Hello all, This email thread discusses the proposal to add const correctness to the MPI C bindings. Below you can find the original proposal and attached is the slides presented at the Jan forum meeting. To be able to present this proposal in the March forum meeting I think that we still need to work on few items, specifically the motivation for this proposal and addressing concerned that were raised before. The TODO list includes, - Post this proposal on the wiki pages - Review/rewrite the executive summary of this proposal. (any suggestions?) - Address issues that were raised as cons for this proposal. Specifically backward compatibility and clarify the (incorrect) claim that the C standard does not allow a function to modify an object passed in as const (using another pointer). - Any other issues raised (or not) that the proposal is missing? - Review the changes to the standard as proposed below for completeness. That is, the proposal below just addresses the API. a) is this complete? b) are there any other required text modification to the standard? - Prototype the proposal (i.e., have a mpi.h header file) and test it. I can test the header with the ANL, Intel test suites plus a handful of others. I think that we need one conf call to discuss this proposal. (I'm shooting for Mon/Tue/Wed next week at 9am PST) Let me know what you think. Thanks, .Erez P.S. note that the list name is misspelled as mpi-conts (not const) when you're sending email. From: owner-mpi-21_at_[hidden] [mailto:owner-mpi-21_at_[hidden]] On Behalf Of Erez Haba Sent: Monday, December 17, 2007 11:48 PM To: mpi-21_at_[hidden] Subject: [mpi-21] Proposal EH2: add const keyword to the C bindings This is the proposal to add the const keyword to the MPI C bindings where appropriate. I think that this change is appropriate for the MPI 2.2 revision. Thanks, .Erez Background: The const keyword in C defines a contract between the library implementer and the library user. Using the const keyword the library contracts that it will not change its input object. This contract enables some compile-time optimization, but more important it provides clearer and const-correct interface to the library user. (more on http://en.wikipedia.org/wiki/Const_correctness) The MPI C bindings as defined by the MPI 1.1 & 2.0 standards are missing the const keyword for many of the input only parameters. Proposal: Add the const keyword to the API's listed below. Rational: The missing const keyword means that the contract between the user and the MPI library is weaker than it should be. Users need to cast away const-ness before calling MPI and compilers cannot optimize the caller or the library code taking into account the const-ness of the parameter. The C++ binding has already implemented a const correct interface. This change catches up with the C++ interface. (I notice that C++ interface implementations cast away the const-ness when calling their C binding). Backward Compatibility: There should not be any backward compatibility issue: already compiled programs should run without a problem with a new dynamically loaded library as there is no change to the size of the parameters. The C linkers do not check for const correctness, thus no error here. Recompiled programs should see no new compilation errors or warning since the const keyword guarantees a stronger contract; any non const pointer is automatically promoted to be const. Cons: Compilers that still do not support the const keyword. MPI library implementations need to change their C header files. Source level backward compatibility issue, in-case I am wrong. Note: adding the const keyword to the point-to-point and collectives API's send buffer depends on the "Proposal EH1: Send buffer access". Adding the const keyword: I have compiled the list of API's that are missing the const keywords. I've grouped these API's by functionality and the