As with standard Unix, a file must be opened before reading or writing. However, we distinguish file handles for both cases. If the system provides a parallel file system, it is possible for several processors to read and write a single file. We call this mode ``parallel''. Otherwise the file is read by a single processor and the data delivered according to the distributed memory layout. The reverse occurs upon writing. We call this mode ``serial''. To allow user choice where the architecture permits, we provide for requesting either mode. However, the request may be overridden if the system permits only one mode. Upon writing, we allow appending to an existing file.