Re: [ctypes-users] Question about strings and pointers

[Ananda R. <ana...@sa...>]

I haven't tested your code but  I thought I will tell you the first thing
that came in my mind.

I think you are trying to pass a python string to a c function as a c
string.

Try this to create the ctypes string in python and then pass pointer  with
your function call.

import ctypes
temp_str = 'this is the substring we would like to print'
my_string = ctypes.create_string_buffer(temp_str, len(temp_str))

Hope it helps,

Ananda


On Thu, Jan 28, 2010 at 1:29 PM, Valentin Haenel <val...@gm...>wrote:

> Hi,
>
> i have been using ctypes quite happily over the last year or so, for a
> variety
> of smaller projects. However last week i stumbled upon an issue that i was
> unable to resolve. To spare you having to look through and understand the
> code
> that i am wrapping i have written a small example to illustrate the type of
> problem i am having:
>
> Imagine i want to use the following funcion from ctypes:
>
> void print_substring(char * first, char * after_last){
>    char * current = first;
>    while (current != after_last){
>        putchar(*current);
>        current = current + 1;
>    }
>    putchar('\n');
>
> }
>
> In this case first and after_last are pointers into the same memory block.
> An
> exmaple use would be:
>
> void main(void){
>    char * string = "this is the substring we would like to print";
>    print_substring(&string[12], &string[21]);
>    return;
> }
>
> In this case first and after last are:
>
> this is the substring we would like to print
>            ^        ^
>
> I have attempted to wrap this with the following python code:
>
> from ctypes import *
> lib = cdll.LoadLibrary('./libdemo-library.so.1')
> lib.print_substring.argtypes = [POINTER(c_char), POINTER(c_char)]
> test = "this is the substring we would like to print"
> lib.print_substring(pointer(c_char(test[12])),pointer(c_char(test[21])))
>
> But as you already guess this does not work. From what i understand the two
> pointers don't point into the same string but into freshly allocated
> instances
> of c_char, although i might be mistaken about that.  In any case  the code
> will
> just print a load of garbage from memory and eventually segfault. I just
> can't
> figure out how to obtain these two pointers to differnt positions in the
> same
> string in ctypes.
>
> If you want to play around with the code, please find it attached (includes
> makefile).
>
> Thanks in advance for your advice.
>
> V-
>
>
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