Compliant Solution
In the first compilation module, which is included by the user, the abstract data type is declared as a pointer to a struct. The struct is declared as an incomplete type.
struct string_mx; typedef struct string_mx *string_m;
In the "private" compilation unit struct string_mx is fully defined but non-visible to a user of the API.
struct string_mx {
size_t size;
size_t maxsize;
unsigned char strtype;
char *cstr;
};