# MPI_Unpack_external

Unpack a buffer (packed with MPI_Pack_external) according to a datatype into contiguous memory

## Synopsis

    int MPI_Unpack_external(const char datarep[],
                          const void *inbuf,
                          MPI_Aint insize,
                          MPI_Aint *position,
                          void *outbuf,
                          int outcount,
                          MPI_Datatype datatype)

## Input Parameters

datarep
  data representation (string)
inbuf
  input buffer start (choice)
insize
  input buffer size, in bytes (address integer)
outcount
  number of output data items (integer)
datatype
  datatype of output data item (handle)

  ## Input/Output Parameters

position
  current position in buffer, in bytes (address integer)

  ## Output Parameters

outbuf
  output buffer start (choice)

  ## 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.

  ## Errors

  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.

MPI_SUCCESS
  No error; MPI routine completed successfully.
MPI_ERR_TYPE
  Invalid datatype argument. Additionally, this error can occur if an uncommitted MPI_Datatype (see `MPI_Type_commit`) is used in a communication call.
MPI_ERR_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).