roll back changes switching from MPI::MPI_CXX to MPI::MPI_C

cmake/CMakeLists.txt

@@ -203,6 +203,10 @@ if((CMAKE_SYSTEM_NAME STREQUAL "Windows") AND BUILD_SHARED_LIBS)
   set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS ON)
 endif()
 
+# do not include the (obsolete) MPI C++ bindings which makes for leaner object files
+# and avoids namespace conflicts. Put this early to increase its visbility.
+set(MPI_CXX_SKIP_MPICXX TRUE)
+
 ########################################################################
 # User input options                                                   #
 ########################################################################
@@ -400,28 +404,24 @@ if(PKG_ADIOS)
 endif()
 
 if(NOT CMAKE_CROSSCOMPILING)
-  find_package(MPI QUIET COMPONENTS C)
+  find_package(MPI QUIET COMPONENTS CXX)
   option(BUILD_MPI "Build MPI version" ${MPI_FOUND})
 else()
   option(BUILD_MPI "Build MPI version" OFF)
 endif()
 
 if(BUILD_MPI)
-  # do not include the (obsolete) MPI C++ bindings which makes
-  # for leaner object files and avoids namespace conflicts
-  set(MPI_CXX_SKIP_MPICXX TRUE)
   # We use a non-standard procedure to cross-compile with MPI on Windows
   if((CMAKE_SYSTEM_NAME STREQUAL "Windows") AND CMAKE_CROSSCOMPILING)
     include(MPI4WIN)
   else()
-    find_package(MPI REQUIRED COMPONENTS C)
+    find_package(MPI REQUIRED COMPONENTS CXX)
     option(LAMMPS_LONGLONG_TO_LONG "Workaround if your system or MPI version does not recognize 'long long' data types" OFF)
     if(LAMMPS_LONGLONG_TO_LONG)
       target_compile_definitions(lammps PRIVATE -DLAMMPS_LONGLONG_TO_LONG)
     endif()
   endif()
-  target_link_libraries(lammps PUBLIC MPI::MPI_C)
+  target_link_libraries(lammps PUBLIC MPI::MPI_CXX)
-  target_compile_definitions(lammps PUBLIC -DMPICH_SKIP_MPICXX -DOMPI_SKIP_MPICXX=1)
 else()
   target_sources(lammps PRIVATE ${LAMMPS_SOURCE_DIR}/STUBS/mpi.cpp)
   add_library(mpi_stubs INTERFACE)
@@ -717,7 +717,6 @@ foreach(PKG_LIB POEMS ATC AWPMD H5MD)
     file(GLOB_RECURSE ${PKG_LIB}_SOURCES CONFIGURE_DEPENDS
       ${LAMMPS_LIB_SOURCE_DIR}/${PKG_LIB}/[^.]*.c ${LAMMPS_LIB_SOURCE_DIR}/${PKG_LIB}/[^.]*.cpp)
     add_library(${PKG_LIB} STATIC ${${PKG_LIB}_SOURCES})
-    target_compile_definitions(${PKG_LIB} PUBLIC -DMPICH_SKIP_MPICXX -DOMPI_SKIP_MPICXX=1)
     set_target_properties(${PKG_LIB} PROPERTIES OUTPUT_NAME lammps_${PKG_LIB}${LAMMPS_MACHINE})
     target_link_libraries(lammps PRIVATE ${PKG_LIB})
     if(PKG_LIB STREQUAL "awpmd")
@@ -743,7 +742,7 @@ if(PKG_ATC)
     message(FATAL_ERROR "The ATC Package is not compatible with -DLAMMPS_BIGBIG")
   endif()
   if(BUILD_MPI)
-    target_link_libraries(atc PRIVATE ${LAPACK_LIBRARIES} MPI::MPI_C)
+    target_link_libraries(atc PRIVATE ${LAPACK_LIBRARIES} MPI::MPI_CXX)
   else()
     target_link_libraries(atc PRIVATE ${LAPACK_LIBRARIES} mpi_stubs)
   endif()
@@ -1057,9 +1056,9 @@ else()
 endif()
 if(BUILD_MPI)
   message(STATUS "<<< MPI flags >>>
--- MPI_defines:      ${MPI_C_COMPILE_DEFINITIONS}
+-- MPI_defines:      ${MPI_CXX_COMPILE_DEFINITIONS}
--- MPI includes:     ${MPI_C_INCLUDE_PATH}
+-- MPI includes:     ${MPI_CXX_INCLUDE_PATH}
--- MPI libraries:    ${MPI_C_LIBRARIES};${MPI_Fortran_LIBRARIES}")
+-- MPI libraries:    ${MPI_CXX_LIBRARIES};${MPI_Fortran_LIBRARIES}")
 endif()
 if(PKG_GPU)
   message(STATUS "<<< GPU package settings >>>

cmake/Modules/LAMMPSInterfacePlugin.cmake

@@ -62,6 +62,9 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "Intel")
 endif()
 set(CMAKE_POSITION_INDEPENDENT_CODE TRUE)
 
