defmt ("de format", short for "deferred formatting") is a highly efficient logging framework that targets resource-constrained devices, like microcontrollers.


  • println!-like formatting
  • Multiple logging levels: error, info, warn, debug, trace
  • Compile-time RUST_LOG-like filtering of logs: include/omit logging levels with module-level granularity
  • Timestamped logs

Current limitations

  • Output object format must be ELF
  • Custom linking (linker script) is required
  • Single, global logger instance (but using multiple channels is possible)

Intended use

In its current iteration defmt mainly targets tiny embedded devices that have no mean to display information to the developer, e.g. a screen. In this scenario logs need to be transferred to a second machine, usually a PC/laptop, before they can be displayed to the developer/end-user.

defmt operating principles, however, are applicable to beefier machines and could be use to improve the logging performance of x86 web server applications and the like.

Operating principle

defmt achieves high performance using deferred formatting and string compression.

Deferred formatting means that formatting is not done on the machine that's logging data but on a second machine. That is, instead of formatting 255u8 into "255" and sending the string, the single-byte binary data is sent to a second machine, the host, and the formatting happens there.

defmt's string compression consists of building a table of string literals, like "Hello, world" or "The answer is {:?}", at compile time. At runtime the logging machine sends indices instead of complete strings.


defmt is part of the Knurling project, Ferrous Systems' effort at improving tooling used to develop for embedded systems.

If you think that our work is useful, consider sponsoring it via GitHub Sponsors.

commit aa731a3 on 2024-05-13 17:49:16