Add dll export to the callback functions.

[jeroen1602]

I was having issues tyring to build this project for windows. It was complaining that the callback functions have already been defined and can thus not add the dllexport attribute.

Here is an example error:

cgo-gcc-export-header-prolog:123:40: error: redeclaration of 'go_formfill_FFI_GetPage_cb' cannot add 'dllexport' attribute
  123 | extern __declspec(dllexport) FPDF_PAGE go_formfill_FFI_GetPage_cb(FPDF_FORMFILLINFO* me, FPDF_DOCUMENT document, int nPageIndex);
      |                                        ^
fpdf_formfill.go:16:18: note: previous declaration is here
   16 | extern FPDF_PAGE go_formfill_FFI_GetPage_cb(struct _FPDF_FORMFILLINFO *this, FPDF_DOCUMENT document, int nPageIndex);

I go this problem when using the zig (cross) compiler and clang (running native on Windows).

So I made a change to add the dllexport required to the first definitions of these methods when targeting windows.

Using this I did get it to compile for Windows.

[Alexufo]

Did you try to compile go-pdfium with mingw64 for windows?

[jeroen1602]

I did not try mingw64. Either before or after this change.

I did eventually not have to use because the FPDF_RenderPage call I needed cgo for. Checks if the device handle is of a specific type. But because this type has its origin in cgo it is unique to this package so you can never create a handle that has the correct type.

See the code here:

go-pdfium/internal/implementation_cgo/fpdfview_win32.go

Lines 28 to 31 in c0fb871

	hdc, ok := request.DC.(C.HDC)
	if !ok {
	return nil, errors.New("DC is not of type C.HDC")
	}

[Alexufo]

tnx. Have you some manual for zig and your pullrequest? I tried without success

[jeroen1602]

I set the C and C++ compiler it should use using

CC="zig cc -target x86_64-windows" CXX="zig c++ -target x86_64-windows"

[Alexufo]

thx, go compiler does not say about wrong path to pkg-config ?

[jeroen1602]

That does sound familiar.

I believe I searched for a windows port of pkg-config which may work. But decided that cross compiling using either WSL, Macos or a real Linux host was a lot easier.