Fix gribhandle on Mac for file with multiple messages

src/metkit/codes/GribHandle.cc

@@ -94,7 +94,7 @@ GribHandle::GribHandle(eckit::DataHandle& handle, eckit::Offset offset):
     FILE* f = handle.openf();
     ASSERT(f);
 
-    handle.seek(offset);
+    fseek(f, offset, SEEK_SET);
 
     h = codes_handle_new_from_file(0, f, PRODUCT_GRIB, &err);
 

tests/CMakeLists.txt

@@ -17,7 +17,8 @@ endif()
 
 ecbuild_get_test_multidata( TARGET grib_get_data
                             DIRNAME grib_api/data
-                            NAMES latlon.grib )
+                            NAMES latlon.grib
+                                  synthetic_2msgs.grib )
 
 ecbuild_get_test_multidata( TARGET metkit_get_odb_data
                             NAMES multiodb.odb
@@ -61,6 +62,15 @@ ecbuild_add_test( TARGET        metkit_test_odbsplitter
                   NO_AS_NEEDED
                   LIBS          metkit )
 
+ecbuild_add_test( TARGET        metkit_test_gribhandle
+                  CONDITION     HAVE_GRIB
+                  SOURCES       test_gribhandle.cc
+                  INCLUDES      "${ECKIT_INCLUDE_DIRS}" "${ECCODES_INCLUDE_DIR}"
+                  LIBS          metkit
+                  TEST_DEPENDS  grib_get_data
+                  NO_AS_NEEDED
+                  ENVIRONMENT   "${metkit_env}")
+
 list(APPEND testFileSuffixes typesfactory expand param_axis steprange_axis time hypercube type_levelist )
 
 foreach(test IN LISTS testFileSuffixes)

tests/test_gribhandle.cc

@@ -0,0 +1,67 @@
+/*
+ * (C) Copyright 1996- ECMWF.
+ *
+ * This software is licensed under the terms of the Apache Licence Version 2.0
+ * which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
+ * In applying this licence, ECMWF does not waive the privileges and immunities
+ * granted to it by virtue of its status as an intergovernmental organisation
+ * nor does it submit to any jurisdiction.
+ */
+
+/// @date   Apr 2024
+/// @author Christopher Bradley
+
+#include "eccodes.h"
+
+#include "eckit/testing/Test.h"
+#include "eckit/io/FileHandle.h"
+
+#include "metkit/codes/GribHandle.h"
+#include "metkit/codes/GribAccessor.h"
+
+using namespace eckit::testing;
+
+namespace metkit {
+namespace grib {
+namespace test {
+
+//-----------------------------------------------------------------------------
+
+// Test that a gribhandle will point to the correct message in a file, given an offset.
+CASE( "File with two messages" ) {
+
+    // The test file has two messages of different packing types, with some junk data in between.
+    eckit::PathName path("synthetic_2msgs.grib");
+
+    off_t* offsets;
+    grib_context* c = nullptr;
+    int n = 0;
+    int err = codes_extract_offsets_malloc(c, path.asString().c_str(), PRODUCT_GRIB, &offsets, &n, 1);
+    EXPECT(!err);
+    EXPECT(n == 2);
+
+    GribAccessor<std::string> packingType("packingType");
+    std::vector<std::string> expected = {"grid_simple", "grid_ccsds"};
+
+    eckit::FileHandle dh(path);
+    dh.openForRead();
+
+    for (int i = 0; i < n; i++) {
+        GribHandle h(dh, offsets[i]);
+        EXPECT(packingType(h) == expected[i]);
+    }
+
+    free(offsets);
+    dh.close();
+}
+
+//-----------------------------------------------------------------------------
+
+}  // namespace test
+}  // namespace grib
+}  // namespace metkit
+
+int main(int argc, char **argv)
+{
+    return run_tests ( argc, argv );
+}