Unless I've been wrong all these years in C you can write a header file which says this is a pointer to T, without ever saying what's in T and C will not allow people to access the elements of T, since it doesn't know what they are, but they can have the pointer since it doesn't need to know how T is laid out to know it's a pointer and all pointers are laid out the same in C.

Then for your internal stuff you define what's inside T and you can use T normally.

Also, even if you're returning an index, learn from Unix's mistake and don't say it's an integer. Give it some other type, even if that type is just an alias for a primitive integer type, because at least you are signalling that these are not integers and you might make a few programmers not muddle these with other integers they've got. A file descriptor is not, in fact, a process ID, a port number, or a retry count, and 5 is only any of those things if you specify which of them it is.