FmtLog

Send a log line where you need it: the library writes nowhere by itself, the sink is supplied by the application, and one message can go to several at once - the serial port while debugging, the network on a working device, a memory card for post-mortem. Levels are set per source, so a chatty part can be quieted without touching the rest, and the check happens before the message is built. Critical and system levels are never suppressed. Ready-made sinks: serial port, coloured terminal output, telnet with history, a web page fed over SSE, SNMP traps for a monitoring station, and Loki for searching logs in Grafana. Telnet and the web page keep their own place per connection, so a client joining late sees the same depth as the first one. Pattern formatting comes from the FmtTiny library.