In my Python (2.7.3) code, I\'m trying to use an ioctl call, accepting a long in

Question

In my Python (2.7.3) code, I'm trying to use an ioctl call, accepting a long int (64 bit) as an argument. I'm on a 64-bit system, so a 64-bit int is the same size as a pointer.

In existing C code, I'd use the call like this (for some file descriptor fd, and request codes IOC_SET_VAL, IOC_GET_VAL):

uint64_t val_in = (uint64_t)1<<32;

int ret;

ret = ioctl(fd,_IOWR(0,IOC_SET_VAL,void*),val_in);

uint64_t val_out;

ret = ioctl(fd,_IOWR(0,IOC_GET_VAL,void*),&val_out); // val_out == val_in

In python, I open the device file, and then I get the following results:

from fcntl import ioctl

ioctl(fd, IOC_SET_VAL, 1<<30)

struct.unpack("L",ioctl(fd, IOC_GET_VAL," "*8)[0] # gives 1<<30

ioctl(fd, IOC_SET_VAL, 1<<32) # gives error:

   # OverflowError: signed integer is greater than maximum

ioctl(fd, IOC_SET_VAL, struct.pack("L", 1<<32))

struct.unpack("L",ioctl(fd, IOC_GET_VAL," "*8)[0] # does not give 1<<32 -

   # gives 64-bit pointer

As far as I can tell, the arg parameter of Python's ioctl() can be either a 32-bit int or a 64-bit pointer - but what I need is to pass a 64-bit int.

Is there any way Python can do this?

Explanation / Answer

It might be possible using Python's built-in ioctl(), but it's really difficult to test anything.

If nothing else works, you should be able to call the underlying C function directly using the ctypes module, but it's platform-specific.

Assuming Linux, something like this ought to work...

from ctypes import *

libc = CDLL('libc.so.6')

fd = c_int(something)
IOC_SET_VAL = c_int(something)
IOC_GET_VAL = c_int(something)
value = c_uint64(something)

# To set the value
libc.ioctl(fd, IOC_SET_VAL, value)

# To get the value
libc.ioctl(fd, IOC_GET_VAL, byref(value))
...but, again, I can't really test it.

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