HP OpenVMS Systems Documentation
Guide to OpenVMS File Applications
5.2.7 The File Name, Type, and Version Components
On Alpha systems, RMS identifies the file name, file type, and file version components from the portion of a file specification that follows any node, device, root and directory components as follows:
The right-most semicolon or period followed by legal version characters begins the version component. A semicolon (unescaped) followed by characters that are not legal for a version component is illegal. The right-most period to the left of the version (if any) begins the type component. The characters to the left of the type are the file name.
Legal forms of a version component are:
On ODS-2 and ODS-5 disks, the numerals in a version component, interpreted as a decimal number, may not exceed 32767.
Note that, although RMS accepts a period as a version delimiter, in output specifications, RMS always uses the semicolon as the delimiter.
The following are some examples of name, type, and version:
Note that, although RMS on Alpha systems allows periods in a file name, files with such names can be created only on ODS-5 disks.
On VAX systems and on Alpha systems with versions previous to OpenVMS V7.2, RMS identifies the file name, file type, and file version components from the portion of a file specification that follows any node, device, root, and directory components as follows:
5.2.8 Leading Hyphens in File and Subdirectory Names (Alpha Only)
On Alpha systems, OpenVMS Version 7.2 supports the use of file names and subdirectory names that begin with hyphens. This is supported on ODS-5 and ODS-2 disks.
No special action is required for specifying a name that begins with a hyphen, with the following exception. A reference to a subdirectory whose name consists only of hyphens must include at least one escaped hyphen (so that RMS can distinguish the reference from a relative directory specification). On output, RMS always displays such a subdirectory name with the first hyphen escaped.
5.2.9 Restrictions and Anomalies
The following sections describe a restriction when using extended file
names and a DCL parsing anomaly.
Using the tilde (~) as the first character in a file name in the Compaq C Run-Time Library (CRTL) allows a programmer to specify both UNIX style and OpenVMS style file specifications to routines such as creat() and fopen().
In UNIX file specifications, a tilde (~) in the first character of a pathname represents the user's home directory. However, in an OpenVMS extended file name, a tilde is legal anywhere in a file name or directory name.
To preserve backward compatibility, the CRTL will continue to interpret a leading tilde (~) to mean the user's home directory. To pass an OpenVMS file name that begins with a tilde (~) to a CRTL routine that accepts UNIX style file specifications, specify the tilde preceded by the escape character (^). For example, ^~.
The following Compaq CRTL functions accept OpenVMS extended file names and require this syntax for a leading tilde (~) in the file specification:
188.8.131.52 DCL Parsing Anomaly
Prior to OpenVMS Version 7.2, DCL parses some file specifications incompletely, and does not detect all illegal ODS-2 file specifications. Specifically, if a file specification has three comoponents separated by two delimiters, DCL does not verify that the second delimiter is a semicolon and the third component is a version number. However, RMS verifies the file specification completely and detects errors that DCL may miss. For example, on an ODS-2 volume:
For compatibility, this behavior has been preserved. However, this can produce unexpected results when accessing an ODS-5 volume under the following conditions:
Under these conditions, the command CREATE A.B.C succeeds on an ODS-5 volume. However, you cannot access the file with a DCL command that requires a version number (such as the DELETE command). For example:
The only way to delete this file when the parsing style is TRADITIONAL is to use wildcards, and for safety, you should use the /CONFIRM qualifier with the DELETE command. However, it is safer to change the parsing style to EXTENDED. For example:
5.3 Logical Names and Parsing
RMS translates a logical name present in a file specification at run time. The use of logical names can be desirable for several reasons, including program simplification, device independence, file independence, and ease of use.
You can specify the file specification at compile (or assembly) time, or the program can prompt for it at run time. By specifying a logical name when you compile a program, you eliminate having to program a terminal input request, and you preserve the flexibility of being able to specify the input file at run time.
