From 27d03c4fb2eaedc0475d19cf94cb0485aec972ba Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Sun, 6 Jun 2021 16:01:35 -0700
Subject: 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.
---
 ffi.c |  2 +-
 txr.1 | 33 ++++++++++++++++++++++++++++++---
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/ffi.c b/ffi.c
index 25d8a8d5..7c001187 100644
--- a/ffi.c
+++ b/ffi.c
@@ -2142,7 +2142,7 @@ static void ffi_closure_put(struct txr_ffi_type *tft, val ptr, mem_t *dst,
   } else if (type == ffi_closure_s) {
     struct txr_ffi_closure *tfcl = ffi_closure_struct(ptr);
     p = tfcl->fptr;
-  } else {
+  } else if (ptr != nil) {
     uw_throwf(error_s, lit("~a: ~s cannot be used as function pointer"),
               self, ptr, nao);
   }
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
-- 
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