252. File Views

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MPI_FILE_SET_VIEW(fh, disp, etype, filetype, datarep, info)
INOUT fhfile handle (handle)
IN dispdisplacement (integer)
IN etypeelementary datatype (handle)
IN filetypefiletype (handle)
IN datarepdata representation (string)
IN infoinfo object (handle)

int MPI_File_set_view(MPI_File fh, MPI_Offset disp, MPI_Datatype etype, MPI_Datatype filetype, char *datarep, MPI_Info info)
void MPI::File::Set_view(MPI::Offset disp, const MPI::Datatype& etype, const MPI::Datatype& filetype, const char* datarep, const MPI::Info& info)

The MPI_FILE_SET_VIEW routine changes the process's view of the data in the file. The start of the view is set to disp; the type of data is set to etype; the distribution of data to processes is set to filetype; and the representation of data in the file is set to datarep. In addition, MPI_FILE_SET_VIEW resets the individual file pointers and the shared file pointer to zero. MPI_FILE_SET_VIEW is collective; the values for datarep and the extents of etype in the file data representation must be identical on all processes in the group; values for disp, filetype, and info may vary. The datatypes passed in etype and filetype must be committed.

The etype always specifies the data layout in the file. If etype is a portable datatype (see Section Semantic Terms , page Semantic Terms ), the extent of etype is computed by scaling any displacements in the datatype to match the file data representation. If etype is not a portable datatype, no scaling is done when computing the extent of etype. The user must be careful when using nonportable etypes in heterogeneous environments; see Section Datatypes for File Interoperability , page Datatypes for File Interoperability for further details.

If MPI_MODE_SEQUENTIAL mode was specified when the file was opened, the special displacement MPI_DISPLACEMENT_CURRENT must be passed in disp. This sets the displacement to the current position of the shared file pointer. MPI_DISPLACEMENT_CURRENT is invalid unless the amode for the file has MPI_MODE_SEQUENTIAL set.

For some sequential files, such as those corresponding to magnetic tapes or streaming network connections, the displacement may not be meaningful. MPI_DISPLACEMENT_CURRENT allows the view to be changed for these types of files. ( End of rationale.)

Advice to implementors.

It is expected that a call to MPI_FILE_SET_VIEW will immediately follow MPI_FILE_OPEN in numerous instances. A high-quality implementation will ensure that this behavior is efficient. ( End of advice to implementors.)
The disp displacement argument specifies the position (absolute offset in bytes from the beginning of the file) where the view begins.

Advice to users.

disp can be used to skip headers or when the file includes a sequence of data segments that are to be accessed in different patterns (see Figure 27 ). Separate views, each using a different displacement and filetype, can be used to access each segment.

Figure 27: Displacements

( End of advice to users.)
An etype ( elementary datatype) is the unit of data access and positioning. It can be any MPI predefined or derived datatype. Derived etypes can be constructed by using any of the MPI datatype constructor routines, provided all resulting typemap displacements are nonnegative and monotonically nondecreasing. Data access is performed in etype units, reading or writing whole data items of type etype. Offsets are expressed as a count of etypes; file pointers point to the beginning of etypes.

Advice to users.

In order to ensure interoperability in a heterogeneous environment, additional restrictions must be observed when constructing the etype (see Section File Interoperability , page File Interoperability ). ( End of advice to users.)
A filetype is either a single etype or a derived MPI datatype constructed from multiple instances of the same etype. In addition, the extent of any hole in the filetype must be a multiple of the etype's extent. These displacements are not required to be distinct, but they cannot be negative, and they must be monotonically nondecreasing.

If the file is opened for writing, neither the etype nor the filetype is permitted to contain overlapping regions. This restriction is equivalent to the ``datatype used in a receive cannot specify overlapping regions'' restriction for communication. Note that filetypes from different processes may still overlap each other.

If filetype has holes in it, then the data in the holes is inaccessible to the calling process. However, the disp, etype and filetype arguments can be changed via future calls to MPI_FILE_SET_VIEW to access a different part of the file.

It is erroneous to use absolute addresses in the construction of the etype and filetype.

The info argument is used to provide information regarding file access patterns and file system specifics to direct optimization (see Section File Info , page File Info ). The constant MPI_INFO_NULL refers to the null info and can be used when no info needs to be specified.

The datarep argument is a string that specifies the representation of data in the file. See the file interoperability section (Section File Interoperability , page File Interoperability ) for details and a discussion of valid values.

The user is responsible for ensuring that all nonblocking requests and split collective operations on fh have been completed before calling MPI_FILE_SET_VIEW---otherwise, the call to MPI_FILE_SET_VIEW is erroneous.

MPI_FILE_GET_VIEW(fh, disp, etype, filetype, datarep)
IN fhfile handle (handle)
OUT dispdisplacement (integer)
OUT etypeelementary datatype (handle)
OUT filetypefiletype (handle)
OUT datarepdata representation (string)

int MPI_File_get_view(MPI_File fh, MPI_Offset *disp, MPI_Datatype *etype, MPI_Datatype *filetype, char *datarep)
void MPI::File::Get_view(MPI::Offset& disp, MPI::Datatype& etype, MPI::Datatype& filetype, char* datarep) const

MPI_FILE_GET_VIEW returns the process's view of the data in the file. The current value of the displacement is returned in disp. The etype and filetype are new datatypes with typemaps equal to the typemaps of the current etype and filetype, respectively.

The data representation is returned in datarep. The user is responsible for ensuring that datarep is large enough to hold the returned data representation string. The length of a data representation string is limited to the value of MPI_MAX_DATAREP_STRING.

In addition, if a portable datatype was used to set the current view, then the corresponding datatype returned by MPI_FILE_GET_VIEW is also a portable datatype. If etype or filetype are derived datatypes, the user is responsible for freeing them. The etype and filetype returned are both in a committed state.

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