How to properly convert a C ioctl call to a python fcntl.ioctl call?

Question

Following an example on resetting a serial port in Linux I wanted to translate the following snippet

fd = open(filename, O_WRONLY);
ioctl(fd, USBDEVFS_RESET, 0);
close(fd);

into valid python code. Here is what I have tried so far

file_handler = open(self._port, 'w')
fcntl.ioctl(file_handler, termios.USBDEVFS_RESET)
file_handler.close()

which ends with an error 'module' object has no attribute 'USBDEVFS_RESET'. The termios documentation is not very helpful in this point, as it does not list the possible properties of termios. See also the fcntl documentation for an example of such a termios property.

How to I 'convert' the C code to python2.7 code correctly?

Answer 1

I came across this when looking how to do a USBDEVFS_RESET and thought I'd share what I found about _IO: http://bugcommunity.com/wiki/index.php/Develop_with_Python#Introduction_to_ioctl_calls_in_python

So, what I have so far is the following:

from fcntl import ioctl

busnum = 1
devnum = 10

filename = "/dev/bus/usb/{:03d}/{:03d}".format(busnum, devnum) 

#define USBDEVFS_RESET             _IO('U', 20)
USBDEVFS_RESET = ord('U') << (4*2) | 20

fd = open(filename, "wb")
ioctl(fd, USBDEVFS_RESET, 0)
fd.close()

You can get the busnum and devnum from lsusb.

Answer 2

ioctl-opt (pypi) is a small python module translating needed C preprocessor macros to python. For a simple usage example, see this hidraw implementation.

Note that defining ctype structures can be needed (depending on call type) so you can actually pass parameters.

Disclosure: I am the author of both modules.

Answer 3

The macro USBDEVFS_RESET is defined in a system header file somewhere.

You can search for it and replace termios.USBDEVFS_RESET with the actual value.