Synchronous connection fails on MacOS

[rrooggiieerr]

I'm migrating a library from pyserial and pyserial-asyncio-fast to serialx. For the async functions that went smoothly, however with the synchronous functions that my library also provides I can't manage to create a connection.

The following code using pyserial works:

import serial

# Create the connection.
connection = serial.Serial(port="/dev/tty.usbserial-110")

# Open the connection.
if not connection.is_open: # connection.is_open is True when using pyserial
    connection.open()

# Close the connection.
connection.close()

The following code using serialx fails to open the connection, it just hangs:

import serialx as serial

# Create the connection.
connection = serial.Serial(port="/dev/tty.usbserial-110")

# Open the connection.
if not connection.is_open: # connection.is_open is False when using serialx
    connection.open() # This just hangs

# Close the connection.
connection.close()

I'm running this using Python 3.14.4 and serialx 1.8.0 on MacOS Tahoe 26.5.1

[puddly]

Interesting! Could you run this with debug logging enabled and post the output? This package does log a lot and may provide context. What serial adapter are you using?

[rrooggiieerr]

This is the debug logging when using serialx:

2026-06-09 20:14:03,420 DEBUG    selector_events.py:64 Using selector: KqueueSelector
2026-06-09 20:14:03,422 DEBUG    serial_posix.py:112 Opening serial port '/dev/tty.usbserial-110'

That's it, no more logs since connection.open() hangs.

I'm using a cheap USB RS-485 controller based on a (fake?) CH340G chip. Similar to https://nl.aliexpress.com/item/1005006448713693.html

[rrooggiieerr]

I just tried it with 5 more CH340G and 1 CP2101 based serial adapter and all the same result

[puddly]

The code is basically:

LOGGER.debug("Opening serial port %r", self._path)
self._fileno = os.open(self._path, os.O_RDWR | os.O_NOCTTY)

LOGGER.debug("Locking serial port %r", self._path)
fcntl.flock(self._fileno, fcntl.LOCK_EX | fcntl.LOCK_NB)

So if you aren't seeing anything get logged after Opening serial port, that means that os.open itself is locking up.

[rrooggiieerr]

Yes, it seems that's exactly what happens. The following also fails:

import os
os.open("/dev/tty.usbserial-110", os.O_RDWR | os.O_NOCTTY)

[puddly]

What about using /dev/cu.usbserial-110?

[rrooggiieerr]

That works.

[puddly]

I don't recall ever getting /dev/tty.* devices to work, only /dev/cu.? https://stackoverflow.com/questions/8632586/whats-the-difference-between-dev-tty-and-dev-cu-on-macos says:

Any process trying to open it blocks within the open() call as long as DCD is not asserted by hardware (i.e. as long as the modem doesn't have a carrier).

That might be what's happening here?

[rrooggiieerr]

That might be, however pyserial does seem to work nicely with /dev/tty.* devices. Also the async functions of serialx work fine. Maybe there is a way to work around the limitations, or detect if a /dev/tty.* device is used? Currently serialx is not a drop in replacement for pyserial it seems.

[puddly]

The only real difference as far as I can tell is the os.O_NONBLOCK flag. I have a PR in the works to improve write timeouts that will implicitly migrate the sync API to use this flag, which should fix this bug as well.

[rrooggiieerr]

Ok, keep me updated and I can give it a try when your PR is implemented