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.TH "MPI_BSEND" "3" "Sep 17, 2026" "" "Open MPI"
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MPI_Bsend — Basic send with user\-specified buffering.
.SH SYNTAX
.SS C Syntax
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.INDENT 3.5
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.EX
#include <mpi.h>

int MPI_Bsend(const void *buf, int count, MPI_Datatype datatype,
   int dest, int tag, MPI_Comm comm)
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.SS Fortran Syntax
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.EX
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq

MPI_BSEND(BUF, COUNT,DATATYPE, DEST, TAG, COMM, IERROR)
    <type>  BUF(*)
    INTEGER COUNT, DATATYPE, DEST, TAG, COMM, IERROR
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.SS Fortran 2008 Syntax
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.EX
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
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.SH INPUT PARAMETERS
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.IP \(bu 2
\fBbuf\fP : Initial address of send buffer (choice).
.IP \(bu 2
\fBcount\fP : Number of entries in send buffer (nonnegative integer).
.IP \(bu 2
\fBdatatype\fP : Datatype of each send buffer element (handle).
.IP \(bu 2
\fBdest\fP : Rank of destination (integer).
.IP \(bu 2
\fBtag\fP : Message tag (integer).
.IP \(bu 2
\fBcomm\fP : Communicator (handle).
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.SH OUTPUT PARAMETERS
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.IP \(bu 2
\fBierror\fP : Fortran only: Error status (integer).
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.SH DESCRIPTION
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MPI_Bsend performs a buffered\-mode, blocking send.
.SH NOTES
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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 \%<#\:mpi-buffer-attach>).
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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:
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.EX
MPI_Buffer_attach( b, n*sizeof(double) + MPI_BSEND_OVERHEAD );
for (i=0; i<m; i++) {
    MPI_Bsend( buf, n, MPI_DOUBLE, ... );
}
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because only enough buffer space is provided for a single send, and the
loop may start a second \fBMPI_Bsend\fP before the first is done making
use of the buffer.
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In C, you can force the messages to be delivered by
\fBMPI_Buffer_detach( &b, &n );\fP \fBMPI_Buffer_attach( b, n );\fP (The
\fBMPI_Buffer_detach\fP will not complete until all buffered messages are
delivered.)
.SH ERRORS
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Almost all MPI routines return an error value; C routines as the return result
of the function and Fortran routines in the last argument.
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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>/MPI_Init_thread \%<#\:mpi-init-thread>, after MPI_Finalize \%<#\: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 \%<#\: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>/MPI_Comm_spawn_multiple \%<#\: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.
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Open MPI includes three predefined error handlers that can be used:
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.IP \(bu 2
\fBMPI_ERRORS_ARE_FATAL\fP
Causes the program to abort all connected MPI processes.
.IP \(bu 2
\fBMPI_ERRORS_ABORT\fP
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 \%<#\:mpi-abort> was called on that communicator. If
called on a window or file, acts as if MPI_Abort \%<#\: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.
.IP \(bu 2
\fBMPI_ERRORS_RETURN\fP
Returns an error code to the application.
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MPI applications can also implement their own error handlers by calling:
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.IP \(bu 2
MPI_Comm_create_errhandler \%<#\:mpi-comm-create-errhandler> then MPI_Comm_set_errhandler \%<#\:mpi-comm-set-errhandler>
.IP \(bu 2
MPI_File_create_errhandler \%<#\:mpi-file-create-errhandler> then MPI_File_set_errhandler \%<#\:mpi-file-set-errhandler>
.IP \(bu 2
MPI_Session_create_errhandler \%<#\:mpi-session-create-errhandler> then MPI_Session_set_errhandler \%<#\:mpi-session-set-errhandler> or at MPI_Session_init \%<#\:mpi-session-init>
.IP \(bu 2
MPI_Win_create_errhandler \%<#\:mpi-win-create-errhandler> then MPI_Win_set_errhandler \%<#\:mpi-win-set-errhandler>
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Note that MPI does not guarantee that an MPI program can continue past
an error.
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See the MPI man page \%<#\:open-mpi> for a full list of MPI error codes \%<#\:open-mpi-errors>\&.
.sp
See the Error Handling section of the MPI\-3.1 standard for
more information.
.SH Copyright
2003-2026, The Open MPI Community
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