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The following example and explanation shows how to use the HP C RTL time functions to print the current time:
#include <stdio.h>
#include <time.h>
main ()
{
time_t t;
t = time((time_t)0);
printf ("The current time is: %s\n",asctime (localtime (&t)));
}
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This example:
OpenVMS Version 8.3 and higher provides Open Group compliant symbolic link support and POSIX pathname processing support. This support will help partners and customers who port UNIX and LINUX applications to OpenVMS or who use a UNIX style development environment to reduce the application development costs and complexity previously associated with such porting efforts.
The following OpenVMS features are provided to support symbolic links and POSIX pathname processing:
A POSIX pathname is a non-empty character string consisting of 0 or more filenames, separated by a / (the path delimiter). The path delimiter can be the first or last character of a pathname and is also the separator between filenames. Multiple consecutive delimiters are valid and are equivalent to a single delimiter.
POSIX filenames can consist of all ASCII characters except
/
(the path delimiter) and NUL, which is used as a string terminator in
many APIs. (On OpenVMS systems, POSIX filenames are also currently
restricted to not contain the
?
or
*
character.) A filename is sometimes referred to as a pathname
component.
12.1.1 POSIX Pathname Interpretation
If the pathname begins with a / , it is considered to be an absolute pathname, and pathname processing begins at the device and directory specified by the POSIX root (see Section 12.5 for the method to define the root).
If the pathname does not begin with a / , it is considered to be a relative pathname, and pathname processing begins with the current working directory.
See Section 12.6 for how OpenVMS interprets the current working
directory.
12.1.1.1 The POSIX Root Directory
The POSIX root directory is a directory that is used in pathname resolution for absolute pathnames.
The root directory, is the top-most node of the hierarchy, and has itself as its parent directory. The pathname of the root directory is / , and the parent directory of the root directory is / . Note that only files and directories actually entered in the root directory are accessible with a POSIX pathname. For additional details, see the note in Section 12.3.1.
See Section 12.5 for defining the POSIX root on OpenVMS.
12.1.1.2 Symbolic Links
A symbolic link is a special kind of file that points to another file. It is a directory entry that associates a filename with a text string that is interpreted as a POSIX pathname when accessed by certain services. Most operations that access a symbolic link are rerouted to the object to which the text contents refer; if that object does not exist, the operation fails. A symbolic link is implemented on OpenVMS as a file of organization SPECIAL and type SYMBOLIC_LINK.
A symbolic link can specify an absolute pathname or a relative pathname. The relative path in a symbolic link is relative to the location of the link. For example, a symbolic link whose content begins with the path " .. " refers to its parent directory. If the symbolic link is moved to another directory, it refers to its new parent directory. Note that the path specified by a symbolic link is a POSIX pathname and is subject to the restrictions described in Section 12.3.1.
See Section 12.2 for symbolic link usage on OpenVMS.
12.1.1.3 Mount Points
A mount point is a location (in fact, a directory) where a file system is attached. The UNIX term file system is essentially equivalent to the OpenVMS concept of a disk volume. After a mount point is established, any contents of the original directory are inaccessible.
Mount points do not persist across reboots.
See Section 12.7 for establishing mount points on OpenVMS.
12.1.1.4 Reserved Filenames . and ..
There are two reserved POSIX filenames:
. (the current directory)
.. (the parent directory)
On UNIX systems, a special file is associated with a particular hardware device or other resource of the computer system. The operating system uses character special files, sometimes called device files, to provide I/O access to specific character devices.
Special files, at first glance, appear to be just like ordinary files, in that they:
However, there is an important difference between the two: an ordinary file is a logical grouping of data recorded on disk, whereas a special file corresponds to a device entity. Examples are:
Only the null special file (
/dev/null
) is supported. This file is created during the GNV configuration
procedure. Data written on the null special file is discarded. Reads
from a null special file always return 0 bytes.
12.1.2 Using POSIX Pathnames with OpenVMS Interfaces
POSIX compliant pathnames can be passed to most OpenVMS applications, utilities, and APIs.
The existing DCL convention of doubling any quote character that is in a quoted string has been adopted.
