Gio: Use callback functions with C linkage

* gio/giomm/socketsource.cc: Add extern "C".
* gio/src/application.[ccg|hg]: Call Application_Class::open_callback()
via a local function with C linkage.
* gio/src/asyncinitable.[ccg|hg]: Call
AsyncInitable_Class::init_async_vfunc_callback() and
init_finish_vfunc_callback() via local functions with C linkage.
* gio/src/dbusobjectmanagerclient.ccg: Use a local function with C linkage
instead of Glib::destroy_notify_delete<SlotProxyType>.
* gio/src/file.ccg: Add extern "C".
* gio/src/liststore.ccg: Add a TODO comment.
* gio/src/memoryinputstream.ccg: Add extern "C".
* gio/src/settings.ccg: Add extern "C".
* gio/src/socketcontrolmessage.[ccg|hg]: Call
SocketControlMessage_Class::deserialize_async_vfunc_callback()
via a local function with C linkage.

Code that mixes up C linkage and C++ linkage has undefined behavior.
Most compilers make no difference between C and C++ linkage, so it
has not been an issue so far. Part of issue #1

1 files changed, 8 insertions, 4 deletions

diff --git a/gio/src/settings.ccg b/gio/src/settings.ccg
index 7343bb64..1651c0d5 100644
--- a/gio/src/settings.ccg
+++ b/gio/src/settings.ccg
@@ -33,7 +33,11 @@ struct SettingsMapSlots
   Gio::Settings::SlotSetMapping from_property_to_setting;
 };
 
-gboolean
+extern "C"
+{
+// Shall be static. Non-static functions with C linkage get external linkage,
+// even if they are defined in an anonymous namespace.
+static gboolean
 Settings_get_mapping_callback(
   GValue* to_value, GVariant* from_variant, gpointer user_data)
 {
@@ -52,7 +56,7 @@ Settings_get_mapping_callback(
   return result;
 }
 
-GVariant*
+static GVariant*
 Settings_set_mapping_callback(
   const GValue* from_value, const GVariantType* expected_type, gpointer user_data)
 {
@@ -71,12 +75,12 @@ Settings_set_mapping_callback(
   return result;
 }
 
-void
+static void
 Settings_map_callback_destroy(gpointer user_data)
 {
   delete static_cast<SettingsMapSlots*>(user_data);
 }
-
+} // extern "C"
 } // anonymous namespace
 
 namespace Gio