MPI_PACK_SIZE(3)				     Open MPI					  MPI_PACK_SIZE(3)

MPI_Pack_size — Returns the upper bound on the amount of space needed to pack a message.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Pack_size(int incount, MPI_Datatype datatype, MPI_Comm comm,
	       int *size)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_PACK_SIZE(INCOUNT, DATATYPE, COMM, SIZE, IERROR)
	       INTEGER INCOUNT, DATATYPE, COMM, SIZE, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Pack_size(incount, datatype, comm, size, ierror)
	       INTEGER, INTENT(IN) :: incount
	       TYPE(MPI_Datatype), INTENT(IN) :: datatype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, INTENT(OUT) :: size
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • incount: Count argument to packing call (integer).

       • datatype: Datatype argument to packing call (handle).

       • comm: Communicator argument to packing call (handle).

OUTPUT PARAMETERS
       • size: Upper bound on size of packed message, in bytes (integer).

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

DESCRIPTION
       MPI_Pack_size  allows  the  application	to  find out how much space is needed to pack a message. A call to
       MPI_Pack_size(incount, datatype, comm, size) returns in size an upper bound on the  increment  in  position
       that would occur in a call to MPI_Pack <#mpi-pack>, with the same values for incount, datatype, and comm.

       Rationale:  The	call  returns  an upper bound, rather than an exact bound, since the exact amount of space
       needed to pack the message may depend on the context (e.g., first message packed in a packing unit may take
       more space).

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.

       See also:

	  • MPI_Pack <#mpi-pack>

	  • MPI_Unpack <#mpi-unpack>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					  MPI_PACK_SIZE(3)
