Variable length arrays in ntdll definitions

[kubkon]

@daurnimator suggested leaving out the variable length arrays (VLAs) from some of our NT definitions; for example, we currently have something like this:

pub const FILE_NAME_INFORMATION = extern struct {
    FileNameLength: ULONG,
    FileName: [1]WCHAR,
};

where FileName is effectively a VLA which cannot be known a priori. The idea here is to weed that out leaving only the static elements, so in the above example, leaving in only FileNameLength. FileName VLA can then be accessed by taking a pointer just behind this static structure.

[kubkon]

For the record: PR where this discussion originated #5993.

[ikskuh]

I think it would be reasonable to include a getter function in structs that use VLAs:

pub const FILE_NAME_INFORMATION = extern struct {
    FileNameLength: ULONG,
    
    pub fn getFileName(self: *@This()) []WCHAR {
        return @intToPtr([*]WCHAR, @ptrToInt(self) + @sizeOf(ULONG))[0..self.FileNameLength];
    }
};

[kubkon]

I think it would be reasonable to include a getter function in structs that use VLAs:

pub const FILE_NAME_INFORMATION = extern struct {
    FileNameLength: ULONG,
    
    pub fn getFileName(self: *@This()) []WCHAR {
        return @intToPtr([*]WCHAR, @ptrToInt(self) + @sizeOf(ULONG))[0..self.FileNameLength];
    }
};

That's a cool idea, I like it!

[daurnimator]

I think it would be reasonable to include a getter function in structs that use VLAs:

Note that I've hit zig bugs doing this in the past.

• unreachable: /deps/zig/src/codegen.cpp:gen_const_val_ptr:6010 #2431 (comment)
• Offset/alignment for 0 bit types #4016
• other issues I can't find right now :(

might be fixed now though

[kubkon]

I think it would be reasonable to include a getter function in structs that use VLAs:

Note that I've hit zig bugs doing this in the past.

• #2431 (comment)
• Offset/alignment for 0 bit types #4016
• other issues I can't find right now :(

might be fixed now though

Thanks for the heads up @daurnimator!

[Pyrolistical]

I wrote a generic version:

pub fn getFileName(comptime T: type, t: *T) []std.os.windows.WCHAR {
    comptime var file_name_offset = 0;
    switch (@typeInfo(T)) {
        .Struct => |struct_info| {
            if (struct_info.layout != .@"extern") {
                @compileError("expected extern struct but was " ++ @tagName(struct_info.layout));
            }
            inline for (struct_info.fields) |field| {
                if (std.mem.eql(u8, field.name, "FileName")) {
                    break;
                }
                file_name_offset += @sizeOf(field.type);
            }
        },
        else => @compileError("expected T to be a struct but was " ++ @typeName(T)),
    }
    const file_name_ptr: [*]std.os.windows.WCHAR = @ptrFromInt(@intFromPtr(t) + file_name_offset);
    const file_name_length_in_bytes = @field(t, "FileNameLength");
    const file_name_length_in_wchars = file_name_length_in_bytes / @sizeOf(std.os.windows.WCHAR);
    return file_name_ptr[0..file_name_length_in_wchars];
}

Works for structs that declare or remove the FileName: [1]WCHAR field.

@MasterQ32 I believe your original code had a bug in it. FileNameLength in the unit of bytes and not the length of the WCHAR FileName