I am following up on Casting a borrowed reference with a lifetime to a raw pointer in Rust, which solved the wrong problem.
Please consider the following code:
extern crate jni;
extern crate ffi;
use jni::JNIEnv;
use jni::objects::JClass;
use jni::sys::{jint, jlong, jobject};
struct CameraAppEngine {
_env: *mut jni::sys::JNIEnv,
_width: i32,
_height: i32
}
impl CameraAppEngine {
pub fn new(_env: *mut jni::sys::JNIEnv, _width: i32, _height: i32) -> CameraAppEngine {
CameraAppEngine { _env, _width, _height }
}
pub fn create_camera_session(&mut self, surface: jobject) {
// error!
let window = ffi::ANativeWindow_fromSurface(self._env, surface);
}
}
fn app_engine_create(env: &JNIEnv, width: i32, height: i32) -> *mut CameraAppEngine {
let engine = CameraAppEngine::new(env.get_native_interface(), width, height);
Box::into_raw(Box::new(engine))
}
#[no_mangle]
pub extern "C" fn Java_io_waweb_cartoonifyit_MainActivity_createCamera(env: JNIEnv<'static>, _: JClass, width:jint, height:jint) -> jlong {
app_engine_create(&env, width, height) as jlong
}
#[no_mangle]
pub extern "C" fn Java_io_waweb_cartoonifyit_MainActivity_onPreviewSurfaceCreated(_: JNIEnv, _: JClass, engine_ptr:jlong, surface:jobject) {
let mut app = unsafe { Box::from_raw(engine_ptr as *mut CameraAppEngine) };
app.create_camera_session(surface);
}
And in the ffi crate we have:
extern "C" {
pub fn ANativeWindow_fromSurface(env: *mut JNIEnv, surface: jobject) -> *mut ANativeWindow;
}
This results in:
error[E0308]: mismatched types
--> native_app/src/lib.rs:24:53
|
| let window = ffi::ANativeWindow_fromSurface(self._env, surface);
| ^^^^^^^^^ expected struct `ffi::JNINativeInterface`, found struct `jni::sys::JNINativeInterface_`
|
= note: expected raw pointer `*mut *const ffi::JNINativeInterface`
found raw pointer `*mut *const jni::sys::JNINativeInterface_`
error[E0308]: mismatched types
--> native_app/src/lib.rs:24:64
|
| let window = ffi::ANativeWindow_fromSurface(self._env, surface);
| ^^^^^^^ expected enum `std::ffi::c_void`, found enum `jni::sys::_jobject`
|
= note: expected raw pointer `*mut std::ffi::c_void`
found raw pointer `*mut jni::sys::_jobject`
The problem is that the JNIEnv type expected by ANativeWindow_fromSurface is actually unrelated to jni::sys::JNIEnv entirely.
It's defined in ffi like so:
pub type JNIEnv = *const JNINativeInterface;
#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct JNINativeInterface {
pub reserved0: *mut ::std::os::raw::c_void,
pub reserved1: *mut ::std::os::raw::c_void,
pub reserved2: *mut ::std::os::raw::c_void,
pub reserved3: *mut ::std::os::raw::c_void,
pub GetVersion: ::std::option::Option<unsafe extern "C" fn(arg1: *mut JNIEnv) -> jint>,
pub DefineClass: ::std::option::Option<
unsafe extern "C" fn(
arg1: *mut JNIEnv,
arg2: *const ::std::os::raw::c_char,
arg3: jobject,
arg4: *const jbyte,
arg5: jsize,
) -> jclass,
>,
pub FindClass: ::std::option::Option<
unsafe extern "C" fn(arg1: *mut JNIEnv, arg2: *const ::std::os::raw::c_char) -> jclass,
>,
pub FromReflectedMethod:
::std::option::Option<unsafe extern "C" fn(arg1: *mut JNIEnv, arg2: jobject) -> jmethodID>,
pub FromReflectedField:
::std::option::Option<unsafe extern "C" fn(arg1: *mut JNIEnv, arg2: jobject) -> jfieldID>,
pub ToReflectedMethod: ::std::option::Option<
unsafe extern "C" fn(
arg1: *mut JNIEnv,
arg2: jclass,
arg3: jmethodID,
arg4: jboolean,
) -> jobject,
>
// etc...
}
Given the glue code shown in the example, how do I get a valid reference to ffi::JNIEnv so that I may pass it to the ANativeWindow_fromSurface method. Bonus points if you give advice on converting jni::sys::jobject to *mut std::os::raw::c_void (lifetime concerns, null pointers, etc.).
