The Great and Terrible implementation of MPI-2

function index


Unpack a buffer according to a datatype into contiguous memory
int MPI_Unpack(
  void *inbuf,
  int insize,
  int *position,
  void *outbuf,
  int outcount,
  MPI_Datatype datatype,
  MPI_Comm comm


[in] input buffer start (choice)
[in] size of input buffer, in bytes (integer)
[in] current position in bytes (integer)
[out] output buffer start (choice)
[in] number of items to be unpacked (integer)
[in] datatype of each output data item (handle)
[in] communicator for packed message (handle)


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.

Advice to users.

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 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. In fact, in a heterogeneous system, this number may not be determined a priori. ( End of advice to users.)
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.


The restriction on "atomic" packing and unpacking of packing units allows the implementation to add at the head of packing units additional information, such as a description of the sender architecture (to be used for type conversion, in a heterogeneous environment)

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.


All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

No error; MPI routine completed successfully.
Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.
Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).
Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

See Also

MPI_Pack, MPI_Pack_size

Example Code

The following sample code illustrates MPI_Unpack.

#include "mpi.h"
#include <stdio.h>

int main(int argc, char *argv[])
    int rank, size;
    int i;
    char c[100];
    char buffer[110];
    int position = 0;
    MPI_Status status;

    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if (size < 2)
        printf("Please run with 2 processes.\n");fflush(stdout);
        return 1;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    if (rank == 0)
        for (i=0; i<100; i++)
            c[i] = i;
        i = 123;
        MPI_Pack(&i, 1, MPI_INT, buffer, 110, &position, MPI_COMM_WORLD);
        MPI_Pack(c, 100, MPI_CHAR, buffer, 110, &position, MPI_COMM_WORLD);
        MPI_Send(buffer, position, MPI_PACKED, 1, 100, MPI_COMM_WORLD);

    if (rank == 1)
        MPI_Recv(buffer, 110, MPI_PACKED, 0, 100, MPI_COMM_WORLD, &status);
        MPI_Unpack(buffer, 110, &position, &i, 1, MPI_INT, MPI_COMM_WORLD);
        MPI_Unpack(buffer, 110, &position, c, 100, MPI_CHAR, MPI_COMM_WORLD);
        printf("i=%d\nc[0] = %d\n...\nc[99] = %d\n", i, (int)c[0], (int)c[99]);fflush(stdout);

    return 0;