# 17.2.136. MPI_File_read_ordered_begin

MPI_File_read_ordered_begin — Reads a file at a location specified by a shared file pointer; beginning part of a split collective routine (nonblocking).

## 17.2.136.1. SYNTAX

### 17.2.136.1.1. C Syntax

    #include <mpi.h>

    int MPI_File_read_ordered_begin(MPI_File fh, void *buf,
         int count, MPI_Datatype datatype)

### 17.2.136.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_FILE_READ_ORDERED_BEGIN(FH, BUF, COUNT, DATATYPE, IERROR)
         <type>  BUF(*)
         INTEGER FH, COUNT, DATATYPE, IERROR

### 17.2.136.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_File_read_ordered_begin(fh, buf, count, datatype, ierror)
         TYPE(MPI_File), INTENT(IN) :: fh
         TYPE(*), DIMENSION(..), ASYNCHRONOUS :: buf
         INTEGER, INTENT(IN) :: count
         TYPE(MPI_Datatype), INTENT(IN) :: datatype
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.136.2. INPUT/OUTPUT PARAMETER

- `fh`: File handle (handle).

## 17.2.136.3. INPUT PARAMETERS

- `count`: Number of elements in buffer (integer).

- `datatype`: Data type of each buffer element (handle).

## 17.2.136.4. OUTPUT PARAMETERS

- `buf`: Initial address of buffer (choice).

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

## 17.2.136.5. DESCRIPTION

MPI_File_read_ordered_begin is the beginning part of a split collective, nonblocking routine that must be called by all processes in the communicator group associated with the file handle fh. Each process may pass different argument values for the datatype and count arguments. Each process attempts to read, from the file associated with fh, a total number of count data items having datatype type into the user’s buffer buf. For each process, the location in the file at which data is read is the position at which the shared file pointer would be after all processes whose ranks within the group are less than that of this process had read their data.

## 17.2.136.6. NOTES

All the nonblocking collective routines for data access are “split” into two routines, each with `_begin` or `_end` as a suffix. These split collective routines are subject to the semantic rules described in Section 9.4.5 of the MPI-2 standard.

## 17.2.136.7. 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.