plucky (3) MPI_Pack.openmpi.3.gz
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Pack(const void *inbuf, int incount, MPI_Datatype datatype,
void *outbuf, int outsize, int *position, MPI_Comm comm)
Fortran Syntax
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_PACK(INBUF, INCOUNT, DATATYPE, OUTBUF,OUTSIZE, POSITION,
COMM, IERROR)
<type> INBUF(*), OUTBUF(*)
INTEGER INCOUNT, DATATYPE, OUTSIZE, POSITION, COMM, IERROR
Fortran 2008 Syntax
USE mpi_f08
MPI_Pack(inbuf, incount, datatype, outbuf, outsize, position, comm, ierror)
TYPE(*), DIMENSION(..), INTENT(IN) :: inbuf
TYPE(*), DIMENSION(..) :: outbuf
INTEGER, INTENT(IN) :: incount, outsize
TYPE(MPI_Datatype), INTENT(IN) :: datatype
INTEGER, INTENT(INOUT) :: position
TYPE(MPI_Comm), INTENT(IN) :: comm
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
INPUT PARAMETERS
• inbuf: Input buffer start (choice).
• incount: Number of input data items (integer).
• datatype: Datatype of each input data item (handle).
• outsize: Output buffer size, in bytes (integer).
• comm: Communicator for packed message (handle).
INPUT/OUTPUT PARAMETER
• position: Current position in buffer, in bytes (integer).
OUTPUT PARAMETERS
• outbuf: Output buffer start (choice).
• ierror: Fortran only: Error status (integer).
DESCRIPTION
Packs the message in the send buffer specified by inbuf, incount, datatype into the buffer space
specified by outbuf and outsize. The input buffer can be any communication buffer allowed in MPI_Send.
The output buffer is a contiguous storage area containing outsize bytes, starting at the address outbuf
(length is counted in bytes, not elements, as if it were a communication buffer for a message of type
MPI_Packed).
The input value of position is the first location in the output buffer to be used for packing. position
is incremented by the size of the packed message, and the output value of position is the first location
in the output buffer following the locations occupied by the packed message. The comm argument is the
communicator that will be subsequently used for sending the packed message.
Example: An example using MPI_Pack:
int myrank, position, i, j, a[2];
char buff[1000];
// ...
MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
if (myrank == 0) {
/* SENDER CODE */
position = 0;
MPI_Pack(&i, 1, MPI_INT, buff, 1000, &position, MPI_COMM_WORLD);
MPI_Pack(&j, 1, MPI_INT, buff, 1000, &position, MPI_COMM_WORLD);
MPI_Send(buff, position, MPI_PACKED, 1, 0, MPI_COMM_WORLD);
} else {
/* RECEIVER CODE */
MPI_Recv(a, 2, MPI_INT, 0, 0, MPI_COMM_WORLD);
}
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.
SEE ALSO:
• MPI_Unpack
• MPI_Pack_size
COPYRIGHT
2003-2025, The Open MPI Community
Feb 17, 2025 MPI_PACK(3)