C structs can't have fields beginning with an uppercase letter

[bgdncz]

expecting identifier 'end', not 'Filename'
Filename, Contents : Void*

[jhass]

I'd say this is a won't fix. Note that the names don't need to match "the real thing" you're mapping, only the sizes do.

[asterite]

Yes, it's as @jhass says, names don't matter for proper C bindings to work. And because C structs look like regular Crystal objects, accessing their fields look like method calls:

str = SomeLib::Struct.new
str.field = 1
str.field

Method names in Crystal can only begin with lowercase, so that's why there's that restriction.