go-delve/delve · error

could not attach to pid %d: already being debugged by pid %d

Error message

could not attach to pid %d: already being debugged by pid %d

What it means

The target process is already being traced by another debugger: /proc/<pid>/status shows a non-zero TracerPid. A process can only have one ptrace tracer at a time, so the attach is rejected with the tracing debugger's pid.

Source

Thrown at service/debugger/debugger_linux.go:52

			}
			if fi.Sys().(*syscall.Stat_t).Uid != uint32(os.Getuid()) {
				return fmt.Errorf("Could not attach to pid %d: current user does not own the process", pid)
			}

			// check if the process is already being traced
			statusfh, err := os.Open(fmt.Sprintf("/proc/%d/status", pid))
			if err != nil {
				return fallbackerr
			}
			defer statusfh.Close()
			scan := bufio.NewScanner(statusfh)
			const tracerPidPrefix = "TracerPid:"
			for scan.Scan() {
				line := scan.Text()
				if strings.HasPrefix(line, tracerPidPrefix) {
					tpid, _ := strconv.Atoi(strings.TrimSpace(line[len(tracerPidPrefix):]))
					if tpid != 0 {
						return fmt.Errorf("could not attach to pid %d: already being debugged by pid %d", pid, tpid)
					}
				}
			}
		}
	}
	return fallbackerr
}

View on GitHub (pinned to a23773e6c3)

Solutions

1. Detach or close the other debugger session (its pid is in the error message)
2. Kill the other tracer process: kill <tracerPid>
3. Check with: cat /proc/<pid>/status | grep TracerPid before attaching
4. Retry attach once the TracerPid becomes 0

Example fix

// before
dlv attach 12345 // already traced by pid 999
// after (shell)
kill 999
dlv attach 12345

out, _ := os.ReadFile(fmt.Sprintf("/proc/%d/status", pid))
alreadyTraced := strings.Contains(string(out), "TracerPid:\t0") == false

err := debugger.Attach(pid, []string{}, nil)
if err != nil && strings.Contains(err.Error(), "already being debugged") {
    // detach/kill the tracer pid parsed from the message, then retry once
}

When it happens

Trigger: 'dlv attach <pid>' while another delve/gdb/strace session already has the process attached (TracerPid: <tpid> in /proc/<pid>/status).

Common situations: Two developers attaching to the same dev server; a forgotten strace or previous delve session left attached; IDE debug session still open holding the process.

Related errors

• could not attach to pid %d: %s
• process must be stopped in order to kill it
• waiting for target execve failed: %s
• could not read proc stat: %v
• no match found using regexp '%s' in /proc/%d/stat

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/be2ccfc8361af61a. Report an issue: GitHub.