ctypes: delay conversion from c_char_p to string

[Zvezdan Petkovic]

On Apr 21, 2010, at 6:29 PM, Brendan Miller wrote:

> Here's the method I was using. Note that tmp_char_ptr is of type
> c_void_p. This should avoid the memory leak, assuming I am
> interpreting the semantics of the cast correctly. Is there a cleaner
> way to do this with ctypes?
> 
>    def get_prop_string(self, prop_name):
>        # Have to work with c_void_p to prevent ctypes from copying to a string
>        # without giving me an opportunity to destroy the original string.
>        tmp_char_ptr = _get_prop_string(self._props, prop_name)
>        prop_val = cast(tmp_char_ptr, c_char_p).value
>        _string_destroy(tmp_char_ptr)
>        return prop_val

Is this what you want?

=====

import ctypes.util


libc = ctypes.CDLL(ctypes.util.find_library('libc'))

libc.free.argtypes = [ctypes.c_void_p]
libc.free.restype = None
libc.strdup.argtype = [ctypes.c_char_p]
libc.strdup.restype = ctypes.POINTER(ctypes.c_char)


def strdup_and_free(s):
    s_ptr = libc.strdup(s)
    print s_ptr.contents
    i = 0
    while s_ptr[i] != '\0':
        print s_ptr[i],
        i += 1
    print
    libc.free(s_ptr)


if __name__ == '__main__':
    strdup_and_free('My string')

=====

That is an equivalent of this C program:

=====

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


void
strdup_and_free(char *s)
{
	char *s_ptr = strdup(s);

	printf("%c\n", *s_ptr);
	printf("%s\n", s_ptr);
	free(s_ptr);
}


int
main(int argc, char *argv[])
{
	strdup_and_free("My string");
	return 0;
}

=====

To prove that the pointer was actually freed try inserting one more
libc.free(s_ptr) at the end of the function.
You should get Abort trap because you are attempting to free storage that has already been freed (I did on Mac OS X 10.6)


If you insert a second free(s_ptr) call in the C program you also get an Abort trap:

strdup2(15785) malloc: *** error for object 0x100100080: pointer being freed was not allocated

(Again, this is an error message on Mac OS X 10.6)


FWIW, this technique for getting pointer *is* mentioned in ctypes documentation.

Best regards,

	Zvezdan


> 
> On Wed, Apr 21, 2010 at 3:15 PM, Brendan Miller <catphive at catphive.net> wrote:
>> I have a function exposed through ctypes that returns a c_char_p.
>> Since I need to deallocate that c_char_p, it's inconvenient that
>> ctypes copies the c_char_p into a string instead of giving me the raw
>> pointer. I believe this will cause a memory leak, unless ctypes is
>> smart enough to free the string itself after the copy... which I
>> doubt.
>> 
>> Is there some way to tell ctypes to return an actual c_char_p, or is
>> my best bet to return a c_void_p and cast to c_char_p when I'm reading
>> to convert to a string?
>> 
>> Thanks
>> Brendan
>> 
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