Calling Delphi stdcall function with pAnisChar from node js

Question

I have a legacy Delphi dll which requires a json string as input (pAnsiChar) and returns an int as success or failure. I have managed to connect to the dll from nodejs using node-ffi. However, i am getting return int value points to invalid json string.

Could someone point me in the direction as to how to call a Delphi dll with pAnsiChar as function arguments from node

Thanks

Answer 1

So far as I can tell, Node FFI does not currently allow you to control the calling convention. And the default is cdecl. So on the Delphi side it looks like this:

function MyFunction(str: PAnsiChar): Integer; cdecl;

On the node-ffi side I think it looks like this:

var ffi = require('ffi');
var mylib = ffi.Library('libname', {
  'MyFunction': [ 'int', [ 'string' ] ]
});
var retval = mylib.MyFunction("some string");

If you cannot modify the legacy DLL then I'm afraid that you may need to wrap it in a DLL that does nothing other than export cdecl functions, and then pass them through to the legacy DLL's stdcall functions.

Answer 2

PAnsiChar in Delphi is a char* in C/C++. In the FFI declaration for the DLL function, simply declare the PAnsiChar parameter as a "string", which is a null-terminated char* in FFI.

For example, given this Delphi function:

function ProcessJson(Json: PAnsiChar): Integer; stdcall;

The node.js code would look something like this:

var ffi = require('ffi');

var mydll = ffi.Library('mydll', {
  'ProcessJson': [ 'int', [ 'string' ] ]
});

var ret = mydll.ProcessJson("json content here");