mpiwrap: make mpwiwrap.h/mpidummy.h includable from plain C

source/adios2/ADIOSMPI.h

@@ -6,8 +6,6 @@
 #ifndef ADIOS2_ADIOSMPI_H_
 #define ADIOS2_ADIOSMPI_H_
 
-#include "adios2/ADIOSConfig.h"
-
 #include "adios2/helper/mpiwrap.h"
 
 #include <climits> //UXXX_MAX

source/adios2/helper/mpidummy.h

@@ -10,8 +10,8 @@
 #ifndef ADIOS2_HELPER_MPIDUMMY_H_
 #define ADIOS2_HELPER_MPIDUMMY_H_
 
-#include <cstdint>
-#include <cstdio>
+#include "stdint.h"
+#include "stdio.h"
 
 #include "adios2/ADIOSConfig.h"
 
@@ -21,15 +21,15 @@
 #include <mpi.h>
 #else
 
-using MPI_Comm = int;
-using MPI_Status = std::uint64_t;
-using MPI_Request = std::uint64_t;
-using MPI_File = std::FILE *;
-using MPI_Info = int;
-using MPI_Datatype = int;
-using MPI_Offset = long int;
-using MPI_Fint = int;
-using MPI_Op = int;
+typedef int MPI_Comm;
+typedef uint64_t MPI_Status;
+typedef uint64_t MPI_Request;
+typedef FILE *MPI_File;
+typedef int MPI_Info;
+typedef int MPI_Datatype;
+typedef long int MPI_Offset;
+typedef int MPI_Fint;
+typedef int MPI_Op;
 
 #define MPI_SUCCESS 0
 #define MPI_ERR_BUFFER 1  /* Invalid buffer pointer */
@@ -101,8 +101,10 @@ namespace helper
 namespace mpidummy
 {
 #else
+#ifdef __cplusplus
 extern "C" {
 #endif
+#endif
 
 int MPI_Init(int *argc, char ***argv);
 int MPI_Finalize();
@@ -177,7 +179,9 @@ int MPI_Allreduce(const void *sendbuf, void *recvbuf, int count,
 } // end namespace helper
 } // end namespace adios
 #else
+#ifdef __cplusplus
 } // end extern "C"
 #endif
+#endif
 
 #endif /* ADIOS2_MPIDUMMY_H_ */

source/adios2/helper/mpiwrap.cpp

@@ -6,6 +6,7 @@
  */
 
 #include "mpiwrap.h"
+#include "mpidummy.h"
 
 static inline bool SMPI_Available()
 {

source/adios2/helper/mpiwrap.h

@@ -9,18 +9,61 @@
 #ifndef ADIOS2_HELPER_MPIWRAP_H_
 #define ADIOS2_HELPER_MPIWRAP_H_
 
+// Overview of the MPI wrapping:
+// =============================
+//
+// Including this header provides a unified way of dealing with the
+// building against an actual MPI library (ADIOS2_HAVE_MPI) vs using
+// a dummy implementation, and also deals with running serially
+// when built against MPI, but not calling MPI_Init
+//
+// MPI types / constants
+// ---------------------
+//
+// If ADIOS2_HAVE_MPI is defined, the actual MPI types (in particular MPI_Comm)
+// from the real MPI are always used, even in the dummy implementations that are
+// used when MPI hasn't been initialized.
+// Otherwise, the definitions from mpidummy.h are used. These types are in the
+// global namespace.
+//
+// MPI_* functions
+// -------------
+//
+// The usual MPI functions are always defined in the global namespace (extern
+// "C"). If ADIOS2_HAVE_MPI is defined, the declarations come from mpi.h.
+// Otherwise, these functions are declared in mpidummy.h, also extern "C".
+//
+// If ADIOS2_HAVE_MPI is defined, the declarations in mpidummy.h are put into
+// adios2::helper::mpidummy in order to not interfere with the extern "C" ones,
+// but this way they are still available to use in the SMPI_* wrappers
+//
+// SMPI_* wrappers
+// ---------------
+// The SMPI wrappers provide a way to write code that works in all three of the
+// following cases:
+// - If ADIOS2_HAVE_MPI is undefined, these wrappers call the corresponding
+//   extern "C" MPI functions, which in this case are the mpidummy
+//   implementations.
+// - If ADIOS2_HAVE_MPI is defined, and MPI has been initialized, these wrappers
+//   call the corresponding extern "C" MPI functions, which in this case are the
+//   real MPI implementations.
+// - If ADIOS2_HAVE_MPI is defined, and MPI has not been initialized, these
+//   wrappers call the corresponding adios2::helper::mpidummy MPI functions,
+//   which are serial only but don't require the real MPI to be initialized.
+
+#include "adios2/ADIOSConfig.h"
+
 // Get either the real MPI or mpidummy into the global namespace
 #ifdef ADIOS2_HAVE_MPI
 #include <mpi.h>
 #else
 #include "mpidummy.h"
 #endif
 
-// SMPI_* work just as the original MPI_* functions, but will also
-// work if MPI_Init has not been called, as long asssuming the communicator is a
-// single-proc communicator, by falling back to mpidummy in that case
-
+#ifdef __cplusplus
 extern "C" {
+#endif
+
 int SMPI_Comm_dup(MPI_Comm comm, MPI_Comm *newcomm);
 int SMPI_Comm_free(MPI_Comm *newcomm);
 int SMPI_Comm_rank(MPI_Comm comm, int *rank);
@@ -36,6 +79,9 @@ int SMPI_Gatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
                  MPI_Datatype recvtype, int root, MPI_Comm comm);
 int SMPI_Reduce(const void *sendbuf, void *recvbuf, int count,
                 MPI_Datatype datatype, MPI_Op op, int root, MPI_Comm comm);
-}
 
+#ifdef __cplusplus
+}
 #endif
+
+#endif /* ADIOS2_HELPER_MPIWRAP_H_ */