A symbol naming a foreign block type defined using define-foreign-block-callable-type.
with-foreign-block allocates a foreign block using type, function and extra-args in the same way as allocate-foreign-block. It then binds foreign-block-var to the foreign block, execute the code of body and frees the foreign block using free-foreign-block, using
If the foreign block is copied in the code of body, the copy may be invoked, and hence the function called, after exiting this macro. See the discussion in Scope of invocation.
LispWorks Foreign Language Interface User Guide and Reference Manual - 29 Sep 2017