FASM - Passing a String to a Delphi DLL

Question

I have a dll coded in Delphi XE2 with the following code:

procedure xMain(MSG:String);export;
begin
  MessageBox(0,PWideChar(MSG),'Title',0);
end;

exports xMain;

Now, I am importing this function in a FASM application like so:

library  dllfile, "testdll.dll"

import   dllfile,\
         xMain,   "xMain"

And utilizing is as so:

section ".data" data readable writeable

szMSG     db "Message from FASM application!",0

section ".code" code readable executable

invoke    xMain,szMSG

But the resulting messagebox appears with distorted characters as so:

That is the exact result of the function call.

How do I go about solving this issue?

Answer 1

Here are two working samples (using FASM 1.70.03 for Windows):

Ansi version,
dll:

library testdll;

uses
  windows;

procedure xMain(MSG: PAnsiChar); export; stdcall;
begin
  MessageBoxA(0, PAnsiChar(MSG), 'Title', 0);
end;

exports xMain;

begin
end.

exe:

format PE CONSOLE 4.0
entry start

include 'win32a.inc'

section '.code' code readable executable
  start:
    invoke xMain, szMSG
    invoke ExitProcess, 0

section '.data' data readable writeable
    szMSG db 'Message from FASM application!', 0

section '.idata' import data readable writeable
  library kernel32, 'kernel32.dll',\
          dllfile, 'testdll.dll'

  include 'api\kernel32.inc'

  import dllfile,\
         xMain, 'xMain'

Unicode version,
dll:

library testdll;

uses
  windows;

procedure xMain(MSG: PChar); export; stdcall;
begin
  MessageBox(0, PChar(MSG), 'Title', 0);
end;

exports xMain;

begin
end.

exe:

format PE CONSOLE 4.0
entry start

include 'win32w.inc'

section '.code' code readable executable
  start:
    invoke xMain, szMSG
    invoke ExitProcess, 0

section '.data' data readable writeable
    szMSG du 'Message from FASM application!', 0

section '.idata' import data readable writeable
  library kernel32, 'kernel32.dll',\
          dllfile, 'testdll.dll'

  include 'api\kernel32.inc'

  import dllfile,\
         xMain, 'xMain'

Answer 2

Your output is what happens when you send ANSI text to a function that is expecting UTF-16 encoded text. From which I conclude that your FASM code is sending an ANSI payload to the DLL. And the DLL is compiled in a Unicode aware Delphi for which string means UnicodeString, Char means WideChar and so on.

You need to make the two sides match up. For example, by changing the Delphi code:

procedure xMain(Msg: PAnsiChar); stdcall;
begin
  MessageBoxA(0, Msg, 'Title', 0);
end;

Some other points to note:

1. You don't need export on the end of your function declaration. It is ignored by modern Delphi compilers.
2. Don't use Delphi's managed string across module boundaries. In any case on the FASM side you declared the parameter to be a null-terminated pointer to ANSI encoded character array. And that's PAnsiChar.
3. Your code uses the Delphi register calling convention. It's hard to believe that FASM uses that. I'd expect stdcall and Sertac's answer backs that up

Answer 3

Are you sure your MSG argument should be a String and not a PChar?