What causes "failed to create server socket: %w" in golang/go?

Free the port or pick a free one: `go tool trace -http=localhost:0 trace.out` lets the OS choose.

failed to create server socket: %w

Error message

failed to create server socket: %w

What it means

After address parsing succeeds, `net.Listen("tcp", addr)` is called to open the viewer's TCP socket. If the kernel refuses the bind (port taken, privileged port without permission, no such interface, address in use), Listen returns an error that is wrapped and fatal. This is an environment/resource error, not a trace-format error.

Source

Thrown at src/cmd/trace/main.go:159

		case "parsed":
			logAndDie(debugProcessedEvents(tracef))
		case "wire":
			logAndDie(debugRawEvents(tracef))
		case "footprint":
			logAndDie(debugEventsFootprint(tracef))
		default:
			logAndDie(fmt.Errorf("invalid debug mode %s, want one of: parsed, wire, footprint", *debugFlag))
		}
	}

	addr, err := listenAddr(*httpFlag)
	if err != nil {
		logAndDie(fmt.Errorf("malformed -http value %q: %v", *httpFlag, err))
	}

	ln, err := net.Listen("tcp", addr)
	if err != nil {
		logAndDie(fmt.Errorf("failed to create server socket: %w", err))
	}

	addr = ln.Addr().String()
	url, simplified, err := addrURL(addr)
	if err != nil {
		logAndDie(fmt.Errorf("failed to compute server URL: %v", err))
	}

	log.Print("Preparing trace for viewer...")
	parsed, err := parseTraceInteractive(tracef, traceSize)
	if err != nil {
		logAndDie(err)
	}
	// N.B. tracef not needed after this point.
	// We might double-close, but that's fine; we ignore the error.
	tracef.Close()

	// Print a nice message for a partial trace.

View on GitHub (pinned to b6b368adc5)

Solutions

Free the port or pick a free one: `go tool trace -http=localhost:0 trace.out` lets the OS choose.
If you need a specific port, find and kill the holder: `lsof -i :<port>` (Unix) then terminate it.
For privileged ports, run with appropriate privileges or pick a port ≥1024.
Check `ulimit -n` and raise the FD limit if you open many traces in a loop.

Example fix

// before
$ go tool trace -http=:80 trace.out  # as non-root
// after
$ go tool trace -http=:8080 trace.out

Defensive patterns

Strategy: retry

Validate before calling

// before binding, probe the port
ln, err := net.Listen("tcp", addr)
if err != nil { return err }
ln.Close() // go tool trace will re-open

Prevention

Prefer -http=localhost:0 to let the OS pick a free port.
Kill prior viewer instances before starting a new one.
Avoid privileged (<1024) ports unless running with appropriate privileges.

When it happens

Trigger: Another process already holds the requested port; binding <1024 as non-root; specifying an IP not present on any local interface; hitting a per-process file descriptor limit; SO_REUSEADDR unavailable on a TIME_WAIT socket.

Common situations: Re-running `go tool trace` while the previous viewer is still open in another terminal; hardcoded port 8080 colliding with a dev server; container restricted from binding the port; macOS/Windows firewall blocking the bind.

Related errors

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/cf56cfd3ce956767. Report an issue: GitHub.