Adding data to a C buffer passed to a Python Ctypes callback

Question

I have a C function in a DLL which is calling back to a Python function via ctypes.

The C function calls the function with the following prototype:

void GetData(u8 Buffer[], u16 Length);

Note that the buffer points to a static data array which is held in the DLL.

I've generated a Python function type for this as the following:

CFUNCTYPE(None, c_char_p, c_int)

And my Python callback function is being called without any obvious problems.

My problem is, I can't work out how to get ctypes to add raw data to a buffer which is external to Python. Is there a way to define a ctypes string buffer and give it the address of the DLL's buffer?

tito · Accepted Answer · 2012-02-09 18:15:38Z

2

You could use memcpy from libc. If your c_char_p is buffer and c_int is bufferlen:

libc = ctypes.cdll.LoadLibrary('libc.so.6')
content = 'My new content'
bufcontent = ctypes.create_string_buffer(content)
libc.memcpy(buffer. newcontent, min(len(content) + 1, bufferlen))

Same as you would do in C. (note that i've used memcpy for possible binary content, but if you don't care about it, use strncpy instead)

answered Feb 9, 2012 at 18:15

tito

13.3k1 gold badge57 silver badges76 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Jon Mills Over a year ago

Thanks for your answer - you really pointed my along the right lines: - I found out that ctypes has a memmove function which wraps memcpy - so I didn't need to import libc. The only other thing I needed to do was to declare my function type as CFUNCTYPE(None, POINTER(c_char), c_int) rather than CFUNCTYPE(None, c_char_p, c_int) .

Collectives™ on Stack Overflow

Adding data to a C buffer passed to a Python Ctypes callback

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related