MPI_BSEND(3)					     Open MPI					      MPI_BSEND(3)

MPI_Bsend — Basic send with user-specified buffering.

SYNTAX
   C Syntax
	  #include <mpi.h>

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

   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

   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

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).

OUTPUT PARAMETERS
       • ierror : Fortran only: Error status (integer).

DESCRIPTION
       MPI_Bsend performs a buffered-mode, blocking send.

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 <#
       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.)

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>/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.

       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 <#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.

       • 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	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • 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>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

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

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

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

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					      MPI_BSEND(3)
