Data Access With Shared File Pointers

Sun MPI I/O maintains one shared file pointer per collective MPI_File_open (shared among processes in the communicator group that opened the file). As with the routines for data access with individual file pointers, you can also use the current value of the shared file pointer to specify the offset of data accesses implicitly. These routines use and update only the shared file pointer; the individual file pointers are neither used nor updated by any of these routines.

These routines have similar semantics to the explicit-offset data-access routines, except:

• The offset is defined here to be the current value of the shared file pointer.

• Multiple calls (one for each process in the communicator group) affect the shared file pointer routines as if the calls were serialized.

• All processes must use the same file view.

After a shared file pointer operation is initiated, it is updated, relative to the current view of the file, to point to the elementary data item immediately following the last one requested, regardless of the number of items actually accessed.

MPI_File_read_shared and MPI_File_write_shared are blocking routines that use the shared file pointer to read and write files, respectively. The order of serialization is not deterministic for these noncollective routines, so you need to use other methods of synchronization if you wish to impose a particular order.

MPI_File_iread_shared and MPI_File_iwrite_shared are the nonblocking versions of MPI_File_read_shared and MPI_File_write_shared, respectively.

MPI_File_read_ordered and MPI_File_write_ordered are the collective versions of MPI_File_read_shared and MPI_File_write_shared. They must be called by all processes in the communicator group associated with the file handle, and the accesses to the file occur in the order determined by the ranks of the processes within the group. After all the processes in the group have issued their respective calls, for each process in the group, these routines determine the position where the shared file pointer would be after all processes with ranks lower than this process's rank had accessed their data. Then data is accessed (read or written) at that position. The shared file pointer is then updated by the amount of data requested by all processes of the group.

The split collective versions of MPI_File_read_ordered and MPI_File_write_ordered are listed in the table at the beginning of this section.

MPI_File_seek_shared is a collective routine, and all processes in the communicator group associated with the particular file handler must call MPI_File_seek_shared with the same file offset and the same update mode. All the processes are synchronized with a barrier before the shared file pointer is updated.

The offset can be negative for backwards seeking, but you cannot seek to a negative position in the file. The current position is defined as the elementary data item immediately following the last-accessed data item, even if that location is a hole.

MPI_File_get_position_shared returns the current position of the shared file pointer relative to the current displacement and file type.