Harden byte order detection

.github/workflows/ci.yml

@@ -112,6 +112,41 @@ jobs:
       - uses: ./.github/actions/build
       - uses: ./.github/actions/ctest
 
+  big-endian:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 45
+    steps:
+      - uses: actions/checkout@v7
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+        with:
+          platforms: s390x
+      - name: Build and test on s390x (big-endian)
+        run: |
+          docker run --rm --platform linux/s390x \
+            -v "${{ github.workspace }}:/src" \
+            -w /src \
+            -e DEBIAN_FRONTEND=noninteractive \
+            debian:bookworm \
+            bash -exc '
+              apt-get update -qq
+              apt-get install -y \
+                cmake \
+                g++ \
+                libprotobuf-dev \
+                make \
+                protobuf-compiler \
+                python3-minimal
+              python3 -c "import sys; assert sys.byteorder == \"big\""
+              mkdir -p build
+              cd build
+              cmake -LA .. \
+                -DCMAKE_BUILD_TYPE=Debug \
+                -DCMAKE_CXX_STANDARD=14
+              cmake --build . --parallel 2
+              ctest --output-on-failure --parallel 2
+            '
+
   ubuntu-latest:
     runs-on: ubuntu-24.04
     timeout-minutes: 30

include/protozero/config.hpp

@@ -22,17 +22,48 @@ 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
 
 // Check whether __builtin_bswap is available