Cargo.toml

[package]
name = "ffitest"
version = "0.1.0"
authors = ["Pedro Rodriguez <ski.rodriguez@gmail.com>"]

[dependencies]

[lib]
name = "ffitest"
crate-type = ["dylib"]

lib.rs

use std::ffi::{CString, CStr};
use std::os::raw::{c_uint, c_char};

#[no_mangle]
pub extern fn substring(str_input: *const c_char, i: c_uint, j: c_uint) -> *const c_char {
    unsafe {
        let slice = CStr::from_ptr(str_input);
        let rust_str = slice.to_str().unwrap();
        let rust_i = i as usize;
        let rust_j = j as usize;
        let rust_substr = &rust_str[rust_i..rust_j];
        let rust_cstr = CString::new(rust_substr).unwrap();
        rust_cstr.as_ptr()
    }
}

test.sh

$ cargo build --release
   Compiling ffitest v0.1.0 (file:///private/tmp/ffitest)
$ ipython
In [1]: import ctypes

In [2]: ffitest = ctypes.cdll.LoadLibrary("target/release/libffitest.dylib")

In [3]: ffitest.substring.argtypes = (ctypes.c_char_p, ctypes.c_int, ctypes.c_int)

In [4]: ffitest.substring.restype = ctypes.c_char_p

In [5]: ffitest.substring(b"testing", 2, 4)
Out[5]: b'st'

In [6]: ffitest.substring(b"testing", 2, 5)
Out[6]: b'sti'

In [7]: ffitest.substring(b"testing", 2, 6)