go-delve/delve · error

malformed qfThreadInfo response

Error message

malformed qfThreadInfo response

What it means

When enumerating target threads via the GDB RSP qfThreadInfo packet, the first character of the reply must be 'l' (end of list) or 'm' (more thread ids follow). Any other first byte means the stub's response cannot be parsed, so the connection returns this error.

Source

Thrown at pkg/proc/gdbserial/gdbserver_conn.go:962

	conn.outbuf.Reset()
	if first {
		conn.outbuf.WriteString("$qfThreadInfo")
	} else {
		conn.outbuf.WriteString("$qsThreadInfo")
	}

	resp, err := conn.exec(conn.outbuf.Bytes(), "thread info")
	if err != nil {
		return nil, err
	}

	switch resp[0] {
	case 'l':
		return nil, nil
	case 'm':
		// parse list...
	default:
		return nil, errors.New("malformed qfThreadInfo response")
	}

	var pid int
	resp = resp[1:]
	for {
		tidbuf := resp
		comma := bytes.Index(tidbuf, []byte{','})
		if comma >= 0 {
			tidbuf = tidbuf[:comma]
		}
		if conn.multiprocess && pid == 0 {
			dot := bytes.Index(tidbuf, []byte{'.'})
			if dot >= 0 {
				pid, _ = strconv.Atoi(string(tidbuf[1:dot]))
			}
		}
		threads = append(threads, string(tidbuf))
		if comma < 0 {

View on GitHub (pinned to a23773e6c3)

Solutions

1. Verify the remote stub supports qfThreadInfo/q s ThreadInfo (test with the stub directly or GDB)
2. Update the stub/qemu/embedded target firmware to a protocol-conforming version
3. Capture the raw packet exchange (Delve log flags, e.g. --log --log-output=rpc) to see what the stub actually returned
4. Use a stub configuration that provides a thread list, or connect with a backend suited to the target

// confirm the stub answers qfThreadInfo before connecting delve:
// $ echo $qfThreadInfo#b6 | nc host 1234   -> expect 'm<tid hex>' or 'l'

if err := connect(target); err != nil {
    if strings.Contains(err.Error(), "malformed qfThreadInfo") {
        log.Printf("stub does not implement qfThreadInfo: %v", err)
        // fall back to a different backend or updated stub
    }
}

When it happens

Trigger: Thrown at pkg/proc/gdbserial/gdbserver_conn.go:962 when the library encounters an invalid state.

Common situations: Connecting to non-conforming GDB stubs (embedded RTOS, custom stubs, old qemu versions) that do not implement qfThreadInfo; stubs that answer with an empty packet meaning 'unsupported'; protocol proxies/relays mangling packets.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• qMemoryRegionInfo response wrapped around the address space
• too many transmit attempts
• could not determine executable path: %v
• malformed qMemoryRegionInfo response packet (start): %v in %
• malformed qMemoryRegionInfo response packet (size): %v in %s

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/c2868c5e1ddc8963. Report an issue: GitHub.