Need a working example of Firefox ctypes output string parameter

Question

Hi I have an XPCOM component which I'm now converting to use ctypes.

I was able to create functions that take a wchar_t* and I define the functions using ctypes.jschar.ptr. This all works great, but how do I use output parameters when I need to create wchar_t pointers and array of pointers?

I have done a lot of reading and I'm confused.

1. How should I allocate the memory inside my C dll? should I use malloc? if so how would that get freed up?
2. How would you allocate and handle an out parameter of wchar_t * ? would I pass it in from javascript as a CData I declate before?
3. How should I handle a wchar_t string array?

Can anyone give me some code examples of say how to handle a function like this? (both on the C side of things, using malloc? or what should I use to allcoate the memory and on the javascript side, how this should be handled)?

int MyFunc(wchar_t** outString, wchar_t*** outStringArray)

Thanks!

Answer 1

I think I can help with your second question.

On the C side, your method which accepts an output parameter looks like:

const char* useAnOutputParam(const char** outParam) {
    const char* str = "You invoked useAnOutputParam\0";
    const char* outStr = "This is your outParam\0";
    *outParam = outStr;
    return str;
}

And on the JavaScript side:

Components.utils.import("resource://gre/modules/ctypes.jsm");

/**
 * PlayingWithJsCtypes namespace.
 */
if ("undefined" == typeof(PlayingWithJsCtypes)) {
  var PlayingWithJsCtypes = {};
};


var myLib = {
    lib: null,

    init: function() {
        //Open the library you want to call
        this.lib = ctypes.open("/path/to/library/libTestLibraryC.dylib");

        //Declare the function you want to call
        this.useAnOutputParam = this.lib.declare("useAnOutputParam",
                        ctypes.default_abi,
                        ctypes.char.ptr,
                        ctypes.char.ptr.ptr);

    },

    useAnOutputParam: function(outputParam) {
        return this.useAnOutputParam(outputParam);
    },

    //need to close the library once we're finished with it
    close: function() {
        this.coreFoundationLib.close();
    }
};

PlayingWithJsCtypes.BrowserOverlay = {

    /*
     * This is called from the XUL code
     */
    doSomething : function(aEvent) {
        myLib.init();

        //Instantiate the output parameter
        let outputParam = new ctypes.char.ptr();

        //Pass through the address of the output parameter
        let retVal = myLib.useAnOutputParam(outputParam.address());
        alert ("retVal.readString() " + retVal.readString());
        alert ("outputParam.toString() " + outputParam.readString());

        myLib.close();
    }
};

This forum post helped: http://old.nabble.com/-js-ctypes--How-to-handle-pointers-and-get-multiple-values-td27959903.html