foreach.c

/* on Linux, compile me with: gcc -shared -o foreach.so foreach.c */

#include <stddef.h>

typedef wchar_t (*transform_wct)(wchar_t);

void foreachwchar(wchar_t *cs, transform_wct func)
{
	while (cs[0] != L'\0')
	{
		cs[0] = func(cs[0]);
		++cs;
	}
}

foreach.py

#!/usr/bin/env python3
import ctypes as ct

def mytransform(c):
	return chr(ord(c) + 1)

# this is the wchar_t * we'll pass to foreachwchar
unibuf = ct.create_unicode_buffer("Sgd\x1fpthbj\x1faqnvm\x1fenw\x1fitlor\x1fnudq\x1fsgd\x1fk`yx\x1fcnf-")

# fetch foreachwchar
foreachlib = ct.CDLL("./foreach.so")
foreachwchar = foreachlib.foreachwchar

# this is effectively the Python version of that typedef
transform_wct = ct.CFUNCTYPE(ct.c_wchar, ct.c_wchar)

# now "convert" mytransform according to the above typedef
transformcallback = transform_wct(mytransform)

# calltime!
foreachwchar(unibuf, transformcallback)

# output!
print(unibuf.value)