HP OpenVMS Systems Documentation

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The OpenVMS Frequently Asked Questions (FAQ)

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10.8 How do I convert between IEEE and VAX floating data?

In OpenVMS V6.1 and later, the routine CVT$CONVERT_FLOAT is documented in the LIB$ Run-Time Library Reference Manual, and can perform floating point conversions between any two of the following floating datatypes: VAX (F,D,G,H), little-endian IEEE (single, double, quad), big-endian IEEE (single, double, quad), CRAY and IBM System\370, etc.

HP Fortran (all OpenVMS platforms) has a feature which will perform automatic conversion of unformatted data during input or output. See the HP Fortran documentation for information on "non-native data in I/O" and the CONVERT= OPEN statement keyword.

There are floating-point conversion source code packages available for various platforms.

For further floating-point related information, see:

10.9 How do I get the argument count in a Fortran routine?

On VAX, many programmers would use a MACRO routine which accessed the AP register of the caller to get the address of the argument list and hence the argument count. This was not guaranteed to work on VAX, but usually did. However, it doesn't work at all on OpenVMS Alpha, as there is no AP register. On Alpha systems, you must use a language's built-in function to retrieve the argument count, if any. In Fortran this is IARGCOUNT, which is also available in DEC Fortran on OpenVMS VAX.

Note that omitting arguments to Fortran routines is non-standard and is unsupported. It will work in many cases - read the DEC Fortran release notes for additional information.

10.10 How do I get a unique system ID for licensing purposes?

Many software developers desire to use a unique hardware ID to "lock" a given copy of their product to a specific system. Most VAX and Alpha systems do not have a unique hardware-set "system ID" that can be used for this purpose. HP OpenVMS products do not use hardware IDs in the licensing methods, as many users consider a hardware-based licensing scheme to be negative attribute when considering software purchases.

HP OpenVMS uses a software-based system called the License Management Facility (LMF). This provides for software keys (Product Authorization Keys or PAKS) which support capacity and user-based license checking. HP offers an LMF PAK Generator to CSA members---see Section 2.13.

For information on licensing, please see Section 12.4.

However, if a hardware-based method is required, the most common method is based on an Ethernet adaptor hardware address. Sample source code for implementing this is available at:

For additional information on the OpenVMS Ask The Wizard (ATW) area and for a pointer to the available ATW Wizard.zip archive, please see Section 3.8.

10.11 What is an executable, shareable, system or UWSS image?

Executable code in OpenVMS typically resides in an image---an image is a file---the file extension is typically .EXE---that contains this code. Common types of images include executable images, shareable images, system images, and protected (UWSS) images.

Executable images are programs that can be directly executed. These images can grant enhanced privileges, with an INSTALL of the image with /PRIVILEGE, or can grant enhanced access with the specification of a subsystem identifier on the ACL associated with the image.

Shareable images contain code executed indirectly, these images are referenced from executable images and/or from other shareable images. These images can not grant enhanced privileges, even with the use of INSTALL with /PRIVILEGE or a subsystem identifier. These shareable images can be dynamically activated (a LINK that occurs at run-time) via the LIB$FIND_IMAGE_SYMBOL run-time library (RTL) routine. (See `protected images' for information on `privileged shareable images'.)

System images are intended to run directly on the VAX or Alpha hardware---these are normally used for the kernel code that comprises an operating system.

Protected images---also refered to as User-Written System Services (UWSS), or as privileged shareable images---are similiar in some ways to a standard shareable images, but these images include a `change mode' handler, and execute in an `inner' processor mode (privileged mode; executive or kernel), and code executing in inner modes has implicit SETPRV privilege. Must be INSTALLed with /PROTECT. Note that inner-mode code has restrictions around calling library routines, around calling various system services, and around calling code located in other protected or shareable images.

Loadable images and device drivers are images that can be used to add code into the OpenVMS kernel. Pseudo-device drivers are a particularly convenient way to add executable code, with associated driver-defined data structures, into the kernel. The pseudo-device driver includes the UCB and DDB data structures, and a calling interface with support for both privileged and unprivileged access to the driver code via sys$qio[w] calls.

A cookbook approach to creating OpenVMS shareable images is available at the URL:

For additional information on the OpenVMS Ask The Wizard (ATW) area and for a pointer to the available ATW Wizard.zip archive, please see Section 3.8.

10.12 How do I do a file copy from a program?

There are several options available for copying files from within a program. Obvious choices include using lib$spawn(), system(), sys$sndjbc() or sys$creprc() to invoke a DCL COPY command. Other common alternatives include using the callable convert routines and the BACKUP application programming interface (V7.1 and later).

