# 18.2.420. MPI_Unpack

MPI_Unpack — Unpacks a datatype into contiguous memory.

## 18.2.420.1. SYNTAX

### 18.2.420.1.1. C Syntax

    #include <mpi.h>

    int MPI_Unpack(const void *inbuf, int insize, int *position,
         void *outbuf, int outcount, MPI_Datatype datatype,
         MPI_Comm comm)

### 18.2.420.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_UNPACK(INBUF, INSIZE, POSITION, OUTBUF, OUTCOUNT,
         DATATYPE, COMM, IERROR)
         <type>  INBUF(*), OUTBUF(*)
         INTEGER INSIZE, POSITION, OUTCOUNT, DATATYPE,
                 COMM, IERROR

### 18.2.420.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Unpack(inbuf, insize, position, outbuf, outcount, datatype, comm,
                 ierror)
         TYPE(*), DIMENSION(..), INTENT(IN) :: inbuf
         TYPE(*), DIMENSION(..) :: outbuf
         INTEGER, INTENT(IN) :: insize, outcount
         INTEGER, INTENT(INOUT) :: position
         TYPE(MPI_Datatype), INTENT(IN) :: datatype
         TYPE(MPI_Comm), INTENT(IN) :: comm
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.420.2. INPUT PARAMETERS

- `inbuf`: Input buffer start (choice).

- `insize`: Size of input buffer, in bytes (integer).

- `outcount`: Number of items to be unpacked (integer).

- `datatype`: Datatype of each output data item (handle).

- `comm`: Communicator for packed message (handle).

## 18.2.420.3. INPUT/OUTPUT PARAMETER

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

## 18.2.420.4. OUTPUT PARAMETERS

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

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

## 18.2.420.5. DESCRIPTION

Unpacks a message into the receive buffer specified by outbuf, outcount, datatype from the buffer space specified by inbuf and insize. The output buffer can be any communication buffer allowed in MPI_Recv. The input buffer is a contiguous storage area containing insize bytes, starting at address inbuf. The input value of position is the first location in the input buffer occupied by the packed message. position is incremented by the size of the packed message, so that the output value of position is the first location in the input buffer after the locations occupied by the message that was unpacked. comm is the communicator used to receive the packed message.

## 18.2.420.6. NOTES

Note the difference between MPI_Recv and MPI_Unpack: In MPI_Recv, the count argument specifies the maximum number of items that can be received. The actual number of items received is determined by the length of the incoming message. In MPI_Unpack, the count argument specifies the actual number of items that are to be unpacked; the “size” of the corresponding message is the increment in position. The reason for this change is that the “incoming message size” is not predetermined since the user decides how much to unpack; nor is it easy to determine the “message size” from the number of items to be unpacked.

To understand the behavior of pack and unpack, it is convenient to think of the data part of a message as being the sequence obtained by concatenating the successive values sent in that message. The pack operation stores this sequence in the buffer space, as if sending the message to that buffer. The unpack operation retrieves this sequence from buffer space, as if receiving a message from that buffer. (It is helpful to think of internal Fortran files or sscanf in C for a similar function.)

Several messages can be successively packed into one packing unit. This is effected by several successive related calls to MPI_Pack, where the first call provides position = 0, and each successive call inputs the value of position that was output by the previous call, and the same values for outbuf, outcount, and comm. This packing unit now contains the equivalent information that would have been stored in a message by one send call with a send buffer that is the “concatenation” of the individual send buffers.

A packing unit can be sent using type MPI_Packed. Any point-to-point or collective communication function can be used to move the sequence of bytes that forms the packing unit from one process to another. This packing unit can now be received using any receive operation, with any datatype: The type-matching rules are relaxed for messages sent with type MPI_Packed.

A message sent with any type (including MPI_Packed) can be received using the type MPI_Packed. Such a message can then be unpacked by calls to MPI_Unpack.

A packing unit (or a message created by a regular, “typed” send) can be unpacked into several successive messages. This is effected by several successive related calls to MPI_Unpack, where the first call provides position = 0, and each successive call inputs the value of position that was output by the previous call, and the same values for inbuf, insize, and comm.

The concatenation of two packing units is not necessarily a packing unit; nor is a substring of a packing unit necessarily a packing unit. Thus, one cannot concatenate two packing units and then unpack the result as one packing unit; nor can one unpack a substring of a packing unit as a separate packing unit. Each packing unit that was created by a related sequence of pack calls or by a regular send must be unpacked as a unit, by a sequence of related unpack calls.

## 18.2.420.7. 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_Pack

- MPI_Pack_size