+#ifdef UNIFDEF_BLOCKS
+.Pp
+The
+.Fn atexit_b
+function is like
+.Fn atexit
+except the callback is a block pointer instead of a function pointer.
+.Bd -ragged -offset indent
+Note: The
+.Fn Block_copy
+function (defined in
+.In Blocks.h )
+is used by
+.Fn atexit_b
+to make a copy of the block, especially for the case when a stack-based
+block might go out of scope when the subroutine returns.
+.Ed
+#endif