Pass char * from javascript to c and return char * from c to javascript in Jsctypes

Question

I have this C code in a library "pruebaChar.so" for use with jsctypes:

char * ejecutarComando(char * miComando){

     return miComando;
 }

Then, I have this other code for call "pruebaChar.so":

function miComando(unComando) {

    var {Cu} = require("chrome");
    var {ctypes} = Cu.import("resource://gre/modules/ctypes.jsm", null);

    var lib = ctypes.open("/home/usuario/pruebaChar.so"); 
    var comandoLib = lib.declare("ejecutarComando",             
                        ctypes.default_abi,
                        ctypes.char,    // return type is correct?
                        ctypes.char.ptr // argument type is correct?

    );

    /* How do I pass the argument to function 
        and save the return value from function?*/

    var  returnString = comandoLib(unComando); // Is this correct?
    return ???;

}

What values should I put in return type and argument type? How do I pass the argument to function and save the return value from function?

Thank you very much :)

Answer 1

Instead of ctypes.char try using ctypes.char.ptr and for first argument of string it looks like it would work. which is also ctypes.char.ptr.

here's an example of someone doing a x11 jsctypes with string as return:

• declaring it: https://github.com/foudfou/FireTray/blob/a0c0061cd680a3a92b820969b093cc4780dfb10c/src/modules/ctypes/linux/gtk.jsm#L138

  lib.lazy_bind("gtk_window_get_title", gobject.gchar.ptr, this.GtkWindow.ptr);
  

  this is just lazy binding but it converts to:

  var blah = ctypes.declare('gtk_window_get_title', ctypes.default_abi, ctypes.char.ptr, ctypes.voidptr_t);
  

• then he uses it like this:https://github.com/foudfou/FireTray/blob/0fee978ec353f1153865541d286fe6c8adacdf4b/src/modules/linux/FiretrayWindow.jsm#L141

   let data = ctypes.cast(userData, _find_data_t.ptr);
   let inTitle = data.contents.inTitle;
   let gtkWin = ctypes.cast(gtkWidget, gtk.GtkWindow.ptr);
   let winTitle = gtk.gtk_window_get_title(gtkWin);
   if (!winTitle.isNull()) {
       log.debug(inTitle+" = "+winTitle);
       if (libc.strcmp(inTitle, winTitle) == 0)
           data.contents.outWindow = gtkWin;
       }
   }
  

  • This looks to me like the returned is not a jsstring, as he's using libc.strcmp for c string comparison. so i think you would have to maybe do winTitle.readString() to get the string into your js. maybe not, im not sure, maybe a a winTitle.contents would work too.

Im not pro a js-ctypes but if you can send me your .so file and tell me a bit about i can get it to work and let you know.