parameter where the const keyword is applied. The last section lists 4 API's where I suggest not adding the const keyword as it would break source level backward compatibility. /* Point-to-point send buffer */ int MPI_Bsend(const void*, int, MPI_Datatype, int, int, MPI_Comm); int MPI_Bsend_init(const void*, int, MPI_Datatype, int,int, MPI_Comm, MPI_Request*); int MPI_Ibsend(const void*, int, MPI_Datatype, int, int, MPI_Comm, MPI_Request*); int MPI_Irsend(const void*, int, MPI_Datatype, int, int, MPI_Comm, MPI_Request*); int MPI_Isend(const void*, int, MPI_Datatype, int, int, MPI_Comm, MPI_Request*); int MPI_Issend(const void*, int, MPI_Datatype, int, int, MPI_Comm, MPI_Request*); int MPI_Rsend(const void*, int, MPI_Datatype, int, int, MPI_Comm); int MPI_Rsend_init(const void*, int, MPI_Datatype, int,int, MPI_Comm, MPI_Request*); int MPI_Send(const void*, int, MPI_Datatype, int, int, MPI_Comm); int MPI_Send_init(const void*, int, MPI_Datatype, int, int, MPI_Comm, MPI_Request*); int MPI_Sendrecv(const void*, int, MPI_Datatype,int, int, void*, int, MPI_Datatype, int, int, MPI_Comm, MPI_Status*); int MPI_Sendrecv_replace(void*, int, MPI_Datatype, int, int, int, int, MPI_Comm, MPI_Status*); int MPI_Ssend(const void*, int, MPI_Datatype, int, int, MPI_Comm); int MPI_Ssend_init(const void*, int, MPI_Datatype, int,int, MPI_Comm, MPI_Request*); /* Collectives send buffer */ int MPI_Accumulate(const void*, int, MPI_Datatype, int, MPI_Aint, int, MPI_Datatype, MPI_Op, MPI_Win); int MPI_Allgather(const void* , int, MPI_Datatype, void*, int, MPI_Datatype, MPI_Comm); int MPI_Allgatherv(const void* , int, MPI_Datatype, void*, const int*, const int*, MPI_Datatype, MPI_Comm); int MPI_Allreduce(const void* , void*, int, MPI_Datatype, MPI_Op, MPI_Comm); int MPI_Alltoall(const void* , int, MPI_Datatype, void*, int, MPI_Datatype, MPI_Comm); int MPI_Alltoallv(const void* , const int*, const int*, MPI_Datatype, void*, const int*, const int*, MPI_Datatype, MPI_Comm); int MPI_Alltoallw(const void*, const int [], const int [], MPI_Datatype [], void*, const int [], const int [], MPI_Datatype [], MPI_Comm); int MPI_Exscan(const void*, void*, int, MPI_Datatype, MPI_Op, MPI_Comm) ; int MPI_Gather(const void* , int, MPI_Datatype, void*, int, MPI_Datatype, int, MPI_Comm); int MPI_Gatherv(const void* , int, MPI_Datatype, void*, const int*, const int*, MPI_Datatype, int, MPI_Comm); int MPI_Reduce(const void* , void*, int, MPI_Datatype, MPI_Op, int, MPI_Comm); int MPI_Reduce_scatter(const void* , void*, const int*, MPI_Datatype, MPI_Op, MPI_Comm); int MPI_Scan(const void* , void*, int, MPI_Datatype, MPI_Op, MPI_Comm ); int MPI_Scatter(const void* , int, MPI_Datatype, void*, int, MPI_Datatype, int, MPI_Comm); int MPI_Scatterv(const void* , const int*, const int*, MPI_Datatype, void*, int, MPI_Datatype, int, MPI_Comm); /* RMA put buffer */ int MPI_Put(const void*, int, MPI_Datatype, int, MPI_Aint, int, MPI_Datatype, MPI_Win); /* File IO write buffer */ int MPI_File_iwrite(MPI_File, const void*, int, MPI_Datatype, MPIO_Request*); int MPI_File_iwrite_at(MPI_File, MPI_Offset, const void*, int, MPI_Datatype, MPIO_Request*); int MPI_File_iwrite_shared(MPI_File, const void*, int, MPI_Datatype, MPIO_Request*); int MPI_File_write(MPI_File, const void*, int, MPI_Datatype, MPI_Status*); int MPI_File_write_all(MPI_File, const void*, int, MPI_Datatype, MPI_Status*); int MPI_File_write_all_begin(MPI_File, const void*, int, MPI_Datatype); int MPI_File_write_all_end(MPI_File, const void*, MPI_Status*); int MPI_File_write_at(MPI_File, MPI_Offset, const void*, int, MPI_Datatype, MPI_Status*); int MPI_File_write_at_all(MPI_File, MPI_Offset, const void*, int, MPI_Datatype, MPI_Status*); int MPI_File_write_at_all_begin(MPI_File, MPI_Offset, const void*, int, MPI_Datatype); int MPI_File_write_at_all_end(MPI_File, const void*, MPI_Status*); int MPI_File_write_ordered(MPI_File, const