Re: How to pass String BYREF to C function?

Marco,
 You are correct exitResult is a pointer to c_char_p.  I don't know
the exact different between ctypes.byref and ctypes.POINTER but the
documentation states that we should use byref as it is much faster...


ctypes.byref(obj[, offset])¶
    Returns a light-weight pointer to obj, which must be an instance
of a ctypes type. offset defaults to zero, and must be an integer that
will be added to the internal pointer value.
    byref(obj, offset) corresponds to this C code:
    (((char *)&obj) + offset)
    The returned object can only be used as a foreign function call
parameter. It behaves similar to pointer(obj), but the construction is
a lot faster.
    New in version 2.6: The offset optional argument was added.

ctypes.POINTER(type)¶
    This factory function creates and returns a new ctypes pointer
type. Pointer types are cached an reused internally, so calling this
function repeatedly is cheap. type must be a ctypes type.

ctypes.pointer(obj)¶
    This function creates a new pointer instance, pointing to obj. The
returned object is of the type POINTER(type(obj)).
    Note: If you just want to pass a pointer to an object to a foreign
function call, you should use byref(obj) which is much faster.


The code below should do the trick, I must not have had enough coffee
this morning, time for another cup...

import ctypes
import os

if __name__ == "__main__":
    if os.name=='nt':
        userExit = ctypes.cdll.LoadLibrary("libUserExits.dll")
        executeUserExit = userExit.executeUserExit
        executeUserExit.argtypes = [ctypes.c_char_p,
ctypes.POINTER(ctypes.c_char_p)]
        executeUserExit.restype = ctypes.c_int
        exitResult = ctypes.c_char_p()
        result = executeUserExit("hello world", ctypes.byref(exitResult))
        print exitResult.value
    print "Finished"








On Wed, Aug 18, 2010 at 9:22 AM, marco cassisa <[email protected]> wrote:
>
>
> 2010/8/18 Andrew Wilson <[email protected]>
>>
>> Hey Lynton,
>>   I guess the casting and create_stream_buffer was for not as pointed out by TP, the following seems to work fine.I'm really a hack at ctypes...
>>
>>
>>         stream = ctypes.c_char_p("hello world")
>>         exitResult = ctypes.c_char_p()
>>
>>
>>         result = executeUserExit(stream, ctypes.byref(exitResult))
>>
> IMHO exitResult - I insist -  must be a pointer to c_char_p() ( and in fact the signature is
>
> int executeUserExit(char *buffer, char **userExitResultBuffer){
>
> ) and correctly Lynton Grice declared the arguments' types
>
> executeUserExit.argtypes = [c.c_char_p, c.POINTER(c.c_char_p)]
>
> in his working code
>
> I can be wrong of course, i am a newbie!
>
> marco

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