ALPHA PREVIEW defmt wire format has not been finalized yet. When using the framework make sure you use the same "version" (commit hash) for all components (target side and host side).

defmt is a highly efficient logging framework that targets resource-constrained devices, like microcontrollers.


  • println!-like formatting
  • Multiple logging levels: error, info, warn, debug, trace
  • Crate-level logging level filters
  • 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)
  • The @ character is not allowed in strings but otherwise UTF-8 is supported 👍
  • No x86 support. The x86 architecture is exclusively used for testing at the moment.

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.