void*, int, MPI_Datatype, MPI_Status*); int MPI_File_write_ordered_begin(MPI_File, const void*, int, MPI_Datatype); int MPI_File_write_ordered_end(MPI_File, const void*, MPI_Status*); int MPI_File_write_shared(MPI_File, const void*, int, MPI_Datatype, MPI_Status*); /* Input string */ int MPI_Add_error_string(int, const char*); int MPI_Close_port(const char*); int MPI_Comm_accept(const char*, MPI_Info, int, MPI_Comm, MPI_Comm*); int MPI_Comm_connect(const char*, MPI_Info, int, MPI_Comm, MPI_Comm*); int MPI_Comm_set_name(MPI_Comm, const char*); int MPI_Comm_spawn(const char*, char*[], int, MPI_Info, int, MPI_Comm, MPI_Comm*, int []); int MPI_File_delete(const char*, MPI_Info); int MPI_File_open(MPI_Comm, const char*, int, MPI_Info, MPI_File*); int MPI_File_set_view(MPI_File, MPI_Offset, MPI_Datatype, MPI_Datatype, const char*, MPI_Info); int MPI_Info_delete(MPI_Info, const char*); int MPI_Info_get(MPI_Info, const char*, int, char*, int*); int MPI_Info_get_valuelen(MPI_Info, const char*, int*, int*); int MPI_Info_set(MPI_Info, const char*, const char*); int MPI_Lookup_name(const char*, MPI_Info, char*); int MPI_Publish_name(const char*, MPI_Info, const char*); int MPI_Type_set_name(MPI_Datatype, const char*); int MPI_Unpublish_name(const char*, MPI_Info, const char*); int MPI_Win_set_name(MPI_Win, const char*); /* Index/Count arrays */ int MPI_Cart_create(MPI_Comm, int, const int*, const int*, int, MPI_Comm*); int MPI_Cart_map(MPI_Comm, int, const int*, const int*, const int*); int MPI_Cart_rank(MPI_Comm, const int*, int*); int MPI_Cart_sub(MPI_Comm, const int*, MPI_Comm*); int MPI_Graph_create(MPI_Comm, int, const int*, const int*, int, MPI_Comm*); int MPI_Graph_map(MPI_Comm, int, const int*, const int*, int*); int MPI_Group_excl(MPI_Group, int, const int*, MPI_Group*); int MPI_Group_incl(MPI_Group, int, const int*, MPI_Group*); int MPI_Group_translate_ranks(MPI_Group, int, const int*, MPI_Group, int*); int MPI_Type_create_darray(int, int, int, const int [], const int [], const int [], const int [], int, MPI_Datatype, MPI_Datatype*); int MPI_Type_create_hindexed(int, const int [], const MPI_Aint [], MPI_Datatype, MPI_Datatype*); int MPI_Type_create_indexed_block(int, int, const int [], MPI_Datatype, MPI_Datatype*); int MPI_Type_create_struct(int, const int [], const MPI_Aint [], const MPI_Datatype [], MPI_Datatype*); int MPI_Type_create_subarray(int, const int [], const int [], const int [], int, MPI_Datatype, MPI_Datatype*); int MPI_Type_hindexed(int, const int*, const MPI_Aint*, MPI_Datatype, MPI_Datatype*); int MPI_Type_indexed(int, const int*, const int*, MPI_Datatype, MPI_Datatype*); int MPI_Type_struct(int, const int*, const MPI_Aint*, MPI_Datatype*, MPI_Datatype*); /* Pack/Unpack datarep string and input buffer */ int MPI_Pack(const void*, int, MPI_Datatype, void*, int, int*, MPI_Comm); int MPI_Pack_external(const char*, const void*, int, MPI_Datatype, void*, MPI_Aint, MPI_Aint*); int MPI_Pack_external_size(const char*, int, MPI_Datatype, MPI_Aint*); int MPI_Unpack(const void*, int, const int*, void*, int, MPI_Datatype, MPI_Comm); int MPI_Unpack_external(const char*, const void*, MPI_Aint, MPI_Aint*, void*, int, MPI_Datatype); /* Miscellany */ int MPI_Address(const void*, MPI_Aint*); int MPI_Get_address(const void*, MPI_Aint*); int MPI_Status_c2f(const MPI_Status*, MPI_Fint*); int MPI_Status_f2c(const MPI_Fint*, MPI_Status*); int MPI_Test_cancelled(const MPI_Status*, int*); /* NOT CHANGING the following API's to keep backward compatibility and ease of use */ int MPI_Comm_spawn(const char*, /*const*/char*[], int, MPI_Info, int, MPI_Comm, MPI_Comm*, int []); int MPI_Comm_spawn_multiple(int, /*const*/char*[], /*const*/char**[], /*const*/int [],/*const*/MPI_Info [], int, MPI_Comm, MPI_Comm*, int []); int MPI_Group_range_excl(MPI_Group, int, /*const*/int [][3], MPI_Group*); int MPI_Group_range_incl(MPI_Group, int, /*const*/int [][3], MPI_Group*); *