog::debug("Elapsed {}", sw);
    spdlog::debug("Elapsed {:.3}", sw);       

Log binary data in hex

// many types of std::container<char> types can be used.
// ranges are supported too.
// format flags:
// {:X} - print in uppercase.
// {:s} - don't separate each byte with space.
// {:p} - don't print the position on each line start.
// {:n} - don't split the output into lines.
// {:a} - show ASCII if :n is not set.

#include "spdlog/fmt/bin_to_hex.h"

void binary_example()
    auto console = spdlog::get("console");
    std::array<char, 80> buf;
    console->info("Binary example: {}", spdlog::to_hex(buf));
    console->info("Another binary example:{:n}", spdlog::to_hex(std::begin(buf), std::begin(buf) + 10));
    // more examples:
    // logger->info("uppercase: {:X}", spdlog::to_hex(buf));
    // logger->info("uppercase, no delimiters: {:Xs}", spdlog::to_hex(buf));
    // logger->info("uppercase, no delimiters, no position info: {:Xsp}", spdlog::to_hex(buf));

Logger with multi sinks - each with a different format and log level

// create a logger with 2 targets, with different log levels and formats.
// The console will show only warnings or errors, while the file will log all.
void multi_sink_example()
    auto console_sink = std::make_shared<spdlog::sinks::stdout_color_sink_mt>();
    console_sink->set_pattern("[multi_sink_example] [%^%l%$] %v");

    auto file_sink = std::make_shared<spdlog::sinks::basic_file_sink_mt>("logs/multisink.txt", true);

    spdlog::logger logger("multi_sink", {console_sink, file_sink});
    logger.warn("this should appear in both console and file");"this message should not appear in the console, only in the file");

User-defined callbacks about log events

// create a logger with a lambda function callback, the callback will be called
// each time something is logged to the logger
void callback_example()
    auto callback_sink = std::make_shared<spdlog::sinks::callback_sink_mt>([](const spdlog::details::log_msg &msg) {
         // for example you can be notified by sending an email to yourself

    auto console_sink = std::make_shared<spdlog::sinks::stdout_color_sink_mt>();
    spdlog::logger logger("custom_callback_logger", {console_sink, callback_sink});"some info log");
    logger.error("critical issue"); // will notify you

Asynchronous logging

#include "spdlog/async.h"
#include "spdlog/sinks/basic_file_sink.h"
void async_example()
    // default thread pool settings can be modified *before* creating the async logger:
    // spdlog::init_thread_pool(8192, 1); // queue with 8k items and 1 backing thread.
    auto async_file = spdlog::basic_logger_mt<spdlog::async_factory>("async_file_logger", "logs/async_log.txt");
    // alternatively:
    // auto async_file = spdlog::create_async<spdlog::sinks::basic_file_sink_mt>("async_file_logger", "logs/async_log.txt");   

Asynchronous logger with multi sinks

#include "spdlog/async.h"
#include "spdlog/sinks/stdout_color_sinks.h"
#include "spdlog/sinks/rotating_file_sink.h"

void multi_sink_example2()
    spdlog::init_thread_pool(8192, 1);
    auto stdout_sink = std::make_shared<spdlog::sinks::stdout_color_sink_mt >();
    auto rotating_sink = std::make_shared<spdlog::sinks::rotating_file_sink_mt>("mylog.txt", 1024*1024*10, 3);
    std::vector<spdlog::sink_ptr> sinks {stdout_sink, rotating_sink};
    auto logger = std::make_shared<spdlog::async_logger>("loggername", sinks.begin(), sinks.end(), spdlog::thread_pool(), spdlog::async_overflow_policy::block);

User-defined types

struct fmt::formatter<my_type> : fmt::formatter<std::string>
    auto format(my_type my, format_context &ctx) const -> decltype(ctx.out())
        return format_to(ctx.out(), "[my_type i={}]", my.i);

void user_defined_example()
    spdlog::info("user defined type: {}", my_type(14));

User-defined flags in the log pattern

// Log patterns can contain custom flags.
// the following example will add new flag '%*' - which will be bound to a <my_formatter_flag> instance.
#include "spdlog/pattern_formatter.h"
class my_formatter_flag : public spdlog::custom_flag_formatter
    void format(const spdlog::details::log_msg &, const std::tm &, spdlog::memory_buf_t &dest) override
        std::string some_txt = "custom-flag";
        dest.append(, + some_txt.size());

    std::unique_ptr<custom_flag_formatter> clone() const override
        return spdlog::details::make_unique<my_formatter_flag>();

void custom_flags_example()
    auto formatter = std::make_unique<spdlog::pattern_formatter>();
    formatter->add_flag<my_formatter_flag>('*').set_pattern("[%n] [%*] [%^%l%$] %v");

Custom error handler

void err_handler_example()
    // can be set globally or per logger(logger->set_error_handler(..))
    spdlog::set_error_handler([](const std::string &msg) { spdlog::get("console")->error("*** LOGGER ERROR ***: {}", msg); });
    spdlog::get("console")->info("some invalid message to trigger an error {}{}{}{}", 3);


#include "spdlog/sinks/syslog_sink.h"
void syslog_example()
    std::string ident = "spdlog-example";
    auto syslog_logger = spdlog::syslog_logger_mt("syslog", ident, LOG_PID);
    syslog_logger->warn("This is warning that will end up in syslog.");