10.13 What is a descriptor?

A descriptor is a data structure that describes a string or an array. Each descriptor contains information that describes the type of the data being referenced, the size of the data, and the address of the data. It also includes a description of the storage used for the data, typically static or dynamic. Descriptors are passed by reference.

The following are examples of creating and using descriptors in C, with the use of the angle brackets normally expected by the C include statements deliberately altered in deference to HTML:

    #include {descrip.h}
    #include {lib$routines.h}
    #include {stsdef.h}
    int RetStat;
    char TxtBuf[TXTSIZ]
    struct dsc$descriptor StaticDsc =
      { 0, DSC$K_DTYPE_T, DSC$K_CLASS_S, NULL };
    struct dsc$descriptor DynDsc =
      { 0, DSC$K_DTYPE_T, DSC$K_CLASS_D, NULL };
    int DynDscLen = 255;
    $DESCRIPTOR( ConstDsc, "This is a string" );

    /* finish setting up a static descriptor */
    StaticDsc.dsc$w_length      = TXTSIZ;
    StaticDsc.dsc$a_pointer     = (void *) TxtBuf;

    /* finish setting up a dynamic descriptor */
    RetStat = lib$sget1_dd( &DynDscLen, &DynDsc );
    if ( !$VMS_STATUS_SUCCESS( RetStat ) )
      return RetStat;

    /* release the dynamic storage */
    RetStat = lib$sfree1_dd( &DynDsc );
    if (!$VMS_STATUS_SUCCESS( RetStat ))
      return RetStat;

Static descriptors reference storage entirely under application program control, and the contents of the descriptor data structure can be modified as required (by the application). OpenVMS routines do not modify the contents of a static descriptor, nor do they alter the address or length values stored in the static descriptor. (The term "static" refers to the descriptor data structure, and not necessarily to the storage referenced by the descriptor.)

Dynamic descriptors reference storage under the control of the run-time library, and the contents of a dynamic descriptor data structure---once initialized---can only be modified under control of run-time library routines. The dynamic storage referenced by the dynamic descriptor is allocated and maintained by the run-time library routines. Various OpenVMS routines do alter the contents of the descriptor data structure, changing the value for the amount and the address of the storage associated with the dynamic descriptor, as required. Routines can obviously access and alter the contents of the storage referenced by the descriptor.

OpenVMS languages that include support for strings or arrays are expected to use descriptors for the particular structure. Most OpenVMS languages, such as Fortran and BASIC, use descriptors entirely transparently. Some, like DEC C, require the programmer to explicitly create and maintain the descriptor.

For further information on string descriptors, see the OpenVMS Programming Concepts manual, part of the OpenVMS documentation set.

Fortran defaults to passing integers by reference and characters by descriptor. The following sites discuss mixing Fortran and C source code in the same application:

10.14 How do I create a process under another username?

Many server processes can operate within the context of the target user using privileges, using calls such as sys$chkpro and (more commonly in this context) sys$check_access as needed to determine if access would be permitted for the specified user within the current security model.

With OpenVMS V6.2 and later, the persona system services (SYS$PERSONA_*) can be used to assume the persona of the specified user---these allow the server to operate as the specified user, in a controlled fashion. The persona services can be used as a "wrapper" around a sys$creprc process creation call, as well---this will create a seperate process entirely under the assumed persona.

Information on the persona system services is included in the OpenVMS V6.2 new features documentation, and in the OpenVMS V7.1 and later system services documentation. These system services exist and are supported in OpenVMS V6.2 and later releases.

Typical mechanisms for creating a process under another username include:

  • personna services around a sys$creprc call. See above.
  • via DECnet task-to-task, using explicit specification of username and password, or using a DECnet proxy. This creates a network-mode job under the target user. The network-mode job might do little more than a RUN/DETACH of an image passed in via task-to-task---task-to-task communications are fully available using strictly DCL-to-DCL processing, or using a compiled language and DCL, etc.)
  • SUBMIT/USER, or the username argument on the sys$sndjbc call. This creates a batch-mode job under the specified username. The batch-mode job might do little more than a RUN/DETACH of an image passed in via a parameter.
  • the UIC argument on the sys$creprc call. This mimics the UIC of the target user, and is certainly not the prefered mechanism for this task.
  • Via pseudo-terminals...

There are likely a few other mechanisms around... There are various tools available from DECUS and other sources that allow various forms of user impersonation, as well. These tools will require version-dependent kernel code and enhanced privileges for some of (or all of) their operations.

10.15 Why do lib$spawn, lib$set_symbol fail in detached processes?

The processing within run-time library (RTL) calls such as lib$attach, lib$disable_ctrl, lib$do_command, lib$enable_ctrl, lib$get_symbol, lib$run_program, lib$set_symbol, lib$set_logical, and lib$spawn, is dependent on and requires the presence of a command language interpreter (CLI), such as DCL. Without a CLI present in the current process, these calls will fail with a "NOCLI, no CLI present to perform function" error.

