common lisp sbcl manual ffi example failed - lisp

Question

I read sbcl manual,and have a problem at the 8.5 Foreign Data Structure Examples chapter.
I use following examples to verify whether it can run correct.
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Or consider this example of an external C variable and some accesses:
struct c_struct {
short x, y;
char a, b;
int z;
c_struct *n;
};
extern struct c_struct *my_struct;
my_struct->x++;
my_struct->a = 5;
my_struct = my_struct->n;
which can be manipulated in Lisp like this:
(define-alien-type nil
(struct c-struct
(x short)
(y short)
(a char)
(b char)
(z int)
(n (* c-struct))))
(define-alien-variable "my_struct" (* c-struct))
(incf (slot my-struct 'x))
(setf (slot my-struct 'a) 5)
(setq my-struct (slot my-struct 'n))
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Now I run above example code on slime,and it signaled a error.
unknown alien type: C-STRUCT
[Condition of type SIMPLE-ERROR]
Restarts:
0: [RETRY] Retry SLIME REPL evaluation request.
1: [ABORT] Return to sldb level 7.
2: [RETRY] Retry SLIME REPL evaluation request.
3: [ABORT] Return to sldb level 6.
4: [RETRY] Retry SLIME REPL evaluation request.
5: [ABORT] Return to sldb level 5.***
What should I do to define such a struct that can contain it's self point.

Answer 1

Note that the manual also says:
Types may be either named or anonymous. With structure and union types, the name is part of the type specifier, allowing recursively defined types such as:
(struct foo (a (* (struct foo))))
I haven't used SBCL's FFI, but I'd guess that means that you should use:
(define-alien-variable "my_struct" (* (struct c-struct)))