Android example

#include "spdlog/sinks/android_sink.h"
void android_example()
    std::string tag = "spdlog-android";
    auto android_logger = spdlog::android_logger_mt("android", tag);
    android_logger->critical("Use \"adb shell logcat\" to view this message.");

Load log levels from the env variable or argv

#include "spdlog/cfg/env.h"
int main (int argc, char *argv[])
    // or from the command line:
    // ./example SPDLOG_LEVEL=info,mylogger=trace
    // #include "spdlog/cfg/argv.h" // for loading levels from argv
    // spdlog::cfg::load_argv_levels(argc, argv);

So then you can:

$ export SPDLOG_LEVEL=info,mylogger=trace
$ ./example

Log file open/close event handlers

// You can get callbacks from spdlog before/after a log file has been opened or closed. 
// This is useful for cleanup procedures or for adding something to the start/end of the log file.
void file_events_example()
    // pass the spdlog::file_event_handlers to file sinks for open/close log file notifications
    spdlog::file_event_handlers handlers;
    handlers.before_open = [](spdlog::filename_t filename) { spdlog::info("Before opening {}", filename); };
    handlers.after_open = [](spdlog::filename_t filename, std::FILE *fstream) { fputs("After opening\n", fstream); };
    handlers.before_close = [](spdlog::filename_t filename, std::FILE *fstream) { fputs("Before closing\n", fstream); };
    handlers.after_close = [](spdlog::filename_t filename) { spdlog::info("After closing {}", filename); };
    auto my_logger = spdlog::basic_logger_st("some_logger", "logs/events-sample.txt", true, handlers);        

Replace the Default Logger

void replace_default_logger_example()
    auto new_logger = spdlog::basic_logger_mt("new_default_logger", "logs/new-default-log.txt", true);
    spdlog::info("new logger log message");

Log to Qt with nice colors

#include "spdlog/spdlog.h"
#include "spdlog/sinks/qt_sinks.h"
MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
    setMinimumSize(640, 480);
    auto log_widget = new QTextEdit(this);
    int max_lines = 500; // keep the text widget to max 500 lines. remove old lines if needed.
    auto logger = spdlog::qt_color_logger_mt("qt_logger", log_widget, max_lines);
    logger->info("Some info message");

Mapped Diagnostic Context

// Mapped Diagnostic Context (MDC) is a map that stores key-value pairs (string values) in thread local storage.
// Each thread maintains its own MDC, which loggers use to append diagnostic information to log outputs.
// Note: it is not supported in asynchronous mode due to its reliance on thread-local storage.
#include "spdlog/mdc.h"
void mdc_example()
    spdlog::mdc::put("key1", "value1");
    spdlog::mdc::put("key2", "value2");
    // if not using the default format, use the %& formatter to print mdc data
    // spdlog::set_pattern("[%H:%M:%S %z] [%^%L%$] [%&] %v");


Below are some benchmarks done in Ubuntu 64 bit, Intel i7-4770 CPU @ 3.40GHz

Synchronous mode

[info] **************************************************************
[info] Single thread, 1,000,000 iterations
[info] **************************************************************
[info] basic_st         Elapsed: 0.17 secs        5,777,626/sec
[info] rotating_st      Elapsed: 0.18 secs        5,475,894/sec
[info] daily_st         Elapsed: 0.20 secs        5,062,659/sec
[info] empty_logger     Elapsed: 0.07 secs       14,127,300/sec
[info] **************************************************************
[info] C-string (400 bytes). Single thread, 1,000,000 iterations
[info] **************************************************************
[info] basic_st         Elapsed: 0.41 secs        2,412,483/sec
[info] rotating_st      Elapsed: 0.72 secs        1,389,196/sec
[info] daily_st         Elapsed: 0.42 secs        2,393,298/sec
[info] null_st          Elapsed: 0.04 secs       27,446,957/sec
[info] **************************************************************
[info] 10 threads, competing over the same logger object, 1,000,000 iterations
[info] **************************************************************
[info] basic_mt         Elapsed: 0.60 secs        1,659,613/sec
[info] rotating_mt      Elapsed: 0.62 secs        1,612,493/sec
[info] daily_mt         Elapsed: 0.61 secs        1,638,305/sec
[info] null_mt          Elapsed: 0.16 secs        6,272,758/sec

Asynchronous mode

[info] -------------------------------------------------
[info] Messages     : 1,000,000
[info] Threads      : 10
[info] Queue        : 8,192 slots
[info] Queue memory : 8,192 x 272 = 2,176 KB 
[info] -------------------------------------------------
[info] *********************************
[info] Queue Overflow Policy: block
[info] *********************************
[info] Elapsed: 1.70784 secs     585,535/sec
[info] Elapsed: 1.69805 secs     588,910/sec
[info] Elapsed: 1.7026 secs      587,337/sec
[info] *********************************
[info] Queue Overflow Policy: overrun
[info] *********************************
[info] Elapsed: 0.372816 secs    2,682,285/sec
[info] Elapsed: 0.379758 secs    2,633,255/sec
[info] Elapsed: 0.373532 secs    2,677,147/sec


Documentation can be found in the wiki pages.

Thanks to JetBrains for donating product licenses to help develop spdlog