Detached processes typically do not have a CLI present.

In place of lib$spawn, sys$creprc can often be used. The context of the parent process (symbols and logical names) will not be propogated into the subprocess when sys$creprc is used, though when there is no CLI present in the process this (lack of) propogation is moot.

To create a detached process with a CLI, you must specify LOGINOUT as the target image as discussed elsewhere in the FAQ, or only use these calls (and any other calls requiring a CLI) from images that are running in an "interactive", "batch", or "other" mode process.

Also note that the lib$spawn and the C system call will fail in a CAPTIVE login environment. The lib$spawn call can be gotten to work in this environment with the specification of the TRUSTED flag.

10.16 Where can I obtain Bliss, and the libraries and supporting files?

The Bliss language compilers and documentation are available on the OpenVMS Freeware distributions.

Bliss language source code that contains the following statement:


or similar requires the presence of the Bliss libraries. These libraries are created on the target system using the Bliss require files, and are built using the following Bliss commands:

STARLET.L32 contains the public interfaces to OpenVMS:


LIB.L32 contains both the public and private interfaces to OpenVMS:


The equivilent files for Bliss64 are created with:


Some Bliss code may also require the OpenVMS VAX architecture flags. The following is the equivilent of the Alpha ARCH_DEFS.REQ module:

! This is the OpenVMS VAX version of ARCH_DEFS.REQ, and
! contains the architectural definitions for conditionally
! compiling OpenVMS Bliss sources for use on VAX systems.
! (If you should encounter compilation errors here, please
! seriously consider upgrading your Bliss compiler.)
MACRO VAX =                     ! = 1 if compiled BLISS/VAX
        %BLISS(BLISS32V)%;      ! = 0 if not compiled BLISS/VAX

MACRO EVAX =                    ! = 1 if compiled BLISS/E* (Obsolete, old name)
        (%BLISS(BLISS32E) OR %BLISS(BLISS64E))%; ! = 0 if compiled /VAX /Inn

MACRO ALPHA =                   ! = 1 if compiled BLISS/E* (New arch name)
        (%BLISS(BLISS32E) OR %BLISS(BLISS64E))%; ! = 0 if compiled /VAX /Inn

MACRO IA64 =                    ! = 1 if compiled BLISS/I* (New arch name)
        (%BLISS(BLISS32I) OR %BLISS(BLISS64I))%; ! = 0 if compiled /VAX or /Ann

        %BPADDR%;               ! = 32 or 64 based on compiler used

Some Bliss code may require the definition files for the OpenVMS older LIBRTL routine lib$tparse, or the newer lib$table_parse call:


10.17 How can I open a file for shared access?

When creating a file, it is often useful to allow other applications and utilities---such as TYPE---to share read access to the file. This permits you to examine the contents of a log file, for instance.

A C source example that demonstrates how to do this is available in topic (2867) in the OpenVMS Ask The Wizard area:

For additional information on the OpenVMS Ask The Wizard (ATW) area and for a pointer to the available ATW Wizard.zip archive, please see Section 3.8.

Depending on the environment, you may need to use C calls such as fsync and fflush, and---in specific cases---the setvbuf(_IONBF) call.

10.18 How can I have common sources for messages, constants?

Use the GNM tools on the OpenVMS Freeware to have common sources for MSG (message) files and SDML (Document) documentation files. Use the DOCUMENT command to convert the SDML documentation into the necessary formats (Text, Postscript, HTML, etc). Use the MESSAGE/SDL tool (latent in OpenVMS) to create an SDL file based on the messages. Then use the SDL tool (available on the OpenVMS Freeware) to convert the SDL file into language-specific definitions. (There is also a converter around to convert SDL into SDML, if you want to get pictures of the data structures for your documentation.)

10.19 How do I activate the OpenVMS Debugger from an application?

#include {lib$routines.h}
#include {ssdef.h}
#include {string.h}

    char ascic_debug_commands[128];
    char *dbgcmd = "*show calls;go;exit";

    strcpy( ascic_debug_commands, dbgcmd );
    ascic_debug_commands[0] = (char) strlen( dbgcmd ) - 1;


    return 1;

Also see Section 10.28 for another related discussion of the OpenVMS Debugger, and of a technique that uses the SS$_DEBUG signal.

10.20 Dealing with Endian-ness?

OpenVMS VAX, OpenVMS Alpha and OpenVMS I64 (as well as all Microsoft Windows implementations) all support and all use the little-endian byte ordering. Certain Alpha microprocessors and certain Intel Itanium processors can be configured to operate in big-endian and potentially in bi-endian mode. HP-UX typically operates big-endian.

