Pointers and arrays in Python ctypes

Question

I have a DLL containing a C function with a prototype like this:

int c_read_block(uint32 addr, uint32 *buf, uint32 num);

I want to call it from Python using ctypes. The function expects a pointer to a chunk of memory, into which it will write the results. I don't know how to construct and pass such a chunk of memory. The ctypes documentation isn't much help.

Constructing an array and passing it "byref", like this:

    cresult = (c_ulong * num)()
    err = self.c_read_block(addr, byref(cresult), num)

Gives this error message:

ArgumentError: argument 3: <type 'exceptions.TypeError'>: expected LP_c_ulong instance instead of pointer to c_ulong_Array_2

I guess that is because the Python ulong array is nothing like a c uint32 array. Should I use create_char_string. If so, how do I persuade Python to "cast" that buffer to an LP_c_ulong?

Solution 1

You can cast with the cast function :)

>>> import ctypes
>>> x = (ctypes.c_ulong*5)()
>>> x
<__main__.c_ulong_Array_5 object at 0x00C2DB20>
>>> ctypes.cast(x, ctypes.POINTER(ctypes.c_ulong))
<__main__.LP_c_ulong object at 0x0119FD00>
>>> 

Solution 2

You can cast the result, but ctypes allows you to use an array in place of a pointer, directly. The issue is the byref in your code (which would be the equivalent of a pointer to a pointer):

So instead of:

cresult = (c_ulong * num)()
err = self.c_read_block(addr, byref(cresult), num)

try:

cresult = (c_ulong * num)()
err = self.c_read_block(addr, cresult, num)

Comments

[Ralph Sinsuat]

I've used ctypes a few times and I've encountered it in the linked documentation before.

[Mike Vella]

Why i it a LP_c_ulong object and not LP_c_ulong_Array_5 object

[Charles Grunwald]

LP_c_ulong_Array_5 would be an address pointing to an address pointing to the first element in an array of uint32s, whereas the OP's function is looking to receive an address pointing to the first element of an array. LP_c_ulong_Array_5 would be for a function like: int c_read_block(uint32 addr, uint32 *buf[], uint32 num);