# 18.2.274. MPI_Pack

MPI_Pack — Packs data of a given datatype into contiguous memory.

## 18.2.274.1. SYNTAX

### 18.2.274.1.1. 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)

### 18.2.274.1.2. 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

### 18.2.274.1.3. 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

## 18.2.274.2. 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).

## 18.2.274.3. INPUT/OUTPUT PARAMETER

- `position`: Current position in buffer, in bytes (integer).

## 18.2.274.4. OUTPUT PARAMETERS

- `outbuf`: Output buffer start (choice).

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

## 18.2.274.5. 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);
    }

## 18.2.274.6. 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.

See also

- MPI_Unpack

- MPI_Pack_size