Fixed potential strict-aliasing violation in big-endian code.

Also added a test.

Tested on: Linux.

Change-Id: I7b3230f8f6043eec139d5e3e8c9cb45814124274

include/flatbuffers/base.h

@@ -186,14 +186,20 @@ template<typename T> T EndianSwap(T t) {
   if (sizeof(T) == 1) {   // Compile-time if-then's.
     return t;
   } else if (sizeof(T) == 2) {
-    auto r = FLATBUFFERS_BYTESWAP16(*reinterpret_cast<uint16_t *>(&t));
-    return *reinterpret_cast<T *>(&r);
+    union { T t; uint16_t i; } u;
+    u.t = t;
+    u.i = FLATBUFFERS_BYTESWAP16(u.i);
+    return u.t;
   } else if (sizeof(T) == 4) {
-    auto r = FLATBUFFERS_BYTESWAP32(*reinterpret_cast<uint32_t *>(&t));
-    return *reinterpret_cast<T *>(&r);
+    union { T t; uint32_t i; } u;
+    u.t = t;
+    u.i = FLATBUFFERS_BYTESWAP32(u.i);
+    return u.t;
   } else if (sizeof(T) == 8) {
-    auto r = FLATBUFFERS_BYTESWAP64(*reinterpret_cast<uint64_t *>(&t));
-    return *reinterpret_cast<T *>(&r);
+    union { T t; uint64_t i; } u;
+    u.t = t;
+    u.i = FLATBUFFERS_BYTESWAP64(u.i);
+    return u.t;
   } else {
     assert(0);
   }

tests/test.cpp

@@ -1803,6 +1803,16 @@ void TypeAliasesTest()
   TEST_EQ(sizeof(ta->f64()), 8);
 }
 
+void EndianSwapTest() {
+  TEST_EQ(flatbuffers::EndianSwap(static_cast<int16_t>(0x1234)),
+          0x3412);
+  TEST_EQ(flatbuffers::EndianSwap(static_cast<int32_t>(0x12345678)),
+          0x78563412);
+  TEST_EQ(flatbuffers::EndianSwap(static_cast<int64_t>(0x1234567890ABCDEF)),
+          0xEFCDAB9078563412);
+  TEST_EQ(flatbuffers::EndianSwap(flatbuffers::EndianSwap(3.14f)), 3.14f);
+}
+
 int main(int /*argc*/, const char * /*argv*/[]) {
   #if defined(FLATBUFFERS_MEMORY_LEAK_TRACKING) && \
       defined(_MSC_VER) && defined(_DEBUG)
@@ -1866,6 +1876,7 @@ int main(int /*argc*/, const char * /*argv*/[]) {
   ConformTest();
   ParseProtoBufAsciiTest();
   TypeAliasesTest();
+  EndianSwapTest();
 
   FlexBuffersTest();