Device independence is more readily attainable if a logical name is used for the device name component. By using a logical name rather than explicitly specifying a physical device, an alternate device (usually containing a recent backup copy of the device) can be substituted by changing the definition of the logical name. Typically, device independence can reduce or eliminate the downtime caused by media failure or scheduled preventive maintenance.
Similarly, when you use a logical name, and the current copy of a file is not available, an alternate file can be used. To locate several files in a defined search order, you can use a search list, which is a form of logical name. Alternatively, you can use wildcard characters to locate several files using one file specification; however, wildcard characters do not allow you to specify a search order.
Using a logical name to represent a complex file specification or a file specification component reduces keystrokes to save time and reduces the chance of error. For example, you could define a logical node name that translates to an actual node name and access control string for use when locating remote files. To keep the password a secret when you use this technique, the logical name should be defined interactively rather than in a command procedure.
RMS attempts to treat the file name component as a logical name if the
file name is the only component in the file specification. Refer to the
OpenVMS User's Manual for additional information on defining logical names. No
logical name translation is done on any logical name that contains a
compound character (containing the RMS escape character: "^").
The following section describes a file specification on ODS-2 and ODS-5
The maximum length of a file specification string is 255 characters, including all separator characters. The following table lists the length limits for each of the component parts of a file specification. Note that although the collective limit exceeds 255 characters, the overriding limitation is on the length of the file specification.
5.4.2 ODS-5 on Alpha Systems
The maximum length of a file specification string is 4095 characters, including all delimiters. The following table lists the length limits for each of the component parts of a file specification.
RMS supports file names for which the sum of the lengths of the file name, the file type, and the file version does not exceed 255 (simple) characters. ODS-5 disks support file names for which the sum of the lengths of the file name and the file type does not exceed 236 bytes.
Be careful when naming files that will be copied or accessed by remote
systems. File name restrictions are generally determined by the file
naming capabilities of the networked systems that require access to
them. These restrictions must be considered part of the overall
application design when network access is required.
RMS supports creating and accessing up to 255 subdirectory levels (in
any combination in the root and directory components) on ODS-2 and
ODS-5 volumes accessed from Alpha systems. For ODS-2 and ODS-5 volumes
accessed from VAX systems, RMS supports up to 8 subdirectory levels
each for the root and directory components.
On VAX systems, RMS allows access to ODS-5 volumes. But, file specifications are allowed on those volumes that RMS on VAX does not support. Some of the limitations are described here.
When a search is done of a directory that contains files whose names are legal for ODS-5, but not for ODS-2, one of two pseudonames, "\PISO_LATIN\.???" or "\PUNICODE\.???" will be displayed. Such files can be accessed via RMS only from an Alpha system.
It is possible to create directory trees on both ODS-5 and ODS-2
volumes that are deeper than RMS can display. In those cases, RMS will
not display any subdirectories that lie beyond its limits.
From DCL, you can determine a disk device's structure level (for example, ODS-2 or ODS-5) via a SHOW DEVICE/FULL command.
A DCL command procedure can determine the structure level with code such as the following:
The structure level can be determined in a program (executable image) via a call to the SYS$GETDVI system service using an item list with the DVI$_ACPTYPE request code. The following example, in the C programming language, displays a disk device's structure level of determining ODS-2 against ODS-5.
5.4.6 Using File Specification Defaults
When you omit file specification components (except for the node name and root name), RMS supplies default values for the missing components. The file specification to which defaults are applied is called the primary file specification. Your program can supply default values for all file specification components using either the default file specification or the related file specification. In addition, the process executing the program supplies specific default values for device and directory components, via the process default specification.
Where applicable, RMS substitutes the translated logical name to the primary file specification before it applies default values. After translating the primary file specification, RMS applies the defaults from the default file specification, then it applies the defaults from the related file specification, if relevant. RMS then applies the process default values, where applicable, for the device and directory to obtain the full file specification it uses to locate the file.
For more information about the application of defaults, refer to Section 6.1.