ES_SIZEOF: safe and portable sizeof

1 files changed, 32 insertions, 0 deletions

diff --git a/src/include/emscripten.h b/src/include/emscripten.h
new file mode 100644
index 00000000..99cd40e3
--- /dev/null
+++ b/src/include/emscripten.h
@@ -0,0 +1,32 @@
+/**
+ * This file contains a few useful things for compiling C/C++ code
+ * with Emscripten, an LLVM-to-JavaScript compiler.
+ *
+ * The code can be used permissively under the MIT license.
+ *
+ * http://emscripten.org
+ */
+
+// ES_SIZEOF
+//
+// A 'safe' sizeof operator. Sadly llvm-gcc calculates sizeof's
+// and stores them hardcoded as raw values, unlike say offsets
+// within a structure which it nicely details using getelementptr.
+//
+// You should always use ES_SIZEOF instead of sizeof when using
+// Emscripten.
+//
+// Note that this only works with types, not values.
+//
+// Sadly
+//        #define ES_SIZEOF(x) int(&((x*)(NULL))[1])
+// does not work, since the compiler parses and hard-codes the
+// value. So we need to trick it with a function call.
+#ifdef EMSCRIPTEN
+  template<class T>
+  int es_sizeof(T* x) { return int(&x[1]); }
+  #define ES_SIZEOF(T) es_sizeof((T*)NULL)
+#else
+  #define ES_SIZEOF(T) sizeof(T)
+#endif
+