# 18.2.317. MPI_Sendrecv

MPI_Sendrecv — Sends and receives a message.

## 18.2.317.1. SYNTAX

### 18.2.317.1.1. C Syntax

    #include <mpi.h>

    int MPI_Sendrecv(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
         int dest, int sendtag, void *recvbuf, int recvcount,
         MPI_Datatype recvtype, int source, int recvtag,
         MPI_Comm comm, MPI_Status *status)

### 18.2.317.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_SENDRECV(SENDBUF, SENDCOUNT, SENDTYPE, DEST, SENDTAG,
                 RECVBUF, RECVCOUNT, RECVTYPE, SOURCE, RECVTAG, COMM,
                 STATUS, IERROR)
         <type>  SENDBUF(*), RECVBUF(*)
         INTEGER SENDCOUNT, SENDTYPE, DEST, SENDTAG
         INTEGER RECVCOUNT, RECVTYPE, SOURCE, RECVTAG, COMM
         INTEGER STATUS(MPI_STATUS_SIZE), IERROR

### 18.2.317.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Sendrecv(sendbuf, sendcount, sendtype, dest, sendtag, recvbuf,
                 recvcount, recvtype, source, recvtag, comm, status, ierror)
         TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
         TYPE(*), DIMENSION(..) :: recvbuf
         INTEGER, INTENT(IN) :: sendcount, dest, sendtag, recvcount, source,
         recvtag
         TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
         TYPE(MPI_Comm), INTENT(IN) :: comm
         TYPE(MPI_Status) :: status
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.317.2. INPUT PARAMETERS

- `sendbuf`: Initial address of send buffer (choice).

- `sendcount`: Number of elements to send (integer).

- `sendtype`: Type of elements in send buffer (handle).

- `dest`: Rank of destination (integer).

- `sendtag`: Send tag (integer).

- `recvcount`: Maximum number of elements to receive (integer).

- `recvtype`: Type of elements in receive buffer (handle).

- `source`: Rank of source (integer).

- `recvtag`: Receive tag (integer).

- `comm`: Communicator (handle).

## 18.2.317.3. OUTPUT PARAMETERS

- `recvbuf`: Initial address of receive buffer (choice).

- `status`: Status object (status). This refers to the receive operation.

- `ierror`: Fortran only: Error status (integer).

## 18.2.317.4. DESCRIPTION

The send-receive operations combine in one call the sending of a message to one destination and the receiving of another message, from another process. The two (source and destination) are possibly the same. A send-receive operation is useful for executing a shift operation across a chain of processes. If blocking sends and receives are used for such a shift, then one needs to order the sends and receives correctly (for example, even processes send, then receive; odd processes receive first, then send) in order to prevent cyclic dependencies that may lead to deadlock. When a send-receive operation is used, the communication subsystem takes care of these issues. The send-receive operation can be used in conjunction with the functions described in the “Process Topologies” chapter in the MPI Standard in order to perform shifts on various logical topologies. Also, a send-receive operation is useful for implementing remote procedure calls.

A message sent by a send-receive operation can be received by a regular receive operation or probed by a probe operation; a send-receive operation can receive a message sent by a regular send operation.

MPI_Sendrecv executes a blocking send and receive operation. Both send and receive use the same communicator, but possibly different tags. The send buffer and receive buffers must be disjoint, and may have different lengths and datatypes.

If your application does not need to examine the status field, you can save resources by using the predefined constant `MPI_STATUS_IGNORE` as a special value for the status argument.

## 18.2.317.5. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

Note that per the “Return Status” section in the “Point-to-Point Communication” chapter in the MPI Standard, MPI errors on messages received by MPI_Sendrecv do not set the `status.MPI_ERROR` field in the returned status. The error code is always passed to the back-end error handler and may be passed back to the caller through the return value of MPI_Sendrecv if the back-end error handler returns it. The pre-defined MPI error handler `MPI_ERRORS_RETURN` exhibits this behavior, for example.

See also

- MPI_Sendrecv_replace