Full source to ffi definitions
Here is the basic helloworld implementation I used to prove the concept in the accepted answer:
use std::ffi::{CString, CStr};
use std::os::raw::{c_char};
/// Expose the JNI interface for android below
#[cfg(target_os="android")]
#[allow(non_snake_case)]
pub mod android {
extern crate ffi;
use super::*;
use self::ffi::{JNIEnv, jclass, jstring, jlong};
#[derive(Debug)]
struct AppEngine {
greeting: *mut c_char
}
unsafe fn rust_greeting(app: *mut AppEngine) -> *mut c_char {
let app = Box::from_raw(app);
app.greeting
}
/// Constructs an AppEngine object.
fn rust_engine_create(to: *const c_char) -> *mut AppEngine {
let c_str = unsafe { CStr::from_ptr(to) };
let recipient = match c_str.to_str() {
Err(_) => "there",
Ok(string) => string,
};
let greeting = CString::new("Hello ".to_owned() + recipient).unwrap().into_raw();
let app = AppEngine{greeting: greeting};
Box::into_raw(Box::new(app))
}
/// Destroys an AppEngine object previously constructed using `rust_engine_create()`.
unsafe fn rust_engine_destroy(app: *mut AppEngine) {
drop(Box::from_raw(app))
}
#[no_mangle]
pub unsafe extern fn Java_io_waweb_cartoonifyit_MainActivity_greeting(env: &mut JNIEnv, _: jclass, app_ptr: jlong) -> jstring {
let app = app_ptr as *mut AppEngine;
let new_string = env.as_ref().unwrap().NewStringUTF.unwrap();
new_string(env, rust_greeting(app))
}
#[no_mangle]
pub unsafe extern fn Java_io_waweb_cartoonifyit_MainActivity_createNativeApp(env: &mut JNIEnv, _: jclass, java_pattern: jstring) -> jlong {
let get_string_chars = env.as_ref().unwrap().GetStringChars.unwrap();
let is_copy = 0 as *mut u8;
rust_engine_create(get_string_chars(env, java_pattern, is_copy) as *const c_char ) as jlong
}
#[no_mangle]
pub unsafe extern "C" fn Java_io_waweb_cartoonifyit_MainActivity_destroyNativeApp(_: JNIEnv, _: jclass, app_ptr: jlong) {
let app = app_ptr as *mut AppEngine;
rust_engine_destroy(app)
}
}
This is just a proof of concept. More care should be taken when casting raw pointers about. Also see notes about Box::leak in the accepted answer.
A JNIEnv is a pointer to a struct used for communication between Java and native code. This communication ABI is implemented by pretty much every JVM(and android). There are multiple versions of the aforementioned structs, which is what the GetVersion field is for.
It seems to me that you are using an external jni crate along with your own ffi crate generated from this wrapper. I would expect your ffi crate to be the most correct since it is using android headers, instead of the standard JVM headers which a most likely being used by the jni crate.
One last note Box::from_raw(engine_ptr as *mut CameraAppEngine), creates a box which will free the memory located at engine_ptr. This is likely not what you want. Consider using Box::leak to leak the created Box and avoid use after frees.
Related
There's the following line in C:
my_struct->ptr = (char*) data
where as you can see, my_struct has a ptr member of type char*
I have this on Rust:
struct MyStruct {
ptr: *mut libc::c_char
}
then I fill it like this:
unsafe{*my_struct}.ptr = Box::into_raw(my_data) as *mut libc::c_char;
unsafe{((*my_struct).ptr as *mut MyData).as_mut()}.unwrap();
but I'm getting either an unwrap panic on the line above. See that I acessed it immeidately after setting it, so no lifetime, out of scope problems for this data.
Here's the actual MyStruct:
https://github.com/mysql/mysql-server/blob/8.0/include/mysql/udf_registration_types.h#L69
I think the problem is resumed here:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=293ca891bedb9528ae840bcdf737777c
or in other words:
#[derive(Clone, Copy)]
struct MyStruct {
ptr: *mut libc::c_char
}
struct MyData{}
fn main() {
let my_struct = &mut MyStruct{
ptr: std::ptr::null_mut()
} as *mut MyStruct;
let my_data = Box::new(MyData{});
unsafe{*my_struct}.ptr = Box::into_raw(my_data) as *mut libc::c_char;
unsafe{((*my_struct).ptr as *mut MyData).as_mut()}.unwrap();
}
By doing unsafe{*my_struct}.ptr, you're copying *my_struct, then overwriting .ptr of that copy, which is why you're observing no change. Instead, write to (*my_struct).ptr:
unsafe {
(*my_struct).ptr = Box::into_raw(my_data) as *mut libc::c_char;
}
I want to call C++ from Rust. C++ then allocates and fills a buffer, then Rust uses it and deallocates. However, I'd like to deliver back the function pointer to deallocate the data. This is what I tried:
In Rust:
extern "C" {
pub fn openvpn_receive(
instance: *mut OpenVpnInstance,
size: *mut size_t,
deallocate: extern "C" fn(*mut u8),
) -> *mut u8;
}
fn main() {
let size: *mut size_t;
let deallocate = extern "C" fn(*mut u8);
let data: *mut u8 = openvpn_receive(self.instance, size, deallocate);
//use data
deallocate(data);
}
In C++:
uint8_t* openvpn_receive(size_t *size, void (*deallocate_function)(uint8_t *))
{
deallocate_function = &cpp_deallocate_u8;
uint8*t data = receive_data(size);
}
But the Rust code is not valid. How can I make such thing happen?
