Pointers to C functions and to malloced data can be hidden in CLISP objects of type machine_type; garbage-collect will not modify its value. But one should not dare to assume that a C stack pointer or the address of a C function in a shared library satisfies the same requirements.

If another pointer is to be viewed as a CLISP object, it is best to box it, e.g. in a SIMPLE-BIT-VECTOR or in an Fpointer (using allocate_fpointer().)