Data Access With Explicit Offsets

To access data at an explicit offset, specify the position in the file where the next data access for each process should begin. For each call to a data-access routine, a process attempts to access a specified number of file types of a specified data type (starting at the specified offset) into a specified user buffer.

The offset is measured in elementary data type units relative to the current view; moreover, "holes" are not counted when locating an offset. The data is read from (in the case of a read) or written into (in the case of a write) those parts of the file specified by the current view. These routines store the number of buffer elements of a particular data type actually read (or written) in the status object, and all the other fields associated with the status object are undefined. The number of elements that are read or written can be accessed using MPI_Get_count.

MPI_File_read_at attempts to read from the file via the associated file handle returned from a successful MPI_File_open. Similarly, MPI_File_write_at attempts to write data from a user buffer to a file. MPI_File_iread_at and MPI_File_iwrite_at are the nonblocking versions of MPI_File_read_at and MPI_File_write_at, respectively.

MPI_File_read_at_all and MPI_File_write_at_all are collective versions of MPI_File_read_at and MPI_File_write_at, in which each process provides an explicit offset. The split collective versions of these nonblocking routines are listed in the table at the beginning of this section.