Maybe it will help You.
https://doc.rust-lang.org/std/keyword.extern.html
https://rust-embedded.github.io/book/interoperability/rust-with-c.html
C++ -> C -> Rust
use libc::{size_t, c_void};
#[repr(C)]
pub struct OpenVpnInstance();
//#[link(name = "your_c_library")]
extern "C" {
pub fn openvpn_receive(
instance: *mut OpenVpnInstance,
size: *mut size_t,
deallocate: *mut c_void,
) -> *mut u8;
}
pub fn my_callback(arg: *mut u8)
{
println!("CB!");
}
fn main()
{
let size: size_t = 42;
let instance = OpenVpnInstance{};
let dummy_arg: u8 = 42;
unsafe {
let cb = my_callback as *mut c_void;
let instance_ptr = &instance as *const _ as *mut OpenVpnInstance;
let arg_ptr = &dummy_arg as *const _ as *mut u8;
openvpn_receive(instance_ptr, size as *mut size_t, cb);
my_callback(arg_ptr)
}
}
Here's some code I generated using c2rust and then cleaned up a bit:
#![feature(libc)]
extern crate libc;
use libc::*;
use std::mem::transmute;
extern "C" {
#[no_mangle]
fn read(__fd: c_int, __buf: *mut c_void, __nbytes: c_ulong) -> c_long;
#[no_mangle]
fn mmap(
__addr: *mut c_void,
__len: c_ulong,
__prot: c_int,
__flags: c_int,
__fd: c_int,
__offset: c_long,
) -> *mut c_void;
}
pub fn main() {
unsafe {
let buf: *mut c_void = mmap(
0 as *mut c_void,
256i32 as c_ulong,
0x1i32 | 0x2i32 | 0x4i32,
0x2i32 | 0x20i32,
-1i32,
0i32 as c_long,
);
read(0i32, buf, 256i32 as c_ulong);
transmute::<*mut c_void, Option<unsafe extern "C" fn() -> ()>>(buf).unwrap()();
}
}
While I understand what it does, I'm not sure how to interpret the last expression. What does Option<unsafe extern "C" fn() -> ()> mean?
We're trying to call an unsafe extern "C" fn() -> (), which is basically a function with no arguments and no return type. My first attempt was to just use the as keyword, as defined in transmute's documentation. I got the following error:
error[E0605]: non-primitive cast: `*mut libc::c_void` as `unsafe extern "C" fn()`
--> wx.rs:32:9
|
32 | (buf as unsafe extern "C" fn() -> ())();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait
It looks like the function is a non-primitive type and this is why I need transmute. I tried the following:
transmute::<
*mut c_void,
unsafe extern "C" fn() -> ()
>(buf)();
And the code compiled and actually ran as expected.
What I still don't understand is why Option was used by c2rust, but the code works fine without it. It appears that unsafe and extern "C" can also be dropped and the code still works, at least for me.
Let's say I have a C function:
void func(char *buf, unsigned int *len);
To call it in Rust, I declared:
pub fn func(buf: *mut ::std::os::raw::c_char, len: *mut ::std::os::raw::c_uint) {
unimplemented!()
}
Then I wrote another wrapper:
pub fn another_func() -> String {
let mut capacity: u32 = 256;
let mut vec = Vec::with_capacity(capacity as usize);
unsafe {
func(vec.as_ptr() as *mut c_char, &capacity as *mut c_uint)
};
String::from_utf8(vec).unwrap();
unimplemented!()
}
But the compiler told me:
error[E0606]: casting `&u32` as `*mut u32` is invalid
--> src/main.rs:...:28
|
307 | &capacity as *mut c_uint)
Why can't I cast capacity into *mut c_unit?
It turns out I have to make the reference mutable.
func(vec.as_ptr() as *mut c_char, &mut capacity as *mut c_uint)
I'm working on a project that uses libcore and no_std, this requires an implementation of various memory manipulation functions. However, when I try to implement memcpy,
#[no_mangle]
#[no_stack_check]
pub unsafe extern fn memcpy(dest: *mut u8, src: *const u8, n: usize) -> *mut u8{
for i in 0..n {
*dest.offset(i as isize) = *src.offset(i as isize);
}
return dest;
}
I get
error: expected ident, found `*`
*dest.offset(i as isize) = *src.offset(i as isize);
^