A type can be used before it is defined; one common way to deal with that situation is just to use a type reference to a type which has not yet been defined.
Another way is with the `x' type descriptor, which is followed by `s' for a structure tag, `u' for a union tag, or `e' for a enumerator tag, followed by the name of the tag, followed by `:'. If the name contains `::' between a `<' and `>' pair (for C++ templates), such a `::' does not end the name--only a single `:' ends the name; see section Defining a Symbol Within Another Type.
For example, the following C declarations:
struct foo; struct foo *bar;
produce:
.stabs "bar:G16=*17=xsfoo:",32,0,0,0
Not all debuggers support the `x' type descriptor, so on some machines GCC does not use it. I believe that for the above example it would just emit a reference to type 17 and never define it, but I haven't verified that.
Modula-2 imported types, at least on AIX, use the `i' type descriptor, which is followed by the name of the module from which the type is imported, followed by `:', followed by the name of the type. There is then optionally a comma followed by type information for the type. This differs from merely naming the type (see section Giving a Type a Name) in that it identifies the module; I don't understand whether the name of the type given here is always just the same as the name we are giving it, or whether this type descriptor is used with a nameless stab (see section The String Field), or what. The symbol ends with `;'.
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