## 9.2.1. Opening a File

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MPI_FILE_OPEN(comm, filename, amode, info, fh)
IN comm | communicator (handle)
IN filename | name of file to open (string)
IN amode | file access mode (integer)
IN info | info object (handle)
OUT fh | new file handle (handle)

`int MPI_File_open(MPI_Comm comm, char *filename, int amode, MPI_Info info, MPI_File *fh)
` `MPI_FILE_OPEN(COMM, FILENAME, AMODE, INFO, FH, IERROR)
CHARACTER*(*) FILENAME
INTEGER COMM, AMODE, INFO, FH, IERROR
` `static MPI::File MPI::File::Open(const MPI::Intracomm& comm, const char* filename, int amode, const MPI::Info& info)
`

MPI_FILE_OPEN opens the file identified by the file name 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. (Values for info may vary.) comm must be an intracommunicator; it is erroneous to pass an intercommunicator to MPI_FILE_OPEN. Errors in MPI_FILE_OPEN are raised using the default file error handler (see Section I/O Error Handling ). 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 (e.g., MPI_SEND). Furthermore, the use of comm will not interfere with I/O behavior.

The format for specifying the file name in the filename argument is implementation dependent and must be documented by the implementation.

Advice to implementors.

An implementation may require that filename include a string or strings specifying additional information about the file. Examples include the type of filesystem (e.g., a prefix of ufs:), a remote hostname (e.g., a prefix of machine.univ.edu:), or a file password (e.g., a suffix of /PASSWORD=SECRET). ( End of advice to implementors.)
Advice to users.

On some implementations of MPI, the file namespace may not be identical from all processes of all applications. For example, ``/tmp/foo'' may denote different files on different processes, or a single file may have many names, dependent on process location. The user is responsible for ensuring that a single file is referenced by the filename argument, as it may be impossible for an implementation to detect this type of namespace error. ( End of advice to users.)
Initially, all processes view the file as a linear byte stream, and each process views data in its own native representation (no data representation conversion is performed). (POSIX files are linear byte streams in the native representation.) The file view can be changed via the MPI_FILE_SET_VIEW routine.

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

- MPI_MODE_RDONLY --- read only,
- MPI_MODE_RDWR --- reading and writing,
- MPI_MODE_WRONLY --- write only,
- MPI_MODE_CREATE --- create the file if it does not exist,
- MPI_MODE_EXCL --- error if creating file that already exists,
- MPI_MODE_DELETE_ON_CLOSE --- delete file on close,
- MPI_MODE_UNIQUE_OPEN --- file will not be concurrently opened elsewhere,
- MPI_MODE_SEQUENTIAL --- file will only be accessed sequentially,
- MPI_MODE_APPEND --- set initial position of all file pointers to end of file.

Advice to users.

C/C++ users can use bit vector OR ( ) to combine these constants; Fortran 90 users can use the bit vector `IOR` intrinsic. Fortran 77 users can use (nonportably) bit vector `IOR` on systems that support it. Alternatively, Fortran users can portably use integer addition to OR the constants (each constant should appear at most once in the addition.). ( End of advice to users.)
Advice to implementors.

The values of these constants must be defined such that the bitwise OR and the sum of any distinct set of these constants is equivalent. ( End of advice to implementors.)
The modes MPI_MODE_RDONLY, MPI_MODE_RDWR, MPI_MODE_WRONLY, MPI_MODE_CREATE, and MPI_MODE_EXCL have identical semantics to their POSIX counterparts [11]. Exactly one of MPI_MODE_RDONLY, MPI_MODE_RDWR, or MPI_MODE_WRONLY, must be specified. It is erroneous to specify MPI_MODE_CREATE or MPI_MODE_EXCL in conjunction with MPI_MODE_RDONLY; it is erroneous to specify MPI_MODE_SEQUENTIAL together with MPI_MODE_RDWR.

The MPI_MODE_DELETE_ON_CLOSE mode causes the file to be deleted (equivalent to performing an MPI_FILE_DELETE) when the file is closed.

The MPI_MODE_UNIQUE_OPEN mode allows an implementation to optimize access by eliminating the overhead of file locking. It is erroneous to open a file in this mode unless the file will not be concurrently opened elsewhere.

Advice to users.

For MPI_MODE_UNIQUE_OPEN, not opened elsewhere includes both inside and outside the MPI environment. In particular, one needs to be aware of potential external events which may open files (e.g., automated backup facilities). When MPI_MODE_UNIQUE_OPEN is specified, the user is responsible for ensuring that no such external events take place. ( End of advice to users.)
The MPI_MODE_SEQUENTIAL mode allows an implementation to optimize access to some sequential devices (tapes and network streams). It is erroneous to attempt nonsequential access to a file that has been opened in this mode.

Specifying MPI_MODE_APPEND only guarantees that all shared and individual file pointers are positioned at the initial end of file when MPI_FILE_OPEN returns. Subsequent positioning of file pointers is application dependent. In particular, the implementation does not ensure that all writes are appended.

Errors related to the access mode are raised in the class MPI_ERR_AMODE.

The info argument is used to provide information regarding file access patterns and file system specifics (see Section File Info ). The constant MPI_INFO_NULL can be used when no info needs to be specified.
Advice to users.

Some file attributes are inherently implementation dependent (e.g., file permissions). These attributes must be set using either the info argument or facilities outside the scope of MPI. ( End of advice to users.)
Files are opened by default using nonatomic mode file consistency semantics (see Section File Consistency ). The more stringent atomic mode consistency semantics, required for atomicity of conflicting accesses, can be set using MPI_FILE_SET_ATOMICITY.

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MPI-2.0 of July 18, 1997
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