2.4.2 Mapping the null value

In place of NULL, there are several constants providing null values for frequently used types, such as $NULL-HANDLE, $NULL-RECT, and $NULL-STRING. Null values for other pointer types may be designated by the expression null-pointer(<FOO>). Use the function null-pointer? to test whether a value is null. Do not use the expression if(ptr)... as is often done in C, since a null pointer is not the same as #f. There are also functions null-handle and null-handle? for creating and testing handles, since conceptually they are not necessarily pointers.

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C FFI and Win 32 Reference - 31 MAR 2000