The foreign type
foreign-block-pointer
corresponds to the opaque "Block" object in C and derived languages that are introduced in CLANG and used by Apple.
A foreign block pointer should be regarded as opaque, and should not be manipulated or used except as described in Block objects in C (foreign blocks).
A foreign block that is allocated directly by the Lisp side (for example by allocate-foreign-block or with-foreign-block) prints as
"lisp-foreign-block-pointer"
.
foreign-block-pointer
is implemented in LispWorks for Macintosh only.
allocate-foreign-block
define-foreign-block-callable-type
define-foreign-block-invoker
foreign-block-copy
foreign-block-release
free-foreign-block
with-foreign-block
with-local-foreign-block
Block objects in C (foreign blocks)
LispWorks Foreign Language Interface User Guide and Reference Manual - 7 Dec 2011