NAME

       MPI_File_open - Opens a file (collective).

SYNTAX

C Syntax

       #include  <mpi.h>  int  MPI_File_open(MPI_Comm comm, const char *filename,      int amode,
       MPI_Info info,      MPI_File *fh)

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

C++ Syntax

       #include <mpi.h>
       static MPI::File MPI::File::Open(const MPI::Intracomm& comm,
            const char* filename, int amode, const MPI::Info& info)

INPUT PARAMETERS

       comm      Communicator (handle).

       filename  Name of file to open (string).

       amode     File access mode (integer).

       info      Info object (handle).

OUTPUT PARAMETERS

       fh        New file handle (handle).

       IERROR    Fortran only: Error status (integer).

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):

         o  MPI_MODE_APPEND

         o  MPI_MODE_CREATE -- Create the file if it does not exist.

         o  MPI_MODE_DELETE_ON_CLOSE

         o  MPI_MODE_EXCL -- Error creating a file that already exists.

         o  MPI_MODE_RDONLY -- Read only.

         o  MPI_MODE_RDWR -- Reading and writing.

         o  MPI_MODE_SEQUENTIAL

         o  MPI_MODE_WRONLY -- Write only.

         o  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.

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. (See NOTE, below.)

       - 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 NOTE, below.)

       -  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 NOTE, below.)

       - 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 NOTE, below.)

       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.

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

       -  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.

ERRORS

       Almost all MPI routines return an error value; C routines as the value of the function and
       Fortran  routines in the last argument. C++ functions do not return errors. If the default
       error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then  on  error  the  C++  exception
       mechanism will be used to throw an MPI::Exception object.

       Before  the  error value is returned, the current MPI error handler is called. For MPI I/O
       function errors, the default error handler is set to MPI_ERRORS_RETURN. The error  handler
       may    be   changed   with   MPI_File_set_errhandler;   the   predefined   error   handler
       MPI_ERRORS_ARE_FATAL may be used to  make  I/O  errors  fatal.  Note  that  MPI  does  not
       guarantee that an MPI program can continue past an error.