no_std_rust_string_ffi_example.rs

no_std_rust_string_ffi_example.rs

#![no_std]

extern crate alloc;
extern crate libc;

use alloc::{ boxed::Box, ffi::CString, string::{ String, ToString, } };
use hashbrown::HashMap;
use libc::{ c_char, c_void, printf, };

#[no_mangle]
pub extern "C" fn hashmap_new() -> *mut c_void {
  let hashmap: HashMap<String, String> = HashMap::new();
  Box::into_raw(Box::new(hashmap)) as *mut c_void
}

#[no_mangle]
pub extern "C" fn hashmap_insert(hashmap_ptr: *mut c_void, key: *const c_char, value: *const c_char) {
  let hashmap = unsafe { &mut *(hashmap_ptr as *mut HashMap<String, String>) };
  let key_str = unsafe { core::str::from_utf8_unchecked(core::slice::from_raw_parts(key as *const u8, libc::strlen(key))) };
  let value_str = unsafe { core::str::from_utf8_unchecked(core::slice::from_raw_parts(value as *const u8, libc::strlen(value))) };
  hashmap.insert(String::from(key_str), String::from(value_str));
}

#[no_mangle]
pub extern "C" fn hashmap_get(hashmap_ptr: *mut c_void, key: *const c_char) -> *mut c_char {
  let hashmap = unsafe { &mut *(hashmap_ptr as *mut HashMap<String, String>) };
  let key_str = unsafe { core::str::from_utf8_unchecked(core::slice::from_raw_parts(key as *const u8, libc::strlen(key))) };
  match hashmap.get(key_str) {
    Some(value) => {
      let c_string = CString::new(value.as_str()).expect("Failed to create CString");
      c_string.into_raw()
    },
    None => core::ptr::null_mut(),
  }
}

#[no_mangle]
pub extern "C" fn hashmap_free(hashmap_ptr: *mut c_void) {
  unsafe { 
    let _ = Box::from_raw(hashmap_ptr as *mut HashMap<String, String>); 
  }
}

#[no_mangle]
pub fn rust_string_to_c_string(s: String) -> *mut c_char {
  let c_string = CString::new(s).expect("CString::new failed");
  c_string.into_raw()
}

#[no_mangle]
pub extern "C" fn free_c_string(s: *mut c_char) {
  unsafe {
    if s.is_null() {
      return;
    }
    let _ = CString::from_raw(s);
  }
}

pub fn print_c_string(c_string: *const c_char) {
  unsafe {
    printf(b"%s\n\0".as_ptr() as *const _, c_string);
  }
}

fn main() {
  let hashmap = hashmap_new();
  let key = rust_string_to_c_string("key".to_string());
  let value = rust_string_to_c_string("value".to_string());
  hashmap_insert(hashmap, key, value);
  let returned_value = hashmap_get(hashmap, key);
  print_c_string(returned_value);
  hashmap_free(hashmap);
  free_c_string(key);
  free_c_string(value);
  free_c_string(returned_value);
}