diff --git a/configure b/configure
index 9e554925..6a6c4331 100755
--- a/configure
+++ b/configure
@@ -1785,6 +1785,52 @@ $as_echo "$ac_res" >&6; }
 
 } # ac_fn_c_check_header_compile
 
+# ac_fn_c_check_decl LINENO SYMBOL VAR INCLUDES
+# ---------------------------------------------
+# Tests whether SYMBOL is declared in INCLUDES, setting cache variable VAR
+# accordingly.
+ac_fn_c_check_decl ()
+{
+  as_lineno=${as_lineno-"$1"} as_lineno_stack=as_lineno_stack=$as_lineno_stack
+  as_decl_name=`echo $2|sed 's/ *(.*//'`
+  as_decl_use=`echo $2|sed -e 's/(/((/' -e 's/)/) 0&/' -e 's/,/) 0& (/g'`
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $as_decl_name is declared" >&5
+$as_echo_n "checking whether $as_decl_name is declared... " >&6; }
+if eval \${$3+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+$4
+int
+main ()
+{
+#ifndef $as_decl_name
+#ifdef __cplusplus
+  (void) $as_decl_use;
+#else
+  (void) $as_decl_name;
+#endif
+#endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  eval "$3=yes"
+else
+  eval "$3=no"
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+eval ac_res=\$$3
+	       { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_res" >&5
+$as_echo "$ac_res" >&6; }
+  eval $as_lineno_stack; ${as_lineno_stack:+:} unset as_lineno
+
+} # ac_fn_c_check_decl
+
 # ac_fn_c_check_type LINENO TYPE VAR INCLUDES
 # -------------------------------------------
 # Tests whether TYPE exists after having included INCLUDES, setting cache
@@ -4958,6 +5004,31 @@ fi
 
 done
 
+#AC_CHECK_HEADERS(machine/endian.h)
+ac_fn_c_check_decl "$LINENO" "ntohll" "ac_cv_have_decl_ntohll" "#include <arpa/inet.h>
+"
+if test "x$ac_cv_have_decl_ntohll" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_NTOHLL $ac_have_decl
+_ACEOF
+
+ac_fn_c_check_decl "$LINENO" "be64toh" "ac_cv_have_decl_be64toh" "#include <arpa/inet.h>
+"
+if test "x$ac_cv_have_decl_be64toh" = xyes; then :
+  ac_have_decl=1
+else
+  ac_have_decl=0
+fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_DECL_BE64TOH $ac_have_decl
+_ACEOF
+
 
 # Checks for typedefs, structures, and compiler characteristics.
 ac_fn_c_check_type "$LINENO" "size_t" "ac_cv_type_size_t" "$ac_includes_default"
diff --git a/configure.ac b/configure.ac
index b6e73bcb..ffd37d87 100644
--- a/configure.ac
+++ b/configure.ac
@@ -18,6 +18,9 @@ AC_CHECK_HEADERS(stdint.h)
 AC_CHECK_HEADERS(malloc/malloc.h)
 AC_CHECK_HEADERS(malloc.h)
 AC_CHECK_HEADERS(endian.h)
+#AC_CHECK_HEADERS(machine/endian.h)
+AC_CHECK_DECLS([ntohll],[], [], [[#include <arpa/inet.h>]])
+AC_CHECK_DECLS([be64toh],[], [], [[#include <arpa/inet.h>]])
 
 # Checks for typedefs, structures, and compiler characteristics.
 AC_TYPE_SIZE_T
diff --git a/lib/Grid_config.h b/lib/Grid_config.h
index 4152540e..c743bba0 100644
--- a/lib/Grid_config.h
+++ b/lib/Grid_config.h
@@ -16,6 +16,14 @@
 /* GRID_COMMS_NONE */
 /* #undef GRID_COMMS_NONE */
 
+/* Define to 1 if you have the declaration of `be64toh', and to 0 if you
+   don't. */
+#define HAVE_DECL_BE64TOH 0
+
+/* Define to 1 if you have the declaration of `ntohll', and to 0 if you don't.
+   */
+#define HAVE_DECL_NTOHLL 1
+
 /* Define to 1 if you have the <endian.h> header file. */
 /* #undef HAVE_ENDIAN_H */
 
diff --git a/lib/Grid_config.h.in b/lib/Grid_config.h.in
index 2dc0bda4..437a6095 100644
--- a/lib/Grid_config.h.in
+++ b/lib/Grid_config.h.in
@@ -15,6 +15,14 @@
 /* GRID_COMMS_NONE */
 #undef GRID_COMMS_NONE
 
+/* Define to 1 if you have the declaration of `be64toh', and to 0 if you
+   don't. */
+#undef HAVE_DECL_BE64TOH
+
+/* Define to 1 if you have the declaration of `ntohll', and to 0 if you don't.
+   */
+#undef HAVE_DECL_NTOHLL
+
 /* Define to 1 if you have the <endian.h> header file. */
 #undef HAVE_ENDIAN_H
 
diff --git a/lib/parallelIO/GridNerscIO.h b/lib/parallelIO/GridNerscIO.h
index 4dfa7979..4094ab70 100644
--- a/lib/parallelIO/GridNerscIO.h
+++ b/lib/parallelIO/GridNerscIO.h
@@ -9,10 +9,42 @@
 
 #ifdef HAVE_ENDIAN_H
 #include <endian.h>
+#endif
+
+
 #include <arpa/inet.h>
-#define ntohll be64toh
+
+// 64bit endian swap is a portability pain
+#ifndef __has_builtin         // Optional of course.
+#define __has_builtin(x) 0  // Compatibility with non-clang compilers.
+#endif
+
+#if HAVE_DECL_BE64TOH 
+#undef Grid_ntohll
+#define Grid_ntohll be64toh
+#endif
+
+#if HAVE_DECL_NTOHLL
+#undef  Grid_ntohll
+#define Grid_ntohll ntohll
+#endif
+
+#ifndef Grid_ntohll
+
+#if BYTE_ORDER == BIG_ENDIAN 
+
+#define Grid_ntohll(A) (A)
+
 #else 
-#include <arpa/inet.h>
+
+#if __has_builtin(__builtin_bswap64)
+#define Grid_ntohll(A) __builtin_bswap64(A)
+#else
+#error
+#endif
+
+#endif
+
 #endif
 
 namespace Grid {
@@ -208,7 +240,7 @@ inline void reconstruct3(LorentzColourMatrix & cm)
  {
    uint64_t * f = (uint64_t *)file_object;
    for(int i=0;i*sizeof(uint64_t)<bytes;i++){  
-     f[i] = ntohll(f[i]);
+     f[i] = Grid_ntohll(f[i]);
    }
  }
  void inline le64toh_v(void *file_object,uint32_t bytes)