From 4ffbee73a313f4850de8d38e26012b4b203488cf Mon Sep 17 00:00:00 2001
From: Kolen Cheung <10970208+ickc@users.noreply.github.com>
Date: Wed, 9 Sep 2026 14:44:07 +0000
Subject: [PATCH 1/2] Link the OpenMP targets when BUILD_OPENMP is enabled

BUILD_OPENMP defaults to ON and ShumOptions.cmake runs
find_package(OpenMP 3.0 REQUIRED), but nothing ever links the resulting
imported targets. No -fopenmp therefore reaches a compile line, _OPENMP
is left undefined, and the OpenMP regions in shum_wgdos_packing,
shum_horizontal_field_interp, shum_latlon_eq_grids, shum_thread_utils
and shum_byteswap are preprocessed away. A CMake build produces a serial
library where the Makefile build, which defaults SHUM_OPENMP to true,
produces a threaded one, and the option gives no indication that it had
no effect.

Link OpenMP::OpenMP_C and OpenMP::OpenMP_Fortran to shum. PRIVATE is
sufficient because _OPENMP is used inside c_shum_byteswap.c and never in
an installed header, so consumers do not need OpenMP flags of their own.

The thread_utils unit tests exercise the OpenMP paths directly, so
shumlib-tests is given the same linkage in its own right rather than
inheriting it.

Verified with GCC that -fopenmp now reaches the C and Fortran compile
lines of both targets, that libshum links libgomp, and that the fruit
test suite still passes; BUILD_OPENMP=OFF continues to configure and
build with no OpenMP flags.
---
 CMakeLists.txt | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4b9588d..bfec365 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -62,6 +62,23 @@ target_compile_definitions(shum
   PUBLIC
   ${SHUM_DEFINES})
 
+# BUILD_OPENMP goes as far as find_package(OpenMP) in ShumOptions.cmake, but
+# the imported targets have to be linked for the option to have any effect.
+# Without this, no -fopenmp reaches a compile line, _OPENMP is never defined
+# and the OpenMP regions are preprocessed away, so a CMake build produces a
+# serial library where the Makefile build (SHUM_OPENMP ?= true) produces a
+# threaded one.
+#
+# PRIVATE is sufficient: _OPENMP is used inside c_shum_byteswap.c, never in an
+# installed header, so consumers of shumlib do not need OpenMP flags of their
+# own.
+if(BUILD_OPENMP)
+  target_link_libraries(shum
+    PRIVATE
+    OpenMP::OpenMP_C
+    OpenMP::OpenMP_Fortran)
+endif()
+
 # Install shumlib and the C header files for the sub-libraries that
 # have them
 install(TARGETS shum
@@ -95,6 +112,16 @@ if(BUILD_TESTS)
     PUBLIC ${CMAKE_Fortran_MODULE_DIRECTORY})
   target_link_libraries(shumlib-tests PUBLIC fruit shum)
 
+  # The thread_utils tests exercise the OpenMP paths directly, and shumlib's
+  # OpenMP linkage is PRIVATE, so the test executable needs the flags in its
+  # own right.
+  if(BUILD_OPENMP)
+    target_link_libraries(shumlib-tests
+      PRIVATE
+      OpenMP::OpenMP_C
+      OpenMP::OpenMP_Fortran)
+  endif()
+
   add_subdirectory(fruit)
   setup_shum_fruit()
 

From ba7b2ec42b8abc2481ec434c24f206e50996b194 Mon Sep 17 00:00:00 2001
From: Kolen Cheung <10970208+ickc@users.noreply.github.com>
Date: Wed, 16 Sep 2026 16:53:21 +0000
Subject: [PATCH 2/2] add self

---
 CONTRIBUTORS.md | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index e4f90b3..4d1f42f 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -1,8 +1,9 @@
 # Contributors
 
-| GitHub user     | Real Name        | Affiliation | Date       |
-| --------------- | ---------------- | ----------- | ---------- |
-| james-bruten-mo | James Bruten     | Met Office  | 2025-12-09 |
-| t00sa           | Sam Clarke-Green | Met Office  | 2026-02-10 |
-| jennyhickson    | Jenny Hickson    | Met Office  | 2026-03-02 |
-| yaswant         | Yaswant Pradhan  | Met Office  | 2026-07-17 |
+| GitHub user     | Real Name        | Affiliation          | Date       |
+| --------------- | ---------------- | -----------          | ---------- |
+| james-bruten-mo | James Bruten     | Met Office           | 2025-12-09 |
+| t00sa           | Sam Clarke-Green | Met Office           | 2026-02-10 |
+| jennyhickson    | Jenny Hickson    | Met Office           | 2026-03-02 |
+| yaswant         | Yaswant Pradhan  | Met Office           | 2026-07-17 |
+| ickc            | Kolen Cheung     | University of Exeter | 2026-09-16 |