Semihosting

Semihosting lets firmware call into the debugger: print to the host console, and exit with a status code. It is how defmt-semihosting, cortex-m-semihosting's hprintln!, and test harnesses report results without a UART or RTT.

While monitoring

session.monitor services semihosting requests and reports them as events. A SYS_EXIT from the firmware ends the monitor:

const exit = await session.monitor(boot, (event) => {
  if (event.kind === 'semihosting') console.log(`[${event.stream}] ${event.data}`); // stdout or stderr
});
// exit: { SemihostingExit: { Ok: null } } or { SemihostingExit: { Err: … } } when the firmware exits

A firmware without an RTT control block does not need an RTT client. Skip createRttClient for it (elfHasRtt(elf) tells you), so the monitor does not scan RAM for a block that is not there.

<probe-semihosting-console> shows the output and the exit status. Point its source at an <probe-rtt-terminal>, which runs the monitor.

While debugging

A Debugger services semihosting halts as they happen and resumes the core, so they do not show up as stops. Output arrives as output events:

dbg.addEventListener('output', (e) => {
  const out = (e as CustomEvent).detail;
  if (out.source === 'semihosting') console.log(out.text);
});

An exit is reported as a stop, with the halt reason naming the exit.

In the browser

Over WebUSB the worker implements console semihosting: opening :tt (stdout and stderr), writing to it, and exit codes. A browser has no host filesystem, so file operations are refused. Through probe-rs serve, semihosting behaves as it does natively.