17.2.29. MPI_Bsend — Open MPI 5.0.10 documentation

# 17.2.29. MPI_Bsend

MPI_Bsend — Basic send with user-specified buffering.

## 17.2.29.1. SYNTAX

### 17.2.29.1.1. C Syntax

    #include <mpi.h>

    int MPI_Bsend(const void *buf, int count, MPI_Datatype datatype,
       int dest, int tag, MPI_Comm comm)

### 17.2.29.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'

    MPI_BSEND(BUF, COUNT,DATATYPE, DEST, TAG, COMM, IERROR)
        <type>  BUF(*)
        INTEGER COUNT, DATATYPE, DEST, TAG, COMM, IERROR

### 17.2.29.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Bsend(buf, count, datatype, dest, tag, comm, ierror)
        TYPE(*), DIMENSION(..), INTENT(IN) :: buf
        INTEGER, INTENT(IN) :: count, dest, tag
        TYPE(MPI_Datatype), INTENT(IN) :: datatype
        TYPE(MPI_Comm), INTENT(IN) :: comm
        INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.29.2. INPUT PARAMETERS

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

- `count` : Number of entries in send buffer (nonnegative integer).

- `datatype` : Datatype of each send buffer element (handle).

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

- `tag` : Message tag (integer).

- `comm` : Communicator (handle).

## 17.2.29.3. OUTPUT PARAMETERS

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

## 17.2.29.4. DESCRIPTION

MPI_Bsend performs a buffered-mode, blocking send.

## 17.2.29.5. NOTES

This send is provided as a convenience function; it allows the user to send messages without worrying about where they are buffered (because the user must have provided buffer space with MPI_Buffer_attach).

In deciding how much buffer space to allocate, remember that the buffer space is not available for reuse by subsequent MPI_Bsends unless you are certain that the message has been received (not just that it should have been received). For example, this code does not allocate enough buffer space:

    MPI_Buffer_attach( b, n*sizeof(double) + MPI_BSEND_OVERHEAD );
    for (i=0; i<m; i++) {
        MPI_Bsend( buf, n, MPI_DOUBLE, ... );
    }

because only enough buffer space is provided for a single send, and the loop may start a second `MPI_Bsend` before the first is done making use of the buffer.

In C, you can force the messages to be delivered by `MPI_Buffer_detach( &b, &n );` `MPI_Buffer_attach( b, n );` (The `MPI_Buffer_detach` will not complete until all buffered messages are delivered.)

## 17.2.29.6. 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.