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       MPI_Sendrecv -  Sends and receives a message


       #include <mpi.h>
       int MPI_Sendrecv(void *sbuf, int scount, MPI_Datatype sdtype,
                       int dest, int stag, void *rbuf, int rcount,
                       MPI_Datatype rdtype, int src, int rtag,
                       MPI_Comm comm, MPI_Status *status)


       sbuf   - initial address of send buffer (choice)
       scount - number of elements in send buffer (integer)
       sdtype - type of elements in send buffer (handle)
       dest   - rank of destination (integer)
       stag   - send tag (integer)
       rcount - number of elements in receive buffer (integer)
       rdtype - type of elements in receive buffer (handle)
       src    - rank of source (integer)
       rtag   - receive tag (integer)
       comm   - communicator (handle)


       rbuf   - initial address of receive buffer (choice)
       status - status object (Status).  This refers to the receive operation.
              Can also be the MPI constant MPI_STATUS_IGNORE , if  the  return
              status is not desired.


       To  dispell  a common misconception: src and dest do not have to be the
       same.  Additionally, a common mistake when using this  function  is  to
       mismatch  the  tags  with  the  source and destination ranks, which can
       result in deadlock.

       This function is guaranteed not to deadlock in situations  where  pairs
       of  blocking  sends  and  receives  may  deadlock.   For  example,  the
       following code may deadlock if all ranks in MPI_COMM_WORLD  execute  it

       int rank, size, to, from;
       MPI_Comm_rank(MPI_COMM_WORLD, &rank);
       MPI_Comm_size(MPI_COMM_WORLD, &size);
       to = (rank + 1) % size;
       from = (rank + size - 1) % size;
       MPI_Send(send_buffer, ..., to, tag, MPI_COMM_WORLD);
       MPI_Recv(recv_buffer, ..., from, tag, MPI_COMM_WORLD);

       If  even  one  rank's  MPI_Send  blocks and never completes, the entire
       operation may deadlock.  One alternative is to use MPI_Sendrecv in this
       situation because it is guaranteed not to deadlock.


       All  MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK ) have
       an additional argument ierr at the end of the argument list.   ierr  is
       an  integer and has the same meaning as the return value of the routine
       in C.  In Fortran, MPI routines are subroutines, and are  invoked  with
       the call statement.

       All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in


       If an error occurs in an MPI function, the current MPI error handler is
       called  to  handle  it.   By default, this error handler aborts the MPI
       job.  The error handler may be changed with  MPI_Errhandler_set  ;  the
       predefined  error  handler MPI_ERRORS_RETURN may be used to cause error
       values to be returned (in C and Fortran; this  error  handler  is  less
       useful  in  with  the  C++  MPI bindings.  The predefined error handler
       MPI::ERRORS_THROW_EXCEPTIONS should be used in C++ if the  error  value
       needs  to  be recovered).  Note that MPI does not guarantee that an MPI
       program can continue past an error.

       All MPI routines (except MPI_Wtime and  MPI_Wtick  )  return  an  error
       value;  C routines as the value of the function and Fortran routines in
       the last argument.  The C++  bindings  for  MPI  do  not  return  error
       values;  instead,  error values are communicated by throwing exceptions
       of type MPI::Exception (but  not  by  default).   Exceptions  are  only
       thrown if the error value is not MPI::SUCCESS .

       Note  that  if  the MPI::ERRORS_RETURN handler is set in C++, while MPI
       functions will return upon an error, there will be no  way  to  recover
       what the actual error value was.
              - No error; MPI routine completed successfully.
              -  Invalid  communicator.   A  common  error  is  to  use a null
              communicator in a call (not even allowed in MPI_Comm_rank ).
              - Invalid count argument.  Count arguments must be non-negative;
              a count of zero is often valid.
              - Invalid datatype argument.  May be an uncommitted MPI_Datatype
              (see MPI_Type_commit ).
              - Invalid tag argument.  Tags must be non-negative;  tags  in  a
              receive  ( MPI_Recv , MPI_Irecv , MPI_Sendrecv , etc.)  may also
              be MPI_ANY_TAG .  The largest tag value is available through the
              the attribute MPI_TAG_UB .

              -  Invalid  source  or  destination rank.  Ranks must be between
              zero and the size of the communicator  minus  one;  ranks  in  a
              receive  (  MPI_Recv , MPI_Irecv , MPI_Sendrecv , etc.) may also
              be MPI_ANY_SOURCE .




       For more information, please see the official MPI Forum web site, which
       contains  the  text  of  both  the  MPI-1  and  MPI-2 standards.  These
       documents contain detailed information about each MPI function (most of
       which is not duplicated in these man pages).


       The  LAM Team would like the thank the MPICH Team for the handy program
       to       generate        man        pages        ("doctext"        from     ),    the    initial
       formatting, and some initial text for most of the MPI-1 man pages.