How to read+write to a character device without geting stuck ?

[zcattacz]

In python os module can enable read+write to character device like below

try:
    d = os.open("/dev/usb/lp0", os.O_RDWR)
    os.write(d, "~!@")
    printer_status = os.read(d, 100) # returns less if remaining buffer is less than 100, without blocking
finally:
    os.close(d)

I wonder how should this be handled in micropython?

EDIT: @dlech kindly suggested "w+" flag, which opens the device correctly, now my new issue becomes if the response has no end marker and size is unknown how should the device be read() without getting blocked.

with open("/dev/usb/lp0", "w+b") as f:
    buf1 = bytearray(sz)
    f.readinto(buf1) # this blocks forever, if sz > remaining buffer size

    buf2 = b''
    while True:
        c += f.read(1) # this blocks forever if read over end of response.
        buf2 += c
        if c == end_marker: # in this case, which is unknown 
            break

   # stuck free scheme should be ?

[dlech]

You can just use with open("/dev/usb/lp0", "w+") as f: as you also can in CPython.

[zcattacz]

Hi @dlech,
Thank you for the advice. The "w+" (or "w+b") mode opens the device without error, but .read() call blocks forever, not returning anything. This behavior is reproducible in CPython3. So far it seems only os.open()/os.write()/os.write() works and successfully returns code from printer without delay.

[zcattacz]

Hi @dlech,
My test code in CPython and micropython is as below.

with open("/dev/usb/lp0", "w+b") as f:
    f.write(b"\r\n")
    f.write(b"~!@")
    print("written, ok")
    print(f.read(100).hex()) # here blocks forever

[zcattacz]

OK, I tried .read(), which blocks, I tried read(100), which also blocks, but works in os.read() in CPython.
os.read(dev, bytes) allows setting larger byte size and return less.

Hmm, with read(4) it works. so my question becomes:
How should I avoid getting stuck when calling filehandle.read() if the response is of variable length and has no end of response marker?

[dlech]

You could use poll() to know if there is data available to read one byte and poll again.

Or you could set the file to non-blocking mode.

https://github.com/micropython/micropython-lib/tree/master/unix-ffi contains fcntl for setting the mode to non-blocking and also has os.read().

[zcattacz]

Hi, could you elaborate on the use of poll(), do you mean select.poll() ?

I checked micropython-pyserial and micropython-serial repo, both use fcntl.
I wonder if there is any out-of-the-box way, since I am not doing low-level setup like baud-rate, flow control etc.

[dlech]

do you mean select.poll() ?

yes

could you elaborate on the use of poll()

https://docs.micropython.org/en/latest/library/select.html

If the timeout is set to 0, then it does not block.

I wonder if there is any out-of-the-box way,

Although probably not very efficient, polling and reading one byte at a time should work "out of the box" without fcntl.