Converting *opaque {} to [*c]struct

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,
1

Hi! hope youre doing well
been porting my c program into zig
im using marelsoft win32zig bindings and a c library for loading opengl function pointers called glad
glad needs device context handler to load function pointers and i cant pass the device context handler from zig binding struct

// HDC i have from zig bindgen
pub const HDC = *opaque {}; 

 // HDC needed inside translate c function
pub const HDC = [*c]struct_HDC__;
pub const struct_HDC__ = extern struct {
    unused: c_int = 0,
    pub const Arc = __root.Arc;
    pub const BitBlt = __root.BitBlt;
    pub const CancelDC = __root.CancelDC;
    pub const Chord = __root.Chord;
    pub const ChoosePixelFormat = __root.ChoosePixelFormat;
    pub const CloseMetaFile = __root.CloseMetaFile;
    pub const CreateCompatibleBitmap = __root.CreateCompatibleBitmap;
    pub const CreateDiscardableBitmap = __root.CreateDiscardableBitmap;
    pub const CreateCompatibleDC = __root.CreateCompatibleDC;
    pub const CreateDIBitmap = __root.CreateDIBitmap;
    pub const DeleteDC = __root.DeleteDC;
    ...
}

// translate c function that needs HDC struct as arg
pub export fn gladLoaderLoadWGL(arg_hdc: HDC) c_int {
    var hdc = arg_hdc;
    _ = &hdc;
    return gladLoadWGLUserPtr(hdc, glad_wgl_get_proc, null);
}
2

you have pointers of different types, so you would need to @ptrCast them

3

tried glad.gladLoaderLoadWGL(@ptrCast(device_context_handle)) and got these errors

run
└─ run exe namad
   └─ compile exe namad Debug native 2 errors
.zig-cache\o\f66a2503ece2aa7ef812a93594ca27df\glad-win32-wgl.zig:63353:210: error: type 'usize' cannot represent integer value '-1'
    if ((@as(?*anyopaque, @ptrCast(@alignCast(@constCast(glad_wglGetExtensionsStringARB)))) == @as(?*anyopaque, null)) or (@as(HANDLE, @ptrCast(@alignCast(hdc))) == @as(HANDLE, @ptrFromInt(@as(usize, @intCast(@as(LONG_PTR, -@as(c_int, 1)))))))) {
                                                                                                                                                                                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
referenced by:
    glad_wgl_find_extensions_wgl: .zig-cache\o\f66a2503ece2aa7ef812a93594ca27df\glad-win32-wgl.zig:63380:57
    gladLoadWGLUserPtr: .zig-cache\o\f66a2503ece2aa7ef812a93594ca27df\glad-win32-wgl.zig:63291:39
    8 reference(s) hidden; use '-freference-trace=10' to see all references
src\nile\nile.zig:217:40: error: @ptrCast increases pointer alignment
            _ = glad.gladLoaderLoadWGL(@ptrCast(device_context_handle));
                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src\nile\nile.zig:217:49: note: '?*win32.graphics.gdi.HDC__opaque_3290' has alignment '1'
            _ = glad.gladLoaderLoadWGL(@ptrCast(device_context_handle));
                                                ^~~~~~~~~~~~~~~~~~~~~
src\nile\nile.zig:217:40: note: '[*c]glad-win32-wgl.struct_HDC__' has alignment '4'
src\nile\nile.zig:217:40: note: use @alignCast to assert pointer alignment
error: 2 compilation errors

tried these two changes bellow but still get some strange errors
gladLoaderLoadWGL(@as(glad.HDC, @alignCast(device_context_handle.?)));
gladLoaderLoadWGL(@ptrCast(@alignCast(device_context_handle.?)));

run
└─ run exe namad
   └─ compile exe namad Debug native 1 errors
.zig-cache\o\f66a2503ece2aa7ef812a93594ca27df\glad-win32-wgl.zig:63353:210: error: type 'usize' cannot represent integer value '-1'
    if ((@as(?*anyopaque, @ptrCast(@alignCast(@constCast(glad_wglGetExtensionsStringARB)))) == @as(?*anyopaque, null)) or (@as(HANDLE, @ptrCast(@alignCast(hdc))) == @as(HANDLE, @ptrFromInt(@as(usize, @intCast(@as(LONG_PTR, -@as(c_int, 1)))))))) {
                                                                                                                                                                                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
referenced by:
    glad_wgl_find_extensions_wgl: .zig-cache\o\f66a2503ece2aa7ef812a93594ca27df\glad-win32-wgl.zig:63380:57
    gladLoadWGLUserPtr: .zig-cache\o\f66a2503ece2aa7ef812a93594ca27df\glad-win32-wgl.zig:63291:39
    7 reference(s) hidden; use '-freference-trace=9' to see all references
error: 1 compilation errors
4

to turn a minus 1 into a usize would require a @bitCast, assuming you mean to fill that memory with all 1s. for basics of type conversion and Zig’s type system like this you’d really be better served by reading the official documentation.