Also, in order to avoid constraints on future use of quoted strings for pathnames (for example, to support another syntactic variation, such as Windows-compatible names), the leading tag ^UP^ is required on the quoted pathname to identify it as a POSIX pathname to DCL, RMS, and OpenVMS utilities. It is not required for the C RTL or GNV.
So, for example, /a/b/c becomes "^UP^/a/b/c" and /a/b"/c becomes "^UP^/a/b""/c" .
Throughout the remainder of this chapter, the term POSIX
pathname refers to the unmodified pathname (for example,
/a/b/c
) and quoted pathname refers to our quoted, tagged pathname
(for example,
"^UP^/a/b/c"
).
12.1.2.1 Special Considerations with POSIX Filenames
When a user creates a file on OpenVMS using a POSIX filename, it would be ideal if the filename viewed as an OpenVMS filename were the same. For example, when a user asks to create a file with POSIX filename a.b , OpenVMS could create a file with the name a.b; (where ; represents the file version).
But because one of the components of an OpenVMS filename is the file type (consisting of at least a period), POSIX filenames without a period cannot be mapped in as direct a fashion (for example, POSIX filename a cannot be mapped to OpenVMS filename a; ). The reasonable solution is for OpenVMS to append a period (representing a null file-type field) so that POSIX filename a becomes OpenVMS filename a.;
Note that OpenVMS appends a period (or null type) even to a POSIX filename that itself ends in a period. This is necessary to distinguish POSIX filenames a. and a from each other.
Furthermore, OpenVMS directories have a type and version of .DIR;1 . When a directory of POSIX filename a is created on OpenVMS, it is created with an OpenVMS filename of a.DIR;1 . And because a.DIR is a valid POSIX filename, it needs to be distinguished from a directory with POSIX filename a residing in the same directory. So OpenVMS adds a period (or null type) to POSIX filenames ending in .DIR (so a file with POSIX filename a.DIR becomes a.DIR.; ).
Keeping the above rules in mind, the following are POSIX filenames that are unmodified on OpenVMS, except for adding a version:
POSIX filename OpenVMS name Type Version Displayed by DIR as
-------------- ------------ ---- ------- -------------------
a.b a .b ; a.b;
a.b; a .b; ; a.b;;
a.b;2 a .b;2 ; a.b;2;
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The following are examples of POSIX filenames that end with a period (.) or .DIR and, therefore, have a period appended when creating the OpenVMS filename:
POSIX filename OpenVMS name Type Version Displayed by DIR as
-------------- ------------ ---- ------- -------------------
a a . ; a.;
a. a. . ; a^..;
a.. a.. . ; a^.^..;
a.b. a.b. . ; a^.b^..;
a.DIR a.DIR . ; a^.DIR.;
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Finally, OpenVMS will add .DIR;1 to the end of any POSIX directory:
POSIX filename OpenVMS name Type Version Displayed by DIR as
-------------- ------------ ---- ------- -------------------
a a .DIR ;1 a.DIR;1
a.dir a.dir .DIR ;1 a^.dir.DIR;1
a. a. .DIR ;1 a^..DIR;1
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Note that both the OpenVMS filenames a.DIR;1 and a.; map to the POSIX filename a . Specifying the POSIX filename a will find either a.DIR;1 or a.; . If both are present, it finds a.; . The ambiguous mapping applies whether a.DIR;1 is a directory or an ordinary file.
All POSIX filenames that contain the
*
or
?
characters are unmappable. This is an open issue to be addressed in a
future release of OpenVMS.
12.1.2.2 Special Considerations with OpenVMS Filenames
When mapping OpenVMS filenames to POSIX filenames (a feature provided by C RTL function readdir ), the following rules apply:
If the OpenVMS filename ends in .DIR;1 and the type of a file is a directory, then .DIR;1 is removed. For all other files, the ending semi-colon and version number are removed from the OpenVMS filename.
If the OpenVMS filename ends in a period, the period is removed.
All OpenVMS filenames that contain the / or NUL characters are unmappable and, therefore, not returned to the caller.
OpenVMS directories ..DIR;1 and ...DIR;1 are unmappable because their corresponding POSIX filenames would be the special filenames . and .. .
OpenVMS file
.
is unmappable because the resulting POSIX filename would be a NULL
string.
