writing to a c_char_p received in a callback

Hey all,

 

I have a callback function that would be defined like this in C:

 

#define MY_DATA = "abcdefg"

int set_data_cb(void *unused, char *buf, int* bufsize)

{

    (*bufsize) = (int)strlen(MY_DATA)

    strncpy(buf, MY_DATA, (*bufsize))

}

 

Of course this is simplified, but the idea is the caller is passing the
callback func a pointer to a string, which the callback function is
responsible for adding data to. I'm having an awfully hard time getting
this to work with ctypes. It seems that ctypes is automatically
converting my c_char_p into a normal python str, which of course does
not allow me to update the original char* that was passed into the
callback.

 

I've tried several different variations:

 

1)      CB_FUNC_INPUT  =CFUNCTYPE(c_int, c_voidp, c_char_p,
POINTER(ctypes.c_int))
# This gives me a python str ... no good



2)      class my_c_char_p(ctypes.c_char_p): pass
CB_FUNC_INPUT  =CFUNCTYPE(c_int, c_voidp, my_c_char_p,
POINTER(ctypes.c_int))
# this gives me a my_c_char_p object, but updating the value property
doesn't seem to get my data back to the caller



3)      CB_FUNC_INPUT  =CFUNCTYPE(c_int, c_voidp, POINTER(c_char_p),
POINTER(ctypes.c_int))
# this gives me a pointer, but settings contents.value also does not get
my data back to the caller



4)      CB_FUNC_INPUT  =CFUNCTYPE(c_int, c_voidp, c_void_p,
POINTER(ctypes.c_int))
# argh, this gives me a long, not a pointer. (I was hoping I could just
cast it, but no luck)

 

I've googled this to death, but either I have not found the magic
combination of search terms, or the answer is so obvious that nobody
else has had this problem.

 

Any help would be appreciated!

TIA

-Matt

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