# 18.2.125. MPI_File_open

MPI_File_open — Opens a file (collective).

## 18.2.125.1. SYNTAX

### 18.2.125.1.1. C Syntax

    #include <mpi.h>

    int MPI_File_open(MPI_Comm comm, const char *filename,
         int amode, MPI_Info info,
         MPI_File *fh)

### 18.2.125.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_FILE_OPEN(COMM, FILENAME, AMODE, INFO, FH, IERROR)
         CHARACTER*(*)   FILENAME
         INTEGER COMM, AMODE, INFO, FH, IERROR

### 18.2.125.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_File_open(comm, filename, amode, info, fh, ierror)
         TYPE(MPI_Comm), INTENT(IN) :: comm
         CHARACTER(LEN=*), INTENT(IN) :: filename
         INTEGER, INTENT(IN) :: amode
         TYPE(MPI_Info), INTENT(IN) :: info
         TYPE(MPI_File), INTENT(OUT) :: fh
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.125.2. INPUT PARAMETERS

- `comm`: Communicator (handle).

- `filename`: Name of file to open (string).

- `amode`: File access mode (integer).

- `info`: Info object (handle).

## 18.2.125.3. OUTPUT PARAMETERS

- `fh`: New file handle (handle).

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

## 18.2.125.4. DESCRIPTION

MPI_File_open opens the file identified by the filename filename on all processes in the comm communicator group. MPI_File_open is a collective routine; all processes must provide the same value for amode, and all processes must provide filenames that reference the same file which are textually identical (note: Open MPI I/O plugins may have restrictions on characters that can be used in filenames. For example, the ROMIO plugin may disallow the colon (“:”) character from appearing in a filename). A process can open a file independently of other processes by using the MPI_COMM_SELF communicator. The file handle returned, fh, can be subsequently used to access the file until the file is closed using MPI_File_close. Before calling MPI_Finalize, the user is required to close (via MPI_File_close) all files that were opened with MPI_File_open. Note that the communicator comm is unaffected by MPI_File_open and continues to be usable in all MPI routines. Furthermore, use of comm will not interfere with I/O behavior.

Initially, all processes view the file as a linear byte stream; that is, the etype and filetype are both MPI_BYTE. The file view can be changed via the MPI_File_set_view routine.

The following access modes are supported (specified in amode, in a bit-vector OR in one of the following integer constants):

- `MPI_MODE_APPEND`

- `MPI_MODE_CREATE`: Create the file if it does not exist.

- `MPI_MODE_DELETE_ON_CLOSE`

- `MPI_MODE_EXCL`: Error creating a file that already exists.

- `MPI_MODE_RDONLY`: Read only.

- `MPI_MODE_RDWR`: Reading and writing.

- `MPI_MODE_SEQUENTIAL`

- `MPI__MODE_WRONLY`: Write only.

- `MPI_MODE_UNIQUE_OPEN`

The modes `MPI_MODE_RDONLY`, `MPI_MODE_RDWR`, `MPI_MODE_WRONLY`, and `MPI_MODE_CREATE` have identical semantics to their POSIX counterparts. It is erroneous to specify `MPI_MODE_CREATE` in conjunction with `MPI_MODE_RDONLY`. Errors related to the access mode are raised in the class `MPI_ERR_AMODE`.

On single-node clusters, files are opened by default using nonatomic mode file consistency semantics. The more stringent atomic-mode consistency semantics, required for atomicity of overlapping accesses, are the default when processors in a communicator group reside on more than one node. This setting can be changed using MPI_File_set_atomicity.

The MPI_File_open interface allows the user to pass information via the info argument. It can be set to `MPI_INFO_NULL`. See the HINTS section for a list of hints that can be set.

## 18.2.125.5. HINTS

The following hints can be used as values for the info argument.

SETTABLE HINTS

- `MPI_INFO_NULL`

- `shared_file_timeout`: Amount of time (in seconds) to wait for access to the shared file pointer before exiting with `MPI_ERR_TIMEDOUT`.

- `rwlock_timeout`: Amount of time (in seconds) to wait for obtaining a read or write lock on a contiguous chunk of a UNIX file before exiting with `MPI_ERR_TIMEDOUT`.

- `noncoll_read_bufsize`: Maximum size of the buffer used by MPI I/O to satisfy multiple noncontiguous read requests in the noncollective data-access routines.

  Note

  A buffer size smaller than the distance (in bytes) in a UNIX file between the first byte and the last byte of the access request causes MPI I/O to iterate and perform multiple UNIX read() or write() calls. If the request includes multiple noncontiguous chunks of data, and the buffer size is greater than the size of those chunks, then the UNIX read() or write() (made at the MPI I/O level) will access data not requested by this process in order to reduce the total number of write() calls made. If this is not desirable behavior, you should reduce this buffer size to equal the size of the contiguous chunks within the aggregate request.

- `noncoll_write_bufsize`: Maximum size of the buffer used by MPI I/O to satisfy multiple noncontiguous write requests in the noncollective data-access routines.

  See the above note in `noncoll_read_bufsize`.

- `coll_read_bufsize`: Maximum size of the buffer used by MPI I/O to satisfy multiple noncontiguous read requests in the collective data-access routines.

  See the above note in `noncoll_read_bufsize`.

- `coll_write_bufsize`: Maximum size of the buffer used by MPI I/O to satisfy multiple noncontiguous write requests in the collective data-access routines.

  See the above note in `noncoll_read_bufsize`.

- `mpiio_concurrency`: (boolean) controls whether nonblocking I/O routines can bind an extra thread to an LWP. .sp

- `mpiio_coll_contiguous`: (boolean) controls whether subsequent collective data accesses will request collectively contiguous regions of the file.

NON-SETTABLE HINTS

- `filename`: Access this hint to get the name of the file.

## 18.2.125.6. 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.