Calling Windows API with libffi on MinGW - c++

Question

I'm trying to have FFI support for my new programming language, which is written in C++ with QT Creator using the MinGW toolchain.
To do this I used a custom-built version of libffi found here: http://ftp.gnome.org/pub/GNOME/binaries/win32/dependencies/libffi-dev_3.0.6-1_win32.zip
I also tried it with another build: http://pkgs.org/fedora-14/fedora-updates-i386/mingw32-libffi-3.0.9-1.fc14.noarch.rpm.html by downloading the SRPM file on Linux, extracting it, and copying the needed files to a Windows partition.
Anyway, I included the required header file, added the import library to the project and put the .dll beside the application's .exe, it compiles and runs, calling MessageBeep() successfully. I tried it next with MessageBoxA(), but it keeps crashing. The debugger doesn't seem to provide much useful information (edit: beside the fact that a call to MessageBoxA did happen) so I keep fiddling with stuff and re-running to no avail.
To isolate the problem from the details of my language, I tried to manually call MessageBoxA by filling myself all the parameters, resulting in the code below, still crashing.
So my question distills to: How can I get the code snippet below to run under QT Creator/MinGW and actually show a message box?
#include "libffi/include/ffi.h"
#include <QLibrary>
void testMessageBox()
{
int n = 4;
ffi_cif cif;
ffi_type **ffi_argTypes = new ffi_type*[n];
void **values = new void*[n];
values[0] = new ulong(0);
values[1] = (void *) "hello";
values[2] = (void *) "mommy";
values[3] = new int32_t(0);
ffi_argTypes[0] = &ffi_type_ulong;
ffi_argTypes[1] = &ffi_type_pointer;
ffi_argTypes[2] = &ffi_type_pointer;
ffi_argTypes[3] = &ffi_type_uint32;
ffi_type *c_retType = &ffi_type_sint32;
int32_t rc; // return value
if (ffi_prep_cif(&cif, FFI_STDCALL, n, c_retType, ffi_argTypes) == FFI_OK)
{
QLibrary lib("user32.dll");
lib.load();
void *msgbox = lib.resolve("MessageBoxA");
ffi_call(&cif, (void (*)()) msgbox, &rc, values);
}
}

Answer 1

you should pass the address to the values array instead of the values. the working code under mingw64 is
#include <stdio.h>
#include <ffi.h>
#include <Windows.h>
int main()
{
ffi_cif cif;
HINSTANCE dllHandle = LoadLibrary("user32.dll");
int n = 4;
ffi_type *ffi_argTypes[4];
void *values[4];
UINT64 a=0;
UINT32 b=0;
TCHAR* s1= "hello";
TCHAR* s2= "hello2";
values[0] = &a;
values[1] = &s1;
values[2] = &s2;
values[3] = &b;
ffi_argTypes[0] = &ffi_type_uint64;
ffi_argTypes[1] = &ffi_type_pointer;
ffi_argTypes[2] = &ffi_type_pointer;
ffi_argTypes[3] = &ffi_type_uint;
ffi_type *c_retType = &ffi_type_sint;
ffi_type rc; // return value
if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 4, &ffi_type_sint, ffi_argTypes) == FFI_OK) {
ffi_call(&cif, FFI_FN(GetProcAddress(dllHandle,"MessageBoxA")), &rc, values);
}
return 0;
}