diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8c6772834..455556f01 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -433,19 +433,14 @@ set(LIBOPENMC_FORTRAN_SRC
   src/tallies/trigger_header.F90
 )
 set(LIBOPENMC_CXX_SRC
-  src/error.h
   src/initialize.cpp
-  src/hdf5_interface.h
   src/hdf5_interface.cpp
   src/message_passing.cpp
   src/random_lcg.cpp
-  src/random_lcg.h
   src/simulation.cpp
   src/surface.cpp
-  src/surface.h
-  src/xml_interface.h
-  src/pugixml/pugixml.cpp
-  src/pugixml/pugixml.hpp)
+  src/xml_interface.cpp
+  src/pugixml/pugixml.cpp)
 add_library(libopenmc SHARED ${LIBOPENMC_FORTRAN_SRC} ${LIBOPENMC_CXX_SRC})
 set_target_properties(libopenmc PROPERTIES OUTPUT_NAME openmc)
 add_executable(${program} src/main.cpp)
diff --git a/src/hdf5_interface.cpp b/src/hdf5_interface.cpp
index cbbd7d7c3..1502ec2e1 100644
--- a/src/hdf5_interface.cpp
+++ b/src/hdf5_interface.cpp
@@ -2,8 +2,8 @@
 
 #include <array>
 #include <cstring>
-#include <string>
 #include <sstream>
+#include <string>
 
 #include "hdf5.h"
 #include "hdf5_hl.h"
diff --git a/src/surface.cpp b/src/surface.cpp
index 9db9db3fa..2c3279604 100644
--- a/src/surface.cpp
+++ b/src/surface.cpp
@@ -11,6 +11,12 @@
 
 namespace openmc {
 
+//==============================================================================
+// Global variables
+//==============================================================================
+
+int32_t n_surfaces;
+
 //==============================================================================
 // Helper functions for reading the "coeffs" node of an XML surface element
 //==============================================================================
@@ -1232,4 +1238,47 @@ read_surfaces(pugi::xml_node *node)
   }
 }
 
+//==============================================================================
+// Fortran compatibility functions
+//==============================================================================
+
+extern "C" Surface* surface_pointer(int surf_ind) {return surfaces_c[surf_ind];}
+
+extern "C" int surface_id(Surface *surf) {return surf->id;}
+
+extern "C" int surface_bc(Surface *surf) {return surf->bc;}
+
+extern "C" bool surface_sense(Surface *surf, double xyz[3], double uvw[3])
+{return surf->sense(xyz, uvw);}
+
+extern "C" void surface_reflect(Surface *surf, double xyz[3], double uvw[3])
+{surf->reflect(xyz, uvw);}
+
+extern "C" double
+surface_distance(Surface *surf, double xyz[3], double uvw[3], bool coincident)
+{return surf->distance(xyz, uvw, coincident);}
+
+extern "C" void surface_normal(Surface *surf, double xyz[3], double uvw[3])
+{return surf->normal(xyz, uvw);}
+
+extern "C" void surface_to_hdf5(Surface *surf, hid_t group)
+{surf->to_hdf5(group);}
+
+extern "C" int surface_i_periodic(PeriodicSurface *surf)
+{return surf->i_periodic;}
+
+extern "C" bool
+surface_periodic(PeriodicSurface *surf, PeriodicSurface *other, double xyz[3],
+                 double uvw[3])
+{return surf->periodic_translate(other, xyz, uvw);}
+
+extern "C" void free_memory_surfaces_c()
+{
+  for (int i = 0; i < n_surfaces; i++) {delete surfaces_c[i];}
+  delete surfaces_c;
+  surfaces_c = nullptr;
+  n_surfaces = 0;
+  surface_dict.clear();
+}
+
 } // namespace openmc
diff --git a/src/surface.h b/src/surface.h
index 627a8fbae..15ac0d5bd 100644
--- a/src/surface.h
+++ b/src/surface.h
@@ -25,15 +25,14 @@ extern "C" const int BC_PERIODIC {3};
 //==============================================================================
 
 extern "C" double FP_COINCIDENT;
-constexpr double INFTY{std::numeric_limits<double>::max()};
+constexpr double INFTY {std::numeric_limits<double>::max()};
 constexpr int C_NONE {-1};
 
 //==============================================================================
 // Global variables
 //==============================================================================
 
-// Braces force n_surfaces to be defined here, not just declared.
-extern "C" {int32_t n_surfaces {0};}
+extern "C" int32_t n_surfaces;
 
 class Surface;
 Surface **surfaces_c;
@@ -384,44 +383,19 @@ public:
 // Fortran compatibility functions
 //==============================================================================
 
