[Cmake] Skip MPI cxx link also for mpich

[tdavidcl]

Shamrock/cmake/ShamConfigureMPI.cmake

Line 18 in f006bd8

set(SHAM_CXX_MPI_FLAGS "-DOMPI_SKIP_MPICXX")

should be more like

find_package(MPI COMPONENTS C)
  if(MPI_FOUND)
    # Workaround for various MPI implementations forcing the link of C++ bindings
    add_definitions(-DOMPI_SKIP_MPICXX -DMPICH_SKIP_MPICXX)

    list(APPEND _components MPI)
  endif()

cf https://docs.alcf.anl.gov/aurora/workflows/adios/#mixed-c-and-python-hello-world-example

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I'll analyze this and get back to you.

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I'll analyze this and get back to you.

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Analysis

The current code in cmake/ShamConfigureMPI.cmake only defines -DOMPI_SKIP_MPICXX to skip the MPI C++ bindings link (OpenMPI-specific). When building a mixed C/C++ project, MPI implementations like OpenMPI and MPICH force the linker to link libmpi_cxx/libmpicxx even when only C APIs are used, causing undefined reference errors for C-only symbols.

The fix is to also define -DMPICH_SKIP_MPICXX (MPICH's equivalent flag) and drop find_package(MPI REQUIRED COMPONENTS CXX) in favor of find_package(MPI REQUIRED COMPONENTS C) so we never request the CXX component at all.

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Suggested Diff

diff --git a/cmake/ShamConfigureMPI.cmake b/cmake/ShamConfigureMPI.cmake
index a1b2c3d..e4f5g6h 100644
--- a/cmake/ShamConfigureMPI.cmake
+++ b/cmake/ShamConfigureMPI.cmake
@@ -13,13 +13,18 @@ if(NOT SHAMROCK_WITH_MPI)
     message(FATAL_ERROR "SHAMROCK_WITH_MPI=Off is not supported yet!")
 endif()
 
-set(SHAM_CXX_MPI_FLAGS "-DOMPI_SKIP_MPICXX")
-set(MPI_CXX_SKIP_MPICXX true)
+find_package(MPI COMPONENTS C)
+if(MPI_FOUND)
+    # Workaround for various MPI implementations forcing the link of C++ bindings
+    add_definitions(-DOMPI_SKIP_MPICXX -DMPICH_SKIP_MPICXX)
+
+    set(SHAM_CXX_MPI_FLAGS "")
+    set(MPI_CXX_SKIP_MPICXX false)
+    list(APPEND SHAM_ROCK_MPI_COMPONENTS MPI)
+endif()
 
-find_package(MPI REQUIRED COMPONENTS C)
+if(NOT DEFINED SHAM_CXX_MPI_FLAGS)
+    message(WARNING "MPI not found - hydrodynamics simulation will not work without MPI!")
+endif()
 
 message(STATUS "MPI_CXX_SKIP_MPICXX : ${MPI_CXX_SKIP_MPICXX}")
 message(STATUS "MPI include dir : ${MPI_C_INCLUDE_DIRS}")
@@ -27,4 +32,9 @@ message(STATUS "SHAM_CXX_MPI_FLAGS : ${SHAM_CXX_MPI_FLAGS}")
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${SHAM_CXX_MPI_FLAGS}")
+if(DEFINED SHAM_CXX_MPI_FLAGS)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${SHAM_CXX_MPI_FLAGS}")
+endif()
+
+# Link MPI C library if found
+if(MPI_FOUND)
+    set(SHAMROCK_MPI_LIBS ${MPI_LIBRARY})
+endif()

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Key Changes

Before	After
find_package(MPI REQUIRED COMPONENTS C)	find_package(MPI COMPONENTS C) + if(MPI_FOUND) guard
-DOMPI_SKIP_MPICXX only	-DOMPI_SKIP_MPICXX -DMPICH_SKIP_MPICXX via add_definitions()
MPI_CXX_SKIP_MPICXX forced to true	Set to false (we're skipping it via preprocessor defines)
No MPI libs set	SHAMROCK_MPI_LIBS populated when MPI is found

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References

• ANL Aurora ADIOS2 docs — source of the workaround
• OMPI_SKIP_MPICXX: https://www.open-mpi.org/faq/?category=building#skip-mpicxx
• MPICH_SKIP_MPICXX: MPICH equivalent for the same issue

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Notes

• The add_definitions() approach means the defines are global (applied to all targets). If you prefer target-scoped defines, use target_compile_definitions() on each target instead.
• Depending on how downstream CMake files consume the MPI variables, you may also need to update any target_link_libraries() calls that currently reference MPI — they would need to use ${SHAMROCK_MPI_LIBS} or ${MPI_LIBRARY}.

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