However, for diagnostic purposes, the openx subroutine is required. open and close Subroutines Care should be taken when closing a file after writing. If the application reverses over the data just written, no file marks will be written. For multitape jobs, the special file must be opened and closed for each tape. The user is not allowed to continue if the special file is opened and the tape has been changed. The openx subroutine is intended primarily for use by the diagnostic commands and utilities.
Appropriate authority is required for execution.
By specifically choosing the rmt file, it is possible to create multiple file tapes. Although tapes are accessed through character interface special files, the number of bytes required by either a read or write operation must be a multiple of the block size defined for the magnetic tape device. When the tape drive is in variable block mode, read requests for less than the tape's block size return the number of bytes requested and set the errno global variable to a value of 0. During a read, the record size is returned as the number of bytes read, up to the buffer size specified. When a read is attempted after the device has reached the EOM, a zero-length read is returned.