-extern "C" Surface* surface_pointer(int surf_ind) {return surfaces_c[surf_ind];}
-
-extern "C" int surface_id(Surface *surf) {return surf->id;}
-
-extern "C" int surface_bc(Surface *surf) {return surf->bc;}
-
-extern "C" bool surface_sense(Surface *surf, double xyz[3], double uvw[3])
-{return surf->sense(xyz, uvw);}
-
-extern "C" void surface_reflect(Surface *surf, double xyz[3], double uvw[3])
-{surf->reflect(xyz, uvw);}
-
-extern "C" double
-surface_distance(Surface *surf, double xyz[3], double uvw[3], bool coincident)
-{return surf->distance(xyz, uvw, coincident);}
-
-extern "C" void surface_normal(Surface *surf, double xyz[3], double uvw[3])
-{return surf->normal(xyz, uvw);}
-
-extern "C" void surface_to_hdf5(Surface *surf, hid_t group)
-{surf->to_hdf5(group);}
-
-extern "C" int surface_i_periodic(PeriodicSurface *surf)
-{return surf->i_periodic;}
-
-extern "C" bool
-surface_periodic(PeriodicSurface *surf, PeriodicSurface *other, double xyz[3],
-                 double uvw[3])
-{return surf->periodic_translate(other, xyz, uvw);}
-
-extern "C" void free_memory_surfaces_c()
-{
-  for (int i = 0; i < n_surfaces; i++) {delete surfaces_c[i];}
-  delete surfaces_c;
-  surfaces_c = nullptr;
-  n_surfaces = 0;
-  surface_dict.clear();
-}
+extern "C" Surface* surface_pointer(int surf_ind);
+extern "C" int surface_id(Surface *surf);
+extern "C" int surface_bc(Surface *surf);
+extern "C" bool surface_sense(Surface *surf, double xyz[3], double uvw[3]);
+extern "C" void surface_reflect(Surface *surf, double xyz[3], double uvw[3]);
+extern "C" double surface_distance(Surface *surf, double xyz[3], double uvw[3],
+                                   bool coincident);
+extern "C" void surface_normal(Surface *surf, double xyz[3], double uvw[3]);
+extern "C" void surface_to_hdf5(Surface *surf, hid_t group);
+extern "C" int surface_i_periodic(PeriodicSurface *surf);
+extern "C" bool surface_periodic(PeriodicSurface *surf, PeriodicSurface *other,
+                                 double xyz[3], double uvw[3]);
+extern "C" void free_memory_surfaces_c();
 
 } // namespace openmc
 #endif // SURFACE_H
diff --git a/src/xml_interface.cpp b/src/xml_interface.cpp
new file mode 100644
index 000000000..aea1c2c17
--- /dev/null
+++ b/src/xml_interface.cpp
@@ -0,0 +1,40 @@
+#include "xml_interface.h"
+
+#include <algorithm>  // for std::transform
+#include <sstream>
+#include <string>
+
+#include "pugixml/pugixml.hpp"
+#include "error.h"
+
+
+namespace openmc {
+
+std::string
+get_node_value(pugi::xml_node node, const char *name)
+{
+  // Search for either an attribute or child tag and get the data as a char*.
+  const pugi::char_t *value_char;
+  if (node.attribute(name)) {
+    value_char = node.attribute(name).value();
+  } else if (node.child(name)) {
+    value_char = node.child_value(name);
+  } else {
+    std::stringstream err_msg;
+    err_msg << "Node \"" << name << "\" is not a member of the \""
+            << node.name() << "\" XML node";
+    fatal_error(err_msg);
+  }
+
+  // Convert to lowercase string.
+  std::string value(value_char);
+  std::transform(value.begin(), value.end(), value.begin(), ::tolower);
+
+  // Remove whitespace.
+  value.erase(0, value.find_first_not_of(" \t\r\n"));
+  value.erase(value.find_last_not_of(" \t\r\n") + 1);
+
+  return value;
+}
+
+} // namespace openmc
diff --git a/src/xml_interface.h b/src/xml_interface.h
index 778497562..52aa6003f 100644
--- a/src/xml_interface.h
+++ b/src/xml_interface.h
@@ -1,8 +1,6 @@
 #ifndef XML_INTERFACE_H
 #define XML_INTERFACE_H
 
-#include <algorithm>  // for std::transform
-#include <sstream>
 #include <string>
 
 #include "pugixml/pugixml.hpp"
@@ -10,39 +8,13 @@
 
 namespace openmc {
 
-bool
+inline bool
 check_for_node(pugi::xml_node node, const char *name)
 {
   return node.attribute(name) || node.child(name);
 }
 
-
-std::string
-get_node_value(pugi::xml_node node, const char *name)
-{
-  // Search for either an attribute or child tag and get the data as a char*.
-  const pugi::char_t *value_char;
-  if (node.attribute(name)) {
-    value_char = node.attribute(name).value();
-  } else if (node.child(name)) {
-    value_char = node.child_value(name);
-  } else {
-    std::stringstream err_msg;
-    err_msg << "Node \"" << name << "\" is not a member of the \""
-            << node.name() << "\" XML node";
-    fatal_error(err_msg);
-  }
-
-  // Convert to lowercase string.
-  std::string value(value_char);
-  std::transform(value.begin(), value.end(), value.begin(), ::tolower);
-
-  // Remove whitespace.
-  value.erase(0, value.find_first_not_of(" \t\r\n"));
-  value.erase(value.find_last_not_of(" \t\r\n") + 1);
-
-  return value;
-}
+std::string get_node_value(pugi::xml_node node, const char *name);
 
 } // namespace openmc
 #endif // XML_INTERFACE_H