ffi: allow nil object to convert under closure type.

* ffi.c (ffi_closure_put): Only diagnose a bad object if it's
not nil, otherwise let the null value of p through.
This is useful because there are sometimes C interfaces which
take an optional function pointer, whereby a null value
indicates that the pointer is not specified.

1 files changed, 30 insertions, 3 deletions

diff --git a/txr.1 b/txr.1
index 58439d26..72cff575 100644
--- a/txr.1
+++ b/txr.1
@@ -73233,11 +73233,29 @@ the foreign function alters the pointer.
 .coNP FFI type @ closure
 The
 .code closure
-type converts two kinds of Lisp objects to a C pointer: the
+type converts three kinds of Lisp objects to a C pointer: the object
+.codn nil ,
+the
 .code cptr
-type, and the special
+type, or the special
 .code ffi-closure
-type, whose instances are produced by the
+type.
+
+When the
+.code nil
+symbol is converted to a
+.code closure
+type, it becomes a null function pointer.
+
+A
+.code cptr
+object of any kind converts to a
+.codn closure ;
+the internal pointer is converted to a function pointer.
+
+Instances of the
+.code ffi-closure
+type are produced by the
 .code ffi-make-closure
 function, or by calls to functions defined by the
 .code deffi-cb
@@ -73246,6 +73264,15 @@ macro. The
 type is useful for passing callbacks to foreign functions: Lisp functions
 which appear to be C functions to foreign code.
 
+In the reverse direction, when a
+.code closure
+object is converted from the foreign function pointer representation
+to a Lisp object, it becomes a
+.code cptr
+object whose tag is the
+.code closure
+symbol.
+
 .coNP FFI type @ void
 The
 .code void