Problem with mjs ffi parse error

majorninth · June 14, 2019, 3:31pm

If you are asking a question, please follow this template:

My goal is: [describe your goal]
to link a C function to MJS
My actions are: [describe your actions - code, commands, etc]
let rxAcc = ‘’; // Accumulated Rx data
let start;
let finish;
let getMsg=ffi(‘int get_msg(char *,int,int *,int *)’);
let ret=getMsg(rxAcc, rxAcc.length, &start, &finish);

C routine: int get_msg(char *buffer, int len, int *start, int *finish)

The result I see is: [show the result - log, etc]
MJS error: parse error at line 92: [&start, &f]
My expectation & question is: [describe your expectation and your question]
getting the values for ret, start and finish returned to the MJS code

nliviu · June 14, 2019, 5:30pm

With the current C function signature (int get_msg(char *buffer, int len, int *start, int *finish)) you have to ffi it like this

let getMsg = ffi('int get_msg(char *, int, void *, void *)');

In the mJS code use Sys.malloc to allocate start and finish, pass the pointers to your function,
create a Dataview for each pointer, get the value and finally Sys.free the pointers.

It’s possible to use another approach if you replace the start and finish parameters with a pointer to a structure. In this case Converting structs to objects is to be used.

majorninth · June 14, 2019, 6:18pm

excuse my ignorance, but how do I go about “create a Dataview for each pointer”?

nliviu · June 14, 2019, 6:22pm

github.com

cesanta/mjs/blob/master/mjs/lib/api_dataview.js

// **DataView API**
//
// See the original API definition at [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView).
//
// mJS DataView diverges from the original in the following ways:
//
// - No `ArrayBuffer`; plain pointers should be used instead;
// - Since there are no constructors in mJS, `DataView.create()` should be
//   used instead;
// - No float support yet (will be added)

let DataView = {
  // ## **`DataView.create(buf, offset, len)`**
  // Create a DataView object instance. `buf` is a pointer to a plain byte
  // array, `offset` is an offset in in this buffer to start dataview from, and
  // `len` is a length managed by dataview.
  //
  // Return value: an object with the methods described below.
  //
  // Example:

This file has been truncated. show original

majorninth · June 14, 2019, 7:20pm

I see this ‘assuming’ in the MJS document [https://github.com/cesanta/mjs],
how do I get it into my MJS code assuming it exists in my C code?

// Assuming s is a foreign pointer to an instance of my_struct, obtained elsewhere.
let sd = ffi(‘void *get_my_struct_descr(void)’)();
let o = s2o(s, sd);
print(o.a, o.b);

nliviu · June 15, 2019, 6:03am

Create and free the structure in C and ffi them.

struct data *create_data(){
...
}
void free_data(struct data *data){
...
}

dilbert168 · November 29, 2020, 9:32pm

I want to do the same.
but instead of using the Sys.malloc, can I simply declare a fix var buffer in JS and pass it the C function ?