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#### 8.7.4 Calling Lisp From C

Calling Lisp functions from C is sometimes possible, but is extremely hackish and poorly supported as of SBCL 0.7.5. See `funcall0` ... `funcall3` in the runtime system. The arguments must be valid SBCL object descriptors (so that e.g. fixnums must be left-shifted by 2.) As of SBCL 0.7.5, the format of object descriptors is documented only by the source code and, in parts, by the old CMUCL `INTERNALS` documentation.

Note that the garbage collector moves objects, and won't be able to fix up any references in C variables. There are three mechanisms for coping with this:

1. The `sb-ext:purify` moves all live Lisp data into static or read-only areas such that it will never be moved (or freed) again in the life of the Lisp session
2. `sb-sys:with-pinned-objects` is a macro which arranges for some set of objects to be pinned in memory for the dynamic extent of its body forms. On ports which use the generational garbage collector (as of SBCL 0.8.3, only the x86) this has a page granularity - i.e. the entire 4k page or pages containing the objects will be locked down. On other ports it is implemented by turning off GC for the duration (so could be said to have a whole-world granularity).
3. Disable GC, using the `without-gcing` macro.