MacOS Open() Fails Occasionally w/ Non-Standard Baud Rate

[jalius]

Describe the problem

Opening a serial port with non-standard baud rate 25000 fails occasionally. The failure occurs in nativeOpen()

go-serial/serial_unix.go

Lines 232 to 247 in f12391c

	settings, err := port.getTermSettings()
	if err != nil {
	port.Close()
	return nil, &PortError{code: InvalidSerialPort, causedBy: fmt.Errorf("error getting term settings: %w", err)}
	}
	// Set raw mode
	setRawMode(settings)
	// Explicitly disable RTS/CTS flow control
	setTermSettingsCtsRts(false, settings)
	if err = port.setTermSettings(settings); err != nil {
	port.Close()
	return nil, &PortError{code: InvalidSerialPort, causedBy: fmt.Errorf("error setting term settings: %w", err)}
	}

due to tcsetattr() rejecting non-standard baudrates.

Scenario:
Sometimes, getTermSettings() populates with a non-standard baud rate. Once the settings struct is populated with a non-standard baud rate, setTermSettings() and thus nativeOpen() will fail with EINVAL. This appears to be a MacOS specific behavior for tcsetattr rejecting non-standard baud rates (see related tcsetattr() issue in avrdude)

Note that a workaround for non-standard baud rate, setSpecialBaudRate(), is already called out in SetMode(), but it does not apply properly when getTermSettings (tcgetattr()) populates with a non-standard baud rate:

go-serial/serial_unix.go

Lines 271 to 276 in f12391c

	// MacOSX require that this operation is the last one otherwise an
	// 'Invalid serial port' error is returned... don't know why...
	if err := port.SetMode(mode); err != nil {
	port.Close()
	return nil, &PortError{code: InvalidSerialPort, causedBy: fmt.Errorf("error configuring port: %w", err)}
	}

This problem also applies to subsequent calls to SetMode w/ non-standard baud rate on a port that was Open()'d successfully. Please see the example code that I attached to reproduce this issue.

Possible Solution: We could replace the Ispeed and Ospeed fields with a standard baud rate before calling setTermSettings() if they are non-standard, then allow the setSpecialBaudRate() to set the non-standard baud rates.

To reproduce

package main

import (
	"fmt"

	"go.bug.st/serial"
)

func t1() error {
	// setting non-standard baud-rate a second time fails on MacOS
	// baud rate was set first in serial.Open()
	// baud rate attempted to set again in SetMode()
	Port := "/dev/tty.usbserial-DK0FPOX4"
	port, err := serial.Open(Port, &serial.Mode{BaudRate: 250000})
	if err != nil {
		fmt.Println("Open() failed", err.Error())
		return err
	}
	defer port.Close()
	err = port.SetMode(&serial.Mode{BaudRate: 250000})
	if err != nil {
		fmt.Println("SetMode() failed", err.Error())
		return err
	}
	fmt.Println("no error")
	return nil
}
func t2() error {
	// setting non-standard baud-rate works once on MacOS
	// baud rate was set first in serial.Open()
	// baud rate attempted to set again in SetMode()
	Port := "/dev/tty.usbserial-DK0FPOX4"

	port, err := serial.Open(Port, &serial.Mode{BaudRate: 9600})
	if err != nil {
		fmt.Println("Open() failed", err.Error())
		return err
	}
	defer port.Close()
	err = port.SetMode(&serial.Mode{BaudRate: 250000})
	if err != nil {
		fmt.Println("SetMode() failed", err.Error())
		return err
	}
	fmt.Println("no error")
	return nil
}
func main() {
	fmt.Println("t1():")
	t1()
	fmt.Println("t2():")
	t2()
}

t1():
SetMode() failed invalid argument
t2():
no error

Please double-check that you have reported each of the following

before submitting the issue.

• I've provided the FULL source code that causes the problem
• I've provided all the actions required to reproduce the problem

Expected behavior

Serial port should Open, and the workaround for MacOS non-standard baud rates should apply even when the port already has a non-standard baudrate.

Operating system and version

MacOS: Sequoia 15.5

Please describe your hardware setup

No response

Additional context

No response

Issue checklist

• I searched for previous requests in the issue tracker
• My request contains all necessary details