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Functions to compute fnctl.ioctl's opt argument

Project description

Pythonified linux asm-generic/ioctl.h .

So you can replicate driver’s code computing fcntl.ioctl’s opt argument.

For example, starting from the following IOCTL declaration (taken from input.h):

#include <sys/ioctl.h>
#define EVIOCGNAME(len) _IOC(_IOC_READ, 'E', 0x06, len) /* get device name */

you could write the following:

from ioctl_opt import IOC, IOC_READ
EVIOCGNAME = lambda length: IOC(IOC_READ, ord('E'), 0x06, length)

The differences are minimal, and all come from python language or coding style:

  • macros/constants to use from ioctl_opt for not start with an underscore

  • defined macro becomes a callable (here a lambda, could be function)

  • IOC’s nr argument has to be an integer, so C’s single-quote char becomes an ord call

  • avoid shadowing built-in len function

The length argument is an object (typically serving as an I/O buffer with the fcntl.ioctl() call), the size of which is incorporated in the generated ioctl opt value. IOC() will calculate the size of the object. Supported object types are:

  • any ctypes type or instance

  • memoryview

  • bytearray

  • struct.Struct

  • array.array

You may want to then write a pythonic function to conveniently access that ioctl:

import ctypes
import fcntl

def getDeviceName(fd, length=1024):
    name = (ctypes.c_char * length)()
    actual_length = fcntl.ioctl(fd, EVIOCGNAME(length), name, True)
    if actual_length < 0:
        raise OSError(-actual_length)
    if name[actual_length - 1] == b'\x00':
        actual_length -= 1
    return name[:actual_length]

More advanced example defining hidraw ioctls, requiring structures (for more on how structures are defined, check python’s ctype documentation for your python version):

import ctypes
from ioctl_opt import IOR, IOC, IOC_READ, IOC_WRITE

# hid.h

# hidraw.h
class hidraw_report_descriptor(ctypes.Structure):
    _fields_ = [
        ('size', ctypes.c_uint),
        ('value', ctypes.c_ubyte * HID_MAX_DESCRIPTOR_SIZE),

class hidraw_devinfo(ctypes.Structure):
    _fields_ = [
        ('bustype', ctypes.c_uint),
        ('vendor', ctypes.c_short),
        ('product', ctypes.c_short),

HIDIOCGRDESCSIZE = IOR(ord('H'), 0x01, ctypes.c_int)
HIDIOCGRDESC = IOR(ord('H'), 0x02, hidraw_report_descriptor)
HIDIOCGRAWINFO = IOR(ord('H'), 0x03, hidraw_devinfo)
HIDIOCGRAWNAME = lambda length: IOC(IOC_READ, ord('H'), 0x04, length)
HIDIOCGRAWPHYS = lambda length: IOC(IOC_READ, ord('H'), 0x05, length)
HIDIOCSFEATURE = lambda length: IOC(IOC_WRITE|IOC_READ, ord('H'), 0x06, length)
HIDIOCGFEATURE = lambda length: IOC(IOC_WRITE|IOC_READ, ord('H'), 0x07, length)

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