Technical data

Using Files and Directories
10.7 Accessing Tape Files
Table 10–5 (Cont.) Comparison of OpenVMS Extended Names and Standard Names
Characteristic OpenVMS Extended Names Standard Names
Format filename.type;version filename.;version (Version is optional.)
Length 39.39; 17.;
Valid
Characters
A through Z; 0 through 9; ampersand
(&), hyphen (-), underscore(_),and
dollar sign ($); wildcard characters
asterisk (*) and percent sign (%)
ASCII ‘‘a’’
1
characters enclosed in quotation
marks (" "). Note that within a file name,
DCL interprets a double set of quotation
marks ("") as a single set ("). If a name has
fewer than 17 characters, the system pads
the name on the right with spaces to arrive
at the 17-character maximum length.
Examples OPENVMS_FILENAME.DAT;23 "GENLABEL#123";2
1
The ASCII ‘‘a’’ character set is defined in Clause 7.4.1 of the ISO 9660 Standard.
10.7.2 Locating Standard-Labeled Tape Files
Before accessing a particular file for a read or write operation, you might
want to search the magnetic tape volume for that file. Use the DCL command
DIRECTORY to locate a file or group of files on a tape volume.
When you specify a file name for a file residing on tape, the tape file system
performs the following tasks:
1. Compares the file name with the file header labels of each file until it finds a
match in the file identifier field of the file header labels.
• If you supply a version number in the file name, the magnetic tape
file system compares it with the generation number and generation
version-number fields in the first file header label.
• If you do not specify a version number, the system neither defaults
a version number nor checks the generation number and generation
version-number fields.
2. The system selects the first file on the magnetic tape whose file name in the
file identifier field matches the specified file name.
The operating system supports neither the directory nor the latest version
number concept for magnetic tape volumes. The system does not search for or
list the latest version of a specified file. The magnetic tape file system cannot
increment version numbers of files written to tape; therefore, two or more
files in the same volume set can have the same file name and version number.
Because the tape file system selects the first matching file name and version
number (if specified), the position of the magnetic tape within the volume set
determines which file is returned on a search operation. A search operation
begins at the current position, so you might want to rewind the volume set
before accessing a file.
3. The search for a matching file and version number (if specified) continues at
the beginning of the header-label set of the next file. The search ends when
the magnetic tape is positioned at the file where the search began.
If the system does not find the requested file on the current volume, it
searches the remaining volumes in the volume set sequentially and then
searches from the beginning of the first volume of the volume set. If the
system still does not find the file name, it reports an error.
10–24 Using Files and Directories