3.5 Pointer Types
As a result type, the address referenced by a _pointer value must not refer to memory managed by the garbage collector (unless the address corresponds to a value that supports interior pointers and that is otherwise referenced to preserve the value from garbage collection). The reference is not traced or updated by the garbage collector. As an argument type, _pointer works for a reference to either GC-managed memory or not.
The equal? predicate equates C pointers (including pointers
for _gcpointer and possibly containing an offset) when they
refer to the same address—
In the BC implementation of Racket, a _gcpointer result pointer can reference to memory that is not managed by the garbage collector, but beware of using an address that might eventually become managed by the garbage collector. For example, if a reference is created by malloc with 'raw and released by free, then the free may allow the memory formerly occupied by the reference to be used later by the garbage collector.
The cpointer-gcable? function returns #t for a cpointer generated via the _gcpointer result type. See cpointer-gcable? for more information.
As a result type with a function type, _racket or _scheme permits multiple values, but multiple values are not allowed in combination with a true value for #:in-original-place? or #:async-apply in _cprocedure or _fun.
A type generated by _cprocedure or _fun builds on _fpointer, and normally _cprocedure or _fun should be used instead of _fpointer.