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?

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> >>>  

Answer 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)