That's not possible - the compiler needs to know the size so that it can allocate space for the structure. You can get away with forward declarations of things

That's not possible - the compiler needs to know the size so that it can allocate space for the structure. You can get away with forward declarations of things if you only ever need a pointer to the struct, because the compiler knows how big a pointer is and only needs to know the size of the structure when you dereference it. You also need to know the size when you allocate it - whether you're using malloc, or allocating statically. As you want to leave control of allocation up to the caller, then the caller needs to know the size. Theoretically I suppose you could allocate through a macro that used an explicit size number in the header, but going to those lengths to take the struct out of the header seems a bit extreme.