python - could ioctl numbers differs from C ioctl numbers?
문제
As far as I know, ioctl numbers are well defined by the drivers and registered in the kernel.
I was playing with some code in python for querying joystick states. I have read this doc about joystick api, this doc about ioctl numbers, and this one from python fcntl module.
I've created a C program for testing and querying values, and the python tests with code that I took from here for implementing the _IOR()
C macro.
Kernel driver define:
monolith@monolith ~/temp $ grep JSIOCGAXES /usr/include/* -r
/usr/include/linux/joystick.h:#define JSIOCGAXES _IOR('j', 0x11, __u8)
C program
#include <stdio.h>
#include <linux/joystick.h>
#include <fcntl.h>
int main() {
int fd = open("/dev/input/js0", O_RDONLY);
printf("Ioctl Number: (int)%d (hex)%x\n", JSIOCGAXES, JSIOCGAXES);
char number;
ioctl(fd, JSIOCGAXES, &number);
printf("Number of axes: %d\n", number);
close(fd);
return 0;
}
C program Output:
monolith@monolith ~/temp $ ./test
Ioctl Number: (int)-2147390959 (hex)80016a11
Number of axes: 6
Python output
# check if _IOR results in the used ioctl number in C
>>> _IOR(ord('j'), 0x11, 'c')
-2147390959
>>> file = open("/dev/input/js0")
# use that integer
>>> fcntl.ioctl(file, -2147390959)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 14] Bad address
# ask what hex value is
>>> "%x" % -2147390959
'-7ffe95ef'
# WHY THIS HEX CONVERSION DIFFERS?
>>> fcntl.ioctl(file, -0x7ffe95ef)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 14] Bad address
# Use the hex value from the C program output
>>> fcntl.ioctl(file, 0x80016a11)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 14] Bad address
Any ideas why I can't query the file descriptor with that ioctl number? ioctl()
and fcntl()
functions take a file descriptor or an object with the fileno()
method implemented so I discart the error from the file
object.
Maybe the problem comes with number conversion and types, no idea...clues?
해결책 2
I'm going to answer my own question.
For some reason, the only way to get the value using ioctl()
from python was issuing this code:
>>> buf = array.array('h', [0])
>>> fcntl.ioctl(file.fileno(), 0x80016a11, buf)
0
>>> buf[0]
6
That is, using a buffer to sotre the result. I should re-read the documentation and understand why fcntl.ioctl(file.fileno(), 0x80016a11)
was not working.
다른 팁
This all boils down to the hex conversions being different - plugging the hex C gives you into Python gives you a different number:
>>> 0x80016a11
2147576337
I'm not sure why Python and C give different hex, but it is likely at least partially related to sign - Python's '%x'
gives a signed hex value 1, printf
s gives unsigned 2.
Using Python's hex value (-7ffe95ef
) is likely to improve things - or, even better, use a variable like you do in C and keep the conversion errors out of it:
op = _IOR(ord('j'), 0x11, 'c')
...
fcntl.ioctl(file, op)