Comment by severino

Thanks for the explanation. In case FILE was opaque in glibc, would the same test (including <stdio.h> and declaring a variable of type FILE) also fail with the unknown storage size error? If so, would linking again some library (-l) be necessary?

EDIT: after some more thinking I assume the key is that we wouldn't be able to have a variable of type FILE, but a pointer, whose size is always known.