spdlog Knowledge Base

spdlog is a very fast, header-only-by-default C++ logging library built on top of fmt. You add one include, call spdlog::info(...), and you have color console output — no build step, no configuration file, no init call required.

It displaced glog, log4cplus, and hand-rolled iostream/macro logging as the default choice for new C++ projects for a simple reason: it is both easier to start with and faster in production. Throughput is measured in millions of lines per second, formatting uses fmt's compile-time-checked {} syntax instead of printf, and every layer above the basics — file rotation, async logging, backtraces, custom sinks — is opt-in rather than mandatory ceremony.

How this is organised

Overview → Basics gets you logging in five minutes. Loggers and Registry and Sinks are the architecture you configure once per application — how loggers, sinks, and the global registry fit together. Formatting, Async Logging, and Performance and Configuration are the tuning layers you reach for once the defaults stop fitting: custom pattern flags, moving I/O off the hot path, and squeezing out the last bit of overhead.

Sections

	Section	What it covers
	Overview	What it is, installing it, the sink architecture, how it compares to glog/Boost.Log
	Basics	The default logger, log levels, fmt-style formatting
	Loggers & Registry	Creating loggers, the global registry, lifetime, multi-sink loggers
	Sinks	Console, file, rotating, daily, syslog, and writing your own
	Formatting & Patterns	Pattern flags, custom flag formatters, source location
	Async Logging	The thread pool, overflow policies, when async pays off
	Performance & Configuration	Compile-time levels, backtrace, flush policies, global setup

Suggested reading paths

• Just want logs in my app: Quick start → Log levels → Console sinks.
• Setting up logging for a real service: Creating loggers → Multi-sink loggers → Rotating and daily sinks → Flush policies.
• Logging is showing up in the profile: Compile-time log level → Async vs sync → Overflow policies.

Quick reference

the 90% of the API

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

spdlog::info("plain message");
spdlog::warn("formatted: {} of {}", done, total);      // fmt syntax

auto file = spdlog::rotating_logger_mt(
    "app", "logs/app.log", 1024 * 1024 * 5, 3);        // 5 MB x 3 files
file->set_level(spdlog::level::debug);
file->set_pattern("[%Y-%m-%d %H:%M:%S.%e] [%n] [%l] %v");
file->flush_on(spdlog::level::err);

spdlog::set_default_logger(file);                      // spdlog::info now goes here
spdlog::shutdown();                                    // at exit

Task	Call
Log at a level	spdlog::trace/debug/info/warn/error/critical(...)
Named console logger	spdlog::stdout_color_mt("name")
Named file logger	spdlog::basic_logger_mt("name", "path.log")
Fetch by name	spdlog::get("name")
Runtime level	logger->set_level(spdlog::level::debug)
Compile-time level	-DSPDLOG_ACTIVE_LEVEL=SPDLOG_LEVEL_DEBUG
Pattern	logger->set_pattern("%+")
Flush	logger->flush() / flush_on(level)