Use callback functins with C linkage

* cairomm/script.cc:
* cairomm/surface.cc: Use local 'extern "C"' callback functions.
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. But see issue glibmm#1.

2 files changed, 33 insertions, 9 deletions

diff --git a/cairomm/script.cc b/cairomm/script.cc
index 1caaf31..2f705c9 100644
--- a/cairomm/script.cc
+++ b/cairomm/script.cc
@@ -80,6 +80,17 @@ cairo_status_t device_write_func_wrapper(void* closure, const unsigned char* dat
   return static_cast<cairo_status_t>((*write_func)(data, length));
 }
 
+//TODO: When we can break ABI, move the code from device_write_func_wrapper()
+// to c_device_write_func_wrapper() and remove device_write_func_wrapper().
+extern "C"
+{
+static cairo_status_t c_device_write_func_wrapper(
+  void* closure, const unsigned char* data, unsigned int length)
+{
+  return device_write_func_wrapper(closure, data, length);
+}
+} // extern "C"
+
 static void set_write_slot(cairo_device_t* surface,
                            Surface::SlotWriteFunc* slot) {
   // the slot will automatically be freed by device_free_slot() when the
@@ -91,8 +102,7 @@ static void set_write_slot(cairo_device_t* surface,
 RefPtr<Script> Script::create_for_stream(const Surface::SlotWriteFunc& write_func)
 {
   auto slot_copy = new Surface::SlotWriteFunc(write_func);
-  auto cobject = cairo_script_create_for_stream(device_write_func_wrapper,
-                                                           slot_copy);
+  auto cobject = cairo_script_create_for_stream(c_device_write_func_wrapper, slot_copy);
   check_status_and_throw_exception(cairo_device_status(cobject));
   set_write_slot(cobject, slot_copy);
   return make_refptr_for_instance<Script>(new Script(cobject, true /* has reference */));
diff --git a/cairomm/surface.cc b/cairomm/surface.cc
index 9982625..275c2ae 100644
--- a/cairomm/surface.cc
+++ b/cairomm/surface.cc
@@ -69,6 +69,21 @@ cairo_status_t write_func_wrapper(void* closure, const unsigned char* data, unsi
   return static_cast<cairo_status_t>((*write_func)(data, length));
 }
 
+//TODO: When we can break ABI, move the code from [read,write]_func_wrapper()
+// to c_[read,write]_func_wrapper() and remove [read,write]_func_wrapper().
+extern "C"
+{
+static cairo_status_t c_read_func_wrapper(void* closure, unsigned char* data, unsigned int length)
+{
+  return read_func_wrapper(closure, data, length);
+}
+
+static cairo_status_t c_write_func_wrapper(void* closure, const unsigned char* data, unsigned int length)
+{
+  return write_func_wrapper(closure, data, length);
+}
+} // extern "C"
+
 Surface::Surface(cairo_surface_t* cobject, bool has_reference)
 : m_cobject(nullptr)
 {
@@ -245,9 +260,8 @@ void Surface::write_to_png_stream(const SlotWriteFunc& write_func)
     delete old_slot;
   auto slot_copy = new SlotWriteFunc(write_func);
   set_write_slot(cobj(), slot_copy);
-  auto status = cairo_surface_write_to_png_stream(cobj(),
-                                                         &write_func_wrapper,
-                                                         slot_copy /*closure*/);
+  auto status = cairo_surface_write_to_png_stream(
+    cobj(), &c_write_func_wrapper, slot_copy /*closure*/);
   check_status_and_throw_exception(status);
 }
 #endif
@@ -333,7 +347,7 @@ RefPtr<ImageSurface> ImageSurface::create_from_png_stream(const SlotReadFunc& re
 {
   auto slot_copy = new SlotReadFunc(read_func);
   auto cobject =
-    cairo_image_surface_create_from_png_stream(&read_func_wrapper, slot_copy);
+    cairo_image_surface_create_from_png_stream(&c_read_func_wrapper, slot_copy);
   check_status_and_throw_exception(cairo_surface_status(cobject));
   set_read_slot(cobject, slot_copy);
   return make_refptr_for_instance<ImageSurface>(new ImageSurface(cobject, true /* has reference */));
@@ -451,7 +465,7 @@ RefPtr<PdfSurface> PdfSurface::create_for_stream(const SlotWriteFunc& write_func
 {
   auto slot_copy = new SlotWriteFunc(write_func);
   auto cobject =
-    cairo_pdf_surface_create_for_stream(write_func_wrapper, slot_copy,
+    cairo_pdf_surface_create_for_stream(c_write_func_wrapper, slot_copy,
                                         width_in_points, height_in_points);
   check_status_and_throw_exception(cairo_surface_status(cobject));
   set_write_slot(cobject, slot_copy);
@@ -519,7 +533,7 @@ RefPtr<PsSurface> PsSurface::create_for_stream(const SlotWriteFunc& write_func,
 {
   auto slot_copy = new SlotWriteFunc(write_func);
   auto cobject =
-    cairo_ps_surface_create_for_stream(write_func_wrapper, slot_copy,
+    cairo_ps_surface_create_for_stream(c_write_func_wrapper, slot_copy,
                                        width_in_points, height_in_points);
   check_status_and_throw_exception(cairo_surface_status(cobject));
   set_write_slot(cobject, slot_copy);
@@ -621,7 +635,7 @@ RefPtr<SvgSurface> SvgSurface::create_for_stream(const SlotWriteFunc& write_func
 {
   auto slot_copy = new SlotWriteFunc(write_func);
   auto cobject =
-    cairo_svg_surface_create_for_stream(write_func_wrapper, slot_copy,
+    cairo_svg_surface_create_for_stream(c_write_func_wrapper, slot_copy,
                                         width_in_points, height_in_points);
   check_status_and_throw_exception(cairo_surface_status(cobject));
   set_write_slot(cobject, slot_copy);