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