+# skip over obsolete MPI-2 C++ bindings
+set(MPI_CXX_SKIP_MPICXX TRUE)
+
 #######
 # helper functions from LAMMPSUtils.cmake
 function(validate_option name values)
@@ -128,9 +131,7 @@ endif()
 ################################################################################
 # MPI configuration
 if(NOT CMAKE_CROSSCOMPILING)
-  set(MPI_CXX_SKIP_MPICXX TRUE)
+  find_package(MPI QUIET COMPONENTS CXX)
-  enable_language(C)
-  find_package(MPI QUIET COMPONENTS C)
   option(BUILD_MPI "Build MPI version" ${MPI_FOUND})
 else()
   option(BUILD_MPI "Build MPI version" OFF)

cmake/Modules/Packages/COLVARS.cmake

@@ -27,8 +27,7 @@ if(BUILD_OMP)
 endif()
 
 if(BUILD_MPI)
-  target_compile_definitions(colvars PUBLIC -DCOLVARS_MPI -DMPICH_SKIP_MPICXX -DOMPI_SKIP_MPICXX=1)
+  target_link_libraries(colvars PUBLIC MPI::MPI_CXX)
-  target_link_libraries(colvars PUBLIC MPI::MPI_C)
 endif()
 
 if(COLVARS_DEBUG)

cmake/Modules/Packages/GPU.cmake

@@ -484,12 +484,11 @@ RegisterFixStyle(${GPU_SOURCES_DIR}/fix_gpu.h)
 
 get_property(GPU_SOURCES GLOBAL PROPERTY GPU_SOURCES)
 if(BUILD_MPI)
-  target_link_libraries(gpu PRIVATE MPI::MPI_C)
+  target_link_libraries(gpu PRIVATE MPI::MPI_CXX)
 else()
   target_link_libraries(gpu PRIVATE mpi_stubs)
 endif()
 
-target_compile_definitions(gpu PRIVATE -DLAMMPS_${LAMMPS_SIZES} -DMPICH_SKIP_MPICXX -DOMPI_SKIP_MPICXX=1)
 set_target_properties(gpu PROPERTIES OUTPUT_NAME lammps_gpu${LAMMPS_MACHINE})
 target_sources(lammps PRIVATE ${GPU_SOURCES})
 target_include_directories(lammps PRIVATE ${GPU_SOURCES_DIR})

cmake/Modules/Packages/ML-HDNNP.cmake

@@ -44,7 +44,7 @@ if(DOWNLOAD_N2P2)
     set(N2P2_PROJECT_OPTIONS "-DN2P2_NO_MPI")
   else()
     # get path to MPI include directory
-    get_target_property(N2P2_MPI_INCLUDE MPI::MPI_C INTERFACE_INCLUDE_DIRECTORIES)
+    get_target_property(N2P2_MPI_INCLUDE MPI::MPI_CXX INTERFACE_INCLUDE_DIRECTORIES)
     foreach (_INCL ${N2P2_MPI_INCLUDE})
       set(N2P2_PROJECT_OPTIONS "${N2P2_PROJECT_OPTIONS} -I${_INCL}")
     endforeach()
@@ -102,9 +102,9 @@ if(DOWNLOAD_N2P2)
   # nnpif library has MPI calls if MPI is enabled, so we must link with MPI libs
   if(BUILD_MPI)
     set_target_properties(LAMMPS::N2P2::LIBNNPIF PROPERTIES
-      INTERFACE_LINK_LIBRARIES MPI::MPI_C)
+      INTERFACE_LINK_LIBRARIES MPI::MPI_CXX)
     if((CMAKE_SYSTEM_NAME STREQUAL Windows) AND CMAKE_CROSSCOMPILING)
-      add_dependencies(LAMMPS::N2P2::LIBNNPIF MPI::MPI_C)
+      add_dependencies(LAMMPS::N2P2::LIBNNPIF MPI::MPI_CXX)
     endif()
   endif()
 

examples/COUPLE/plugin/CMakeLists.txt

@@ -28,9 +28,6 @@ if(MSVC)
 endif()
 
 find_package(MPI REQUIRED COMPONENTS C)
-# do not include the (obsolete) MPI C++ bindings which makes
-# for leaner object files and avoids namespace conflicts
-set(MPI_CXX_SKIP_MPICXX TRUE)
 
 ##########################
 

examples/COUPLE/simple/CMakeLists.txt

@@ -25,10 +25,10 @@ if(MSVC)
   add_compile_definitions(_CRT_SECURE_NO_WARNINGS)
 endif()
 
-find_package(MPI QUIET COMPONENTS C)
 # do not include the (obsolete) MPI C++ bindings which makes
 # for leaner object files and avoids namespace conflicts
 set(MPI_CXX_SKIP_MPICXX TRUE)
+find_package(MPI QUIET COMPONENTS C CXX)
 
 ##########################
 
@@ -38,7 +38,7 @@ if(NOT LAMMPS_SOURCE_DIR)
 endif()
 
 add_executable(simpleCC simple.cpp)
-target_link_libraries(simpleCC PRIVATE LAMMPS::lammps MPI::MPI_C)
+target_link_libraries(simpleCC PRIVATE LAMMPS::lammps MPI::MPI_CXX)
 
 add_executable(simpleC simple.c)
 target_link_libraries(simpleC PRIVATE LAMMPS::lammps MPI::MPI_C)

examples/plugins/LAMMPSInterfaceCXX.cmake

@@ -33,10 +33,11 @@ endif()
 
 ################################################################################
 # MPI configuration
-if(NOT CMAKE_CROSSCOMPILING)
+# do not include the (obsolete) MPI C++ bindings which makes
-  enable_language(C)
+# for leaner object files and avoids namespace conflicts
 set(MPI_CXX_SKIP_MPICXX TRUE)
-  find_package(MPI QUIET COMPONENTS C)
+if(NOT CMAKE_CROSSCOMPILING)
+  find_package(MPI QUIET COMPONENTS CXX)
   option(BUILD_MPI "Build MPI version" ${MPI_FOUND})
 else()
   option(BUILD_MPI "Build MPI version" OFF)