diff --git a/CMakeLists.txt b/CMakeLists.txt
index fe8ffc8..ea40581 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -18,6 +18,15 @@ endif ()
 include_directories ("${CMAKE_CURRENT_BINARY_DIR}")
 
 
+if (WIN32)
+    set(VK_PLATFORM "win32")
+elseif(LINUX)
+    set(VK_PLATFORM "xcb")
+else()
+    message(FATAL_ERROR "unhandled platform")
+endif()
+
+
 ##-----------------------------------------------------------------------------
 add_custom_command (
 OUTPUT
@@ -27,8 +36,9 @@ COMMENT
 COMMAND
     "${PYTHON_EXECUTABLE}"
     "${CMAKE_CURRENT_SOURCE_DIR}/tools/spec.py"
-    "${CMAKE_CURRENT_SOURCE_DIR}/specs/xml/vk.xml"
-    "${CMAKE_CURRENT_BINARY_DIR}/vk.hpp"
+    "--src" "${CMAKE_CURRENT_SOURCE_DIR}/specs/xml/vk.xml"
+    "--dst" "${CMAKE_CURRENT_BINARY_DIR}/vk.hpp"
+    "--platform" "${VK_PLATFORM}"
 DEPENDS
     "${CMAKE_CURRENT_SOURCE_DIR}/tools/spec.py"
     "${CMAKE_CURRENT_SOURCE_DIR}/specs/xml/vk.xml"
diff --git a/tools/spec.py b/tools/spec.py
index 00a3309..e72b586 100644
--- a/tools/spec.py
+++ b/tools/spec.py
@@ -42,7 +42,7 @@ class registry:
 
         return result
 
-    def serialise(self, platform:str):
+    def serialise(self, platform:Set[str]):
         required = []
 
         for (_,f) in self.features.items():
@@ -490,18 +490,18 @@ class extension(type):
             else:
                 raise "Unknown extension node"
 
-    def apply(self, reg:registry, platform:str):
+    def apply(self, reg:registry, platform:Set[str]):
         if self.name in reg.applied:
             return []
         reg.applied.add(self.name)
 
-        if self.platform and self.platform != platform:
+        if self.platform and self.platform not in platform:
             return []
 
         required = []
 
         for dep in self.depends:
-            required = reg.extensions[dep].apply (reg, platform)
+            required = reg.extensions[dep].apply(reg, platform)
 
         logging.info("Applying extension:", self.name, file=sys.stderr)
         for node in self.requires:
@@ -650,8 +650,14 @@ if __name__ == '__main__':
     logging.getLogger().setLevel(logging.WARNING)
 
     parser = argparse.ArgumentParser(description='Transform XML API specification into C++ headers')
-    parser.add_argument('src', type=str, help='the path to the XML file to transform')
-    parser.add_argument('dst', type=str, help='the output path for the result')
+    parser.add_argument('--src', type=str, help='the path to the XML file to transform')
+    parser.add_argument('--dst', type=str, help='the output path for the result')
+    parser.add_argument(
+        '--platform',
+        type=str,
+        action='append',
+        help='a platform to generate output for. may be specific multiple times"'
+    )
 
     args = parser.parse_args()
 
@@ -681,7 +687,7 @@ if __name__ == '__main__':
 
     #reg.extensions['VK_KHR_surface'].apply(reg, platform='xcb')
     extensions = ["VK_KHR_swapchain", "VK_EXT_debug_report", "VK_KHR_external_memory"]
-    q = reg.serialise(platform='xcb')
+    q = reg.serialise(args.platform)
 
 
     dst.write("#pragma once\n")