Do not export GC_call_with_reader_lock API symbol in single-threaded …

…build

(fix of commit 08e4e25)

Issue #473 (bdwgc).

* include/gc/gc.h [!GC_THREADS] (GC_call_with_reader_lock): Define as
a macro.
* misc.c (GC_call_with_reader_lock): Do not define (implement) unless
THREADS macro is defined.

include/gc/gc.h

@@ -1567,9 +1567,13 @@ GC_API void * GC_CALL GC_call_with_alloc_lock(GC_fn_type /* fn */,
 /* (shared) mode.  The 3rd argument (release), if non-zero, indicates   */
 /* that fn wrote some data that should be made visible to the thread    */
 /* which acquires the allocator lock in the exclusive mode later.       */
-GC_API void * GC_CALL GC_call_with_reader_lock(GC_fn_type /* fn */,
+#ifdef GC_THREADS
+  GC_API void * GC_CALL GC_call_with_reader_lock(GC_fn_type /* fn */,
                                 void * /* client_data */,
                                 int /* release */) GC_ATTR_NONNULL(1);
+#else
+# define GC_call_with_reader_lock(fn, cd, r) ((void)(r), (fn)(cd))
+#endif
 
 /* These routines are intended to explicitly notify the collector       */
 /* of new threads.  Often this is unnecessary because thread creation   */

misc.c

@@ -2172,28 +2172,6 @@ GC_API unsigned GC_CALL GC_new_proc(GC_mark_proc proc)
     return result;
 }
 
-GC_API void *GC_CALL GC_call_with_reader_lock(GC_fn_type fn,
-                                              void *client_data, int release)
-{
-    void *result;
-
-    READER_LOCK();
-    result = fn(client_data);
-#   ifdef HAS_REAL_READER_LOCK
-      if (release) {
-        READER_UNLOCK_RELEASE();
-#       ifdef LINT2
-          GC_noop1((unsigned)release);
-#       endif
-        return result;
-      }
-#   else
-      UNUSED_ARG(release);
-#   endif
-    READER_UNLOCK();
-    return result;
-}
-
 GC_API void * GC_CALL GC_call_with_alloc_lock(GC_fn_type fn, void *client_data)
 {
     void * result;
@@ -2214,6 +2192,29 @@ GC_API void * GC_CALL GC_call_with_alloc_lock(GC_fn_type fn, void *client_data)
   {
     UNLOCK();
   }
+
+  GC_API void *GC_CALL GC_call_with_reader_lock(GC_fn_type fn,
+                                                void *client_data,
+                                                int release)
+  {
+    void *result;
+
+    READER_LOCK();
+    result = fn(client_data);
+#   ifdef HAS_REAL_READER_LOCK
+      if (release) {
+        READER_UNLOCK_RELEASE();
+#       ifdef LINT2
+          GC_noop1((unsigned)release);
+#       endif
+        return result;
+      }
+#   else
+      UNUSED_ARG(release);
+#   endif
+    READER_UNLOCK();
+    return result;
+  }
 #endif /* THREADS */
 
 GC_API void * GC_CALL GC_call_with_stack_base(GC_stack_base_func fn, void *arg)