21.3.2. Opaque Handles

Handles for MPI objects are defined to be incomplete struct pointers, which allows for C compilers to do type-checking, while also satisfying the existing requirements, such as equality comparison.

Rationale.

Integer handles do not provide type safety, while struct or union handles fail to satisfy the existing API requirements. ( End of rationale.)
The following handle type definitions are part of the MPI ABI:

typedef struct MPI_ABI_Comm* MPI_Comm; 
typedef struct MPI_ABI_Datatype* MPI_Datatype; 
typedef struct MPI_ABI_Errhandler* MPI_Errhandler; 
typedef struct MPI_ABI_File* MPI_File; 
typedef struct MPI_ABI_Group* MPI_Group; 
typedef struct MPI_ABI_Info* MPI_Info; 
typedef struct MPI_ABI_Message* MPI_Message; 
typedef struct MPI_ABI_Op* MPI_Op; 
typedef struct MPI_ABI_Request* MPI_Request; 
typedef struct MPI_ABI_Session* MPI_Session; 
typedef struct MPI_ABI_Win* MPI_Win;