Harden byte order detection

Use __BYTE_ORDER__ for GCC/Clang, and include Linux/BSD-specific headers for endian detection, and a constexpr static_assert instead of silently defaulting to little-endian.

Fixes #135.

Author: kkaefer

include/protozero/config.hpp

@@ -22,19 +22,66 @@ documentation.
 #define PROTOZERO_BIG_ENDIAN    4321
 
 // Find out which byte order the machine has.
-#if defined(__BYTE_ORDER)
-# if (__BYTE_ORDER == __LITTLE_ENDIAN)
+
+// __BYTE_ORDER__ is set by GCC and Clang.
+#if !defined(PROTOZERO_BYTE_ORDER) && defined(__BYTE_ORDER__)
+# if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
 #  define PROTOZERO_BYTE_ORDER PROTOZERO_LITTLE_ENDIAN
-# endif
-# if (__BYTE_ORDER == __BIG_ENDIAN)
+# elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
 #  define PROTOZERO_BYTE_ORDER PROTOZERO_BIG_ENDIAN
 # endif
-#else
-// This probably isn't a very good default, but might do until we figure
-// out something better.
+#endif // defined(__BYTE_ORDER__)
+
+// On Linux, we can use endian.h.
+#if !defined(PROTOZERO_BYTE_ORDER) && defined(__linux__)
+# include <endian.h>
+# if defined(__BYTE_ORDER)
+#  if (__BYTE_ORDER == __LITTLE_ENDIAN)
+#   define PROTOZERO_BYTE_ORDER PROTOZERO_LITTLE_ENDIAN
+#  elif (__BYTE_ORDER == __BIG_ENDIAN)
+#   define PROTOZERO_BYTE_ORDER PROTOZERO_BIG_ENDIAN
+#  endif
+# endif
+#endif // !defined(PROTOZERO_BYTE_ORDER) && defined(__linux__)
+
+// On BSD, we can use <sys/endian.h>
+#if !defined(PROTOZERO_BYTE_ORDER) && \
+    (defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__))
+# include <sys/endian.h>
+# if defined(BYTE_ORDER)
+#  if (BYTE_ORDER == LITTLE_ENDIAN)
+#   define PROTOZERO_BYTE_ORDER PROTOZERO_LITTLE_ENDIAN
+#  elif (BYTE_ORDER == BIG_ENDIAN)
+#   define PROTOZERO_BYTE_ORDER PROTOZERO_BIG_ENDIAN
+#  endif
+# endif
+#endif
+
+// On Windows, we assume little endian.
+#if !defined(PROTOZERO_BYTE_ORDER) && defined(_MSC_VER)
 # define PROTOZERO_BYTE_ORDER PROTOZERO_LITTLE_ENDIAN
+#endif // !defined(PROTOZERO_BYTE_ORDER) && defined(_MSC_VER)
+
+#if !defined(PROTOZERO_BYTE_ORDER)
+# error "Could not determine byte order; define PROTOZERO_BYTE_ORDER"
 #endif
 
+namespace protozero {
+namespace detail {
+
+constexpr bool is_little_endian() noexcept {
+    return static_cast<unsigned char const&>(
+                static_cast<unsigned int const&>(1u)) == 1u;
+}
+
+} // namespace detail
+} // namespace protozero
+
+static_assert(
+    protozero::detail::is_little_endian() ==
+        (PROTOZERO_BYTE_ORDER == PROTOZERO_LITTLE_ENDIAN),
+    "Could not determine byte order correctly; define PROTOZERO_BYTE_ORDER");
+
 // Check whether __builtin_bswap is available
 #if defined(__GNUC__) || defined(__clang__)
 # define PROTOZERO_USE_BUILTIN_BSWAP