Use __BYTE_ORDER__ to test the endianness when available
GH:
#ifndef OSSL_INTERNAL_ENDIAN_H
# define OSSL_INTERNAL_ENDIAN_H
+/*
+ * IS_LITTLE_ENDIAN and IS_BIG_ENDIAN can be used to detect the endiannes
+ * at compile time. To use it, DECLARE_IS_ENDIAN must be used to declare
+ * a variable.
+ *
+ * L_ENDIAN and B_ENDIAN can be used at preprocessor time. They can be set
+ * in the configarion using the lib_cppflags variable. If neither is
+ * set, it will fall back to code works with either endianness.
+ */
+
+# if defined(__BYTE_ORDER__) && defined(__ORDER_LITTLE_ENDIAN__)
+# define DECLARE_IS_ENDIAN const int ossl_is_little_endian = __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+# define IS_LITTLE_ENDIAN (ossl_is_little_endian)
+# define IS_BIG_ENDIAN (!ossl_is_little_endian)
+# if defined(L_ENDIAN) && (__BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__)
+# error "L_ENDIAN defined on a big endian machine"
+# endif
+# if defined(B_ENDIAN) && (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
+# error "B_ENDIAN defined on a little endian machine"
+# endif
+# if !defined(L_ENDIAN) && (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
+# define L_ENDIAN
+# endif
+# if !defined(B_ENDIAN) && (__BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__)
+# define B_ENDIAN
+# endif
+# else
# define DECLARE_IS_ENDIAN \
const union { \
long one; \
# define IS_LITTLE_ENDIAN (ossl_is_endian.little != 0)
# define IS_BIG_ENDIAN (ossl_is_endian.little == 0)
+# endif
#endif
