NAME

       MPI_Sendrecv -  Sends and receives a message

SYNOPSIS

       #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)

INPUT PARAMETERS

       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)

OUTPUT PARAMETERS

       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.

NOTES

       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 simultaneously

       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.

NOTES FOR FORTRAN

       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 Fortran.

ERRORS

       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.
       MPI_SUCCESS
              - No error; MPI routine completed successfully.
       MPI_ERR_COMM
              - Invalid communicator.  A common error is to use a null  communicator  in  a  call
              (not even allowed in MPI_Comm_rank ).
       MPI_ERR_COUNT
              - Invalid count argument.  Count arguments must be non-negative; a count of zero is
              often valid.
       MPI_ERR_TYPE
              -  Invalid  datatype  argument.   May   be   an   uncommitted   MPI_Datatype   (see
              MPI_Type_commit ).
       MPI_ERR_TAG
              -  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 .

       MPI_ERR_RANK
              -  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 .

SEE ALSO

       MPI_Sendrecv_replace

MORE INFORMATION

       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).

       http://www.mpi-forum.org/

ACKNOWLEDGEMENTS

       The  LAM  Team  would  like the thank the MPICH Team for the handy program to generate man
       pages  ("doctext"  from  ftp://ftp.mcs.anl.gov/pub/sowing/sowing.tar.gz  ),  the   initial
       formatting, and some initial text for most of the MPI-1 man pages.

LOCATION

       sendrecv.c