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fmtlog is a high-performance, asynchronous C++ logging library that uses fmt-style formatting with nanosecond-level latency. It supports both single-threaded synchronous and multi-threaded asynchronous modes, with customizable output to files or callbacks.
Key facts
Objective fields from the source. Values we can't verify are shown as “Unknown” rather than guessed.
| Field | Value |
|---|---|
| Repository | MengRao/fmtlog |
| Owner | MengRao |
| Primary language | C++ |
| License | MIT — OSI-approved |
| Stars | 1k |
| Forks | 155 |
| Open issues | 37 |
| Latest release | v2.3.0 (2025-02-13) |
| Last updated | 2025-02-13 |
| Source | https://github.com/MengRao/fmtlog |
What fmtlog is
fmtlog is a header-only or compiled C++17 library providing asynchronous multi-threaded logging with fmt-compatible formatting, in-order message delivery, pointer argument support (including shared_ptr/unique_ptr), runtime log-level filtering, and configurable flush policies (buffer size, delay, or log level). Dependencies: fmtlib.
Get the fmtlog source
Clone the repository and explore it locally.
git clone https://github.com/MengRao/fmtlog.gitcd fmtlog# follow the project's README for install & configurationNeed it deployed, integrated, or customized instead? DEV.co ships production installs.
Best use cases
Implementation considerations
- Requires explicit fmtlog::poll() calls or startPollingThread(interval); design application polling/flushing strategy upfront to avoid silent log loss.
- Pointer arguments (raw, shared_ptr, unique_ptr) require careful lifetime management; dangling pointers in async context are undefined behavior.
- Single polling thread enforced; use external synchronization if multiple threads need to drive log flushing.
- Default flush delay is 3 seconds; tune setFlushDelay() and setFlushBufSize() for acceptable log staleness in production.
- Header-only and compiled library versions available; choose based on build system and linking preferences.
When to avoid it — and what to weigh
- Requires immediate log visibility — Asynchronous design means logs are queued; explicit poll() calls or background thread required. Not suitable if logs must be flushed immediately after every statement.
- Multi-consumer polling scenario — Library enforces single-threaded polling; if your architecture requires multiple threads calling poll(), you must serialize externally or use an alternative.
- Color/ANSI terminal output needed — README explicitly states color output is not supported (note: fmtlib supports it, but fmtlog does not).
- Non-C++17 legacy codebase — Requires C++17 minimum; integration into older C++ projects will require language upgrade.
License & commercial use
MIT License permits commercial and proprietary use with attribution. No copyleft obligations.
MIT is a permissive OSI-approved license. Commercial use, modification, and distribution allowed provided original license and copyright notice are retained. No patent grant or warranty implied; review your own risk tolerance and obtain legal review if required.
DEV.co evaluation signals
Editorial assessment — not user reviews. Directional, with an explicit confidence level.
| Signal | Assessment |
|---|---|
| Maintenance | Active |
| Documentation | Adequate |
| License clarity | Clear |
| Deployment complexity | Low |
| DEV.co fit | Good |
| Assessment confidence | High |
No security audit data provided. Asynchronous queue-based design mitigates some DoS vectors from blocking I/O. Pointer argument feature introduces use-after-free risk if caller is careless with object lifetimes; review code using pointer args carefully. Callback mechanism allows external handlers; validate callback implementations. No encryption, authentication, or access control features; treat logs as plaintext.
Alternatives to consider
spdlog
Mature, widely-adopted C++ logging library with similar async support, more color/format options, and extensive ecosystem integrations; fmtlog claims higher throughput in benchmarks (not independently verified).
NanoLog
Focus on ultra-low latency via compile-time format string encoding; fmtlog claims lower latency than NanoLog in benchmarks (not independently verified); less actively maintained.
Boost.Log
Mature, battle-tested C++ logging framework with rich filtering, sinks, and attributes; heavier dependency and more complex API; higher adoption in legacy enterprise codebases.
Build on fmtlog with DEV.co software developers
fmtlog's nanosecond-level latency and asynchronous design make it ideal for performance-critical systems. Review the technical considerations above and test in your environment. Contact Devco for guidance on integration or to assess alternatives suited to your architecture.
Talk to DEV.coRelated open-source tools
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Related on DEV.co
Explore the category and the services that help you build with it.
fmtlog FAQ
Does fmtlog block the calling thread?
Can I use fmtlog in multiple threads simultaneously?
What happens if I don't call poll()?
Is fmtlog suitable for embedded systems?
Custom software development services
Bringing fmtlog into production usually means new services, new interfaces, and new failure modes. DEV.co is a software development agency that builds all three and stays on for the open-source observability maintenance.
Evaluate fmtlog for Your Low-Latency Logging Needs
fmtlog's nanosecond-level latency and asynchronous design make it ideal for performance-critical systems. Review the technical considerations above and test in your environment. Contact Devco for guidance on integration or to assess alternatives suited to your architecture.