signal.alarm() handler causing problem with pyserial

Question

so i have a motion sensor connected to an avr micro that is communicating with my python app via usb. im using pyserial to do the comm. during my script i have an infinate loop checking for data from the avr micro. before this loop i start a timer with signal.alarm() that will call a function to end a subprocess. when this alarm goes it interrupts the pyserial comm and the program exits completly. i get the error that pyserial read() is interrupted. is there any way around this issue. any help would be awesome

Answer 1

The problem is that your alarm will interrupt the read from the serial port, which isn't at all what you want.

It sounds like you probably want to break this into two threads that do work separately.

Answer 2

You are using alarm(), which send a signal, and pyserial, which does reads and writes to a serial port. When you are reading or writing to a device like that, and the SIGALRM signal is received, a read() or a write() call is interrupted so the signal can be handled.

As signals are handled in userspace, and reading and writing is actually handled by the kernel, this makes things rather ugly. This is a know wart of the way signals are handled, and dates back to the very early UNIX days.

Code that handles signals correctly in python may look like:

import errno
while True:
    try:
        data = read_operation()
    except OSError, e:
        if getattr(e, 'errno', errno.EINTR):
            continue
        raise
    else:
        break