12.2 Using Symbolic Links
There are several ways to create a symbolic link:
The following examples show how to create and access symbolic links using DCL commands:
| #1 |
|---|
$ create/symlink="a/b.txt" link_to_b.txt
$
$ dir/date link_to_b.txt
Directory DKB0:[TEST]
link_to_b.txt -> a/b.txt
27-MAY-2005 09:45:15.20
|
This example creates the symbolic link file link_to_b.txt with contents equal to the specified text string representing the pathname to which the symbolic link points, a/b.txt . Note that a/b.txt may or may not exist. Also note that the symlink file must be reachable from the POSIX root.
| #2 |
|---|
$ type link_to_b.txt
%TYPE-W-OPENIN, error opening DKB0:[TEST]LINK_TO_B.TXT; as input
-RMS-E-FNF, file not found
|
To access the file referenced in a symbolic link, specify the symbolic link name on the command. The TYPE command error message in this example indicates that the file a/b.txt , referenced through the symbolic link link_to_b.txt created in the first example, does not exist.
| #3 |
|---|
$ create [.a]b.txt
This is a text file.
Exit
$
$ type link_to_b.txt
This is a text file
$
|
This example creates the missing file b.txt . Once the referenced file exists, the TYPE command through the symbolic link is successful.
Alternatively, you can create the file through the symbolic link:
$ create link_to_b.txt This is a text file. Exit $ dir [.a] Directory DKB0:[TEST.A] b.txt;1 $ $ type link_to_b.txt This is a text file. $
You can create a symbolic link within the GNV bash shell with the ln utility:
bash$ ln -s filename slinkname |
Most utilities and commands that reference slinkname are redirected so that they act on filename. Some commands, such as ls , are symbolic link-aware, and show information about the link itself (such as its contents).
A link can be removed with the rm command (which cannot be made to follow the link and remove what it points to).
The examples that follow show how to manage symbolic links through GNV POSIX commands as compared with DCL commands.
| #1 |
|---|
From GNV:
bash$ cd /symlink_example
bash$ echo This is a test. > text
bash$ cat text
This is a test.
bash$ ln -s text LINKTOTEXT
bash$
From DCL:
$ set def DISK$XALR:[PSX$ROOT.symlink_example]
$ create text.
This is a test.
Exit
$ type text.
This is a test.
$ create/symlink="text" LINKTOTEXT
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This example creates a symbolic link called LINKTOTEXT that points to a text file called text .
| #2 |
|---|
From GNV:
bash$ cat LINKTOTEXT
This is a test.
bash$
From DCL:
$ type LINKTOTEXT.
This is a test.
$
|
This example displays the contents of the text file using the symbolic link.
| #3 |
|---|
From GNV:
bash$ ls
LINKTOTEXT text
bash$ ls -l
total 1
lrwxr-x--- 1 SYSTEM 1 4 May 12 08:41 LINKTOTEXT -> text
-rw-r----- 1 SYSTEM 1 16 May 12 08:40 text
bash$
From DCL:
$ DIR
Directory DISK$XALR:[PSX$ROOT.symlink_example]
LINKTOTEXT.;1 text.;1
Total of 2 files.
$ DIR/DATE
Directory DISK$XALR:[PSX$ROOT.symlink_example]
LINKTOTEXT.;1 -> /symlink_example/text
12-MAY-2005 08:46:31.40
text.;1 12-MAY-2005 08:43:47.53
Total of 2 files.
$
|
This example displays the content of a symbolic link. Notice that when you do an ls or a DIR using switches that show any file attribute, then the content of the symbolic link is also displayed.
| #4 |
|---|
From GNV:
bash$ rm text
bash$ cat LINKTOTEXT
cat: linktotext: i/o error
bash$ rm LINKTOTEXT
From DCL:
$ DEL text.;1
$ TYPE LINKTOTEXT.
%TYPE-W-OPENIN, error opening
DISK$XALR:[PSX$ROOT.symlink_example]LINKTOTEXT.;1 as input
-RMS-E-ACC, ACP file access failed
-SYSTEM-F-FILNOTACC, file not accessed on channel
$ DEL LINKTOTEXT.;1
$
|
This example deletes files.
The following sections describe POSIX-compliant pathname and symbolic link support in the C RTL.
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