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POpython - could ioctl numbers differs from C ioctl numbers?
 

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  1. POpython - could ioctl numbers differs from C ioctl numbers?
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    <p>As far as I know, ioctl numbers are well defined by the drivers and registered in the kernel.</p> <p>I was playing with some code in python for querying joystick states. I have read <a href="http://www.kernel.org/doc/Documentation/input/joystick-api.txt" rel="nofollow">this doc about joystick api</a>, <a href="http://www.mjmwired.net/kernel/Documentation/ioctl-number.txt" rel="nofollow">this doc about ioctl numbers</a>, and <a href="http://docs.python.org/library/fcntl.html" rel="nofollow">this one from python fcntl module</a>.</p> <p>I've created a C program for testing and querying values, and the python tests with code that I took <a href="http://www.hsg-kl.de/faecher/inf/material/bonsai/util/ppdev.py" rel="nofollow">from here</a> for implementing the <code>_IOR()</code> C macro.</p> <h2>Kernel driver define:</h2> <pre><code>monolith@monolith ~/temp $ grep JSIOCGAXES /usr/include/* -r /usr/include/linux/joystick.h:#define JSIOCGAXES _IOR('j', 0x11, __u8) </code></pre> <h2>C program</h2> <pre><code>#include &lt;stdio.h&gt; #include &lt;linux/joystick.h&gt; #include &lt;fcntl.h&gt; 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, &amp;number); printf("Number of axes: %d\n", number); close(fd); return 0; } </code></pre> <h2>C program Output:</h2> <pre><code>monolith@monolith ~/temp $ ./test Ioctl Number: (int)-2147390959 (hex)80016a11 Number of axes: 6 </code></pre> <h2>Python output</h2> <pre><code># check if _IOR results in the used ioctl number in C &gt;&gt;&gt; _IOR(ord('j'), 0x11, 'c') -2147390959 &gt;&gt;&gt; file = open("/dev/input/js0") # use that integer &gt;&gt;&gt; fcntl.ioctl(file, -2147390959) Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; IOError: [Errno 14] Bad address # ask what hex value is &gt;&gt;&gt; "%x" % -2147390959 '-7ffe95ef' # WHY THIS HEX CONVERSION DIFFERS? &gt;&gt;&gt; fcntl.ioctl(file, -0x7ffe95ef) Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; IOError: [Errno 14] Bad address # Use the hex value from the C program output &gt;&gt;&gt; fcntl.ioctl(file, 0x80016a11) Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; IOError: [Errno 14] Bad address </code></pre> <p>Any ideas why I can't query the file descriptor with that ioctl number? <code>ioctl()</code> and <code>fcntl()</code> functions take a file descriptor or an object with the <code>fileno()</code> method implemented so I discart the error from the <code>file</code> object.</p> <p>Maybe the problem comes with number conversion and types, no idea...clues?</p>
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    1. COUsing `fcntl.fcntl()` I get this error: `OverflowError: signed integer is greater than maximum`. Now I need to know why C `ioctl()` is managing the argument correctly. C `ioctl()` takes an `int` as an argument. Would tracking the `int` offset be an approach to determine the `signed integer` in python?
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