During OPEN processing, and depending on the label structure of the file, the labels are checked in the sequence:
- Volume header labels
- File header labels
If the result of a check is negative, DMS issues a message which informs the operator or user about what is wrong and gives him/her a chance to react to the error. Possible reactions:
repeat the label check
branch to an exit routine in the program, or
abort the program.
Whether or not error messages are issued depends on the TAPE-ACCESS flag in the user catalog and on the SECLEV operand in the FILE/FCB macro or the PROTECTION-LEVEL operand in the ADD-FILE-LINK command.
For both input and output files, the sequence of label checking varies slightly for the various values in the LABEL and LABEL-TYPE operands of the macro and command, respectively. With LABEL=STD or LABEL=(STD,n) in the macro, or LABEL-TYPE=*STD or LABEL-TYPE=*STD(DIN-REVISION-NUMBER=n) in the command, the user specifies that a file with standard labels is to be processed. The specifications LABEL=NSTD and LABEL-TYPE=*NON-STD indicate that the file to be processed will have nonstandard labels, and label checking by DMS is executed accordingly. The same applies to the specifications LABEL=NO and LABEL-TYPE=*NO: DMS then expects a file with no labels.
In the following sections, label handling during OPEN processing is described in the following order: label handling for input files is described first, followed by label handling for output files, in each case in the sequence: files with standard labels, files with nonstandard labels, and files without labels.
The structure of the routines in which user labels or nonstandard labels are processed is described in section "EXLST exits for processing labels for tape files".