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?

Asked By: Andrew Bainbridge

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Source

Answer 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>
>>>

Answered By: Mark Rushakoff

Answer 2

There is a typo in the solution. In order to get a pointer to an array of ulongs you need to cast to a POINTER(list of ulong)

In [33]: ptr = ctypes.cast(x, ctypes.POINTER(ctypes.c_ulong*5))
In [34]: ptr
Out[34]: <__main__.LP_c_ulong_Array_5 at 0x23e2560>

Answered By: bemehow

Answer 3

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)

Answered By: ephemer

Pointers and arrays in Python ctypes

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