Add dll example

[Jakobzs]

Hello,

I have created an example for a Windows DLL that displays a messagebox upon being loaded.

Extra notes:
Looking into the window-rs documentation I was not able to locate DWORD nor LPVOID for DllMain so I opted to check the winapi crate, and found:

1. DWORD is equivalent to c_ulong which is equivalent to u32
2. LPVOID is equivalent to *mut c_void

[ghost]

All CLA requirements met.

[tim-weis]

DllMain returns a BOOL, which maps to an int in C, corresponding to an i32 in Rust.

[tim-weis]

DllMain runs under the loader lock, making lots of things problematic, like "call[ing] functions in User32.dll or Gdi32.dll". A safe alternative for MessageBoxA would be OutputDebugStringA.

[tim-weis]

Needs to be adjusted to match the correct return type (see above).

[tim-weis]

I'm not convinced that this example is complete enough. The primary reason to produce a DLL is to have clients consume it. In its current shape, the DLL doesn't export any useful functionality, and there is no client code to consume the library either. The latter is particularly interesting, and how this should be done is still under investigation.

It's usually a good idea to submit an issue to discuss the scope and implementation of an example before pushing a PR.

[Jakobzs]

I'm not convinced that this example is complete enough. The primary reason to produce a DLL is to have clients consume it. In its current shape, the DLL doesn't export any useful functionality, and there is no client code to consume the library either. The latter is particularly interesting, and how this should be done is still under investigation.

It's usually a good idea to submit an issue to discuss the scope and implementation of an example before pushing a PR.

Thanks for the feedback Tim.

You are indeed right about BOOL and bool being a mistake since bool only occupies 1 byte, whereas BOOL being an i32 occupies 4 bytes. I have made use of the BOOL struct as found in windows-rs to resolve it.

Previously for testing I used a basic program that called LoadLibrary, but instead now I have created a project dll_loader that takes a filename (or path) as argument and calls LoadLibrary on it.

Lastly I switched from MessageBox to println! based on the reasoning you also specified. I tried OutputDebugStringA but it did not work for me, although I also feel it may not be needed for the example.

In hindsight: Yes, an issue should have been created to discuss the details.

[kennykerr]

You can of course create a DLL with Rust, but that does not require anything from the Windows crate. Once I have #81 completed, we should be able to add a component example that illustrates how to build a DLL that actually implements various kinds of APIs. Until then I don't think it will be beneficial to add a DLL example.

By the way, writing your own DllMain is almost always a bad idea.