Re: How to get binary data to callback function

Peter Silva <[email protected]> writes:
> On Tue, Dec 22, 2009 at 6:13 PM, Nikolaus Rath <[email protected]> wrote:
>
>> Hello,
>>
>> When I define a callback function with a prototype of
>>
>>   ctypes.CFUNCTYPE(None, ctypes.c_char_p, ctypes.c_int)
>>
>> The function receives a true Python string. However, my data isn't
>> actually \0 terminated, and I would like to call string_at(par1, par2)
>> to get the binary data. But how do I get ctypes to pass the c_char_p
>> object to my function, rather than the unwrapped string?
>>
>
> is this what you mean?
>
> these are pre-allocating space for a return value to be filled by the C
> function:
>
>     sense_buffer = create_string_buffer(64)
>     self.mx_sb_len = len(sense_buffer)
>     #io_hdr.sbp=addressof(sense_buffer)
>     io_hdr.sbp=cast(sense_buffer,c_char_p)
>     io_hdr.sb_len_wr = 0 # initialize just in case...
>
>     page_buffer=create_string_buffer(sz)
>     io_hdr.dxfer_len = sz
>     #io_hdr.dxferp = addressof(page_buffer)
>     io_hdr.dxferp = cast(page_buffer,c_char_p)
>
>     if self.debug & Drobo.DBG_HWDialog:
>       print "4 before ioctl, sense_buffer_len=", io_hdr.mx_sb_len
>
>     i=ioctl(self.sg_fd, sg_io_hdr.SG_IO, io_hdr)

I think that's not what I mean, because I don't want to preallocate
anything. But I'm not sure, because I don't understand your code (I don't even
see any callback function being called).

Here is a short example that demonstrates my problem.

C-Library:

#include <stdio.h>
typedef void cb_fun(char*, int);
void callback(cb_fun* f) {
    char* c = "Hello\0World";
    int len = 11;
    
    (*f)(c, len);
}


Python code:

from ctypes import *
lib = CDLL('./lib.so')
def test(buf, len):
    print buf
    print string_at(buf, len)
lib.callback.restype = None
lib.callback(CFUNCTYPE(None, c_char_p, c_int)(test))


When 'test' is called, it prints 'Hello' two times. What I want it to
print is 'Hello\0World'.


Best,

   -Nikolaus

-- 
 »Time flies like an arrow, fruit flies like a Banana.«

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