With little-endian byte order, the least significant byte is always the first byte; the byte at the lowest address. With big-endian byte ordering, the byte storage order in memory is dependent on the size of the data (byte, word, longword) that is being referenced.

Endian-ness is a problem has been solved many times before. Some of the typical solutions include htonl/htons and ntohl/ntohs in the standard C library and the TCP/IP Services XDR (eXternal Data Representation) libraries. One of the more recently introduced network formats, and one that is seeing extensive press and marketing coverage, is XML.

10.21 How to resolve LINK-I-DATMISCH errors?

The message LINK-I-DATMISCH is informational, and indicates that the version of the specified shareable image found in the system shareable image directory does not match the version of the shareable image that was originally loaded into IMAGELIB.OLB, one of the OpenVMS libraries typically searched by the LINKER.

From a privileged username, you can usually completely repair this via the following DCL command:


This command assumes that the shareable image that was found in the SYS$SHARE: area is valid and upward-compatiable, and that the image has simply replaced an older version without also updating IMAGELIB.

10.22 HP C and other OpenVMS C Programming Considerations?

VAX C V3.2 was released for OpenVMS VAX systems in 1991. DEC C V4.0 replaced VAX C V3.2 in 1993 as the HP C compiler for OpenVMS VAX systems. HP C is the ANSI C compiler for OpenVMS Alpha systems. VAX C predates the ANSI C standards, and has various areas that are not compliant with ANSI C requirements. HP C is an ANSI C compiler, and can also compile most VAX C code when /STANDARD=VAXC is specified. Versions of this compiler between V3.2 and V6.5 (exclusive) were known as DEC C, DIGITAL C, and Compaq C.

Both compilers can be installed at the same time on the same OpenVMS VAX system, allowing a migration from VAX C to DEC C, and allowing the same DEC C code to be used on OpenVMS VAX and OpenVMS Alpha.

The system manager can choose the system default C compiler when HP C is installed on a system with VAX C, and a C programmer can explicitly select the required compiler for a any particular compilation.

A current "C" license PAK allows access to both VAX C and HP C on the same OpenVMS VAX system.

Various HP C versions can be installed on OpenVMS VAX V5.5-2 and later. OpenVMS VAX releases such as V5.5-2 and V6.0 will require the installation of a HP C RTL kit, a kit that is included with the HP C compiler. OpenVMS VAX versions V6.1 and later do not require a seperate RTL kit, but HP C RTL ECO kits are available to resolve problems found with the C RTL on various OpenVMS releases.

With HP C, for automatic resolution of the standard C library routines by the LINKER utility, use the /PREFIX qualifier, such as /PREFIX=ALL_ENTRIES. If a particular application program replaces an existing C library routine, use /PREFIX=(ALL_ENTRIES,EXCEPT=(...)). (VAX C required explicit specification of an RTL shareable image or C object library during the link.)

When the /PREFIX is requested, the compiler generates a "decc$" prefix on the specified symbols. This prefix allows the LINKER to resolve the external symbols against the symbols present in the DECC$SHR library. The DECC$SHR library is included in the IMAGELIB.OLB shareable image library, and IMAGELIB is searched by default when any program (written in any language) is LINKed. Because the standard C library routine names are very likely to match application routines written in other languages, a prefix "decc$" is added to the C symbol names to assure their uniqueness; to prevent symbol naming conflicts. C programs, however, can sometimes have private libraries for various purposes, and the external routines share the same names as the library routines. (This is not recommended, but there are applications around that use this technique.) Thus the need to explicity specify whether or not the "decc$" prefix should be prepended to the external symbol names by the compiler.

The qualifiers, and most (all?) with associated pragmas, that may be of interest when migrating VAX C code to HP C include:

  • Failure to specify the prefixing qualifier (on certain and usually older versions of C) can cause the compiler to not add the prefixes for the names of the C library routines into the references placed in the object module, which can in turn cause problems resolving the external symbols in the library when the object code is linked:

  • Some VAX C programs erroneously write to the string literals. By default, HP C does not allow the constants to change.

  • Enables sharing ("shr") of globals and of extern variables. HP C sets externs as non-shareable ("noshr"), VAX C as "shr".

  • VAX C assumes common block model for external linkages.

  • Refers to the padding placed between member elements within a struct. Disabling member alignment packs the data more tightly into memory, but this packaging has performance implications, both on OpenVMS VAX and particularly on OpenVMS Alpha systems.

  • Enable all manner of useful compiler diagnostics:


    You can disable extraneous diagnostics with the following:

    #ifdef __DECC
    #pragma message save
    #pragma message disable /* insert message tag here */

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