when I read mpich2-1.4.1 code, I saw code as follow: Code: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 typedef int MPI_Datatype; #define MPI_CHAR           ((MPI_Datatype)0x4c000101) #define MPI_SIGNED_CHAR    ((MPI_Datatype)0x4c000118) #define MPI_UNSIGNED_CHAR  ((MPI_Datatype)0x4c000102) #define MPI_BYTE           ((MPI_Datatype)0x4c00010d) #define MPI_WCHAR          ((MPI_Datatype)0x4c00040e) #define MPI_SHORT          ((MPI_Datatype)0x4c000203) #define MPI_UNSIGNED_SHORT ((MPI_Datatype)0x4c000204) #define MPI_INT            ((MPI_Datatype)0x4c000405) #define MPI_UNSIGNED       ((MPI_Datatype)0x4c000406) #define MPI_LONG           ((MPI_Datatype)0x4c000407) #define MPI_UNSIGNED_LONG  ((MPI_Datatype)0x4c000408) #define MPI_FLOAT          ((MPI_Datatype)0x4c00040a) #define MPI_DOUBLE         ((MPI_Datatype)0x4c00080b) #define MPI_LONG_DOUBLE    ((MPI_Datatype)0x4c000c0c) #define MPI_LONG_LONG_INT  ((MPI_Datatype)0x4c000809) #define MPI_UNSIGNED_LONG_LONG ((MPI_Datatype)0x4c000819) #define MPI_LONG_LONG      MPI_LONG_LONG_INT Anyone can tell me what is the meaning of those hex values? and why does the author use them? why do they define "MPI_CHAR" as ((MPI_Datatype)0x4c000101) instead of "char"? thx