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Please first review the OpenVMS Programming Concepts Manual and the Guide to Modular Programming manuals. Both are available within the OpenVMS documentation set, and provide details of the expected norms for OpenVMS programs.
OpenVMS provides a registry for facility prefixes and for MESSAGE message compiler codes. To request a prefix and a message facility code for a product you distributinng to other customer sites, send your request in a mail message addressed to product[-at-sign-]hylndr.sqp.zko.dec.com, requesting the submission form and details of the registration process.
Please do not request facility prefixes for products that local to your business, your site, or your system. Facility prefixes and message codes and the facility registration process are intended solely for HP products and Partner Products (and yes, even OpenVMS Freeware packages) that will be distributed across multiple OpenVMS customer sites. |
For a list of common coding bugs, please see the remainder of this section of the FAQ and specifically Section 10.22, please also see the Ask The Wizard topic (1661), and for information on debugging an OpenVMS application, please see topic (7552).
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. ATW has been superceded (for new questions) by the ITRC
discussion forums; the area remains available for reference.
10.2 Can I have a source code example of calling...?
Please use the available SEARCH command on OpenVMS, and please remember to search the available resources, including the support databases and the newsgroup archives. Please also realize that most OpenVMS system services use similar calling sequences, meaning that an example of calling sys$getjpi can be used as an example for sys$getsyi and sys$getdvi. Students: please do not expect folks to write your homework for you. As for search resources:
OpenVMS programming documentation, including the numerous example programs found in recent versions of the OpenVMS Programming Concepts manual, is available:
As for details of argument passing, most OpenVMS system services and RTL routines pass string arguments by descriptor. Languages which support native string data types create descriptors automatically; those which do not (eg., C) require that you set them up explicitly. For further details on using descriptors and particularly for using descriptors from C, please see Section 10.13.
There is extensive information available on how to call OpenVMS system services and OpenVMS Run-Time Library routines, including examples in numerous languages. Among the best available references are:
In addition, you can also locate answers, source code examples and related discussions in the James Search Assistant (formerly NLSA (Natural Language Search Assistant), itself formerly known as Ask Compaq (AskQ)) database:
In this area, you will find the source code of programming examples for calls to many of the OpenVMS system services (and from various programming languages), including calls to core services sys$getjpi[w], sys$getsyi[w] and sys$qio[w], as well as source code examples for calls to many other system services and run-time library routines, and examples of one of the more difficult calling interfaces found on OpenVMS systems, that of the smg$create_menu routine.
Arne Vajhøj has put together a collection of OpenVMS example programs. It can be found at:
Additional information and examples for OpenVMS are available via:
For additional information on the OpenVMS Ask The Wizard (ATW) area and for a pointer to the available ATW Wizard.zip archive, please see Section 3.8. ATW has been superceded (for new questions) by the ITRC discussion forums; the area remains available for reference. and via:
If you're writing a program and want to accept arguments from a foreign command, you can use LIB$GET_FOREIGN to get the command line and parse it yourself, or if you're programming in C, use the normal argc/argv method.
To write an application which uses the normal DCL verb/qualifier/parameter syntax for invocation, see the description of the CLI$ routines in the OpenVMS Callable Utility Routines Reference Manual.
It is possible to write an application which can be used both ways; if a DCL verb isn't used to invoke the image, the application parses the command line itself. One way to do this is to call CLI$GET_VALUE for a required parameter. If it is not present (or you get an error), call LIB$GET_FOREIGN to get the command line and do the manual parse.
See also Section 8.2.
10.4 How do I get a formatted error message in a variable?
Use the SYS$PUTMSG system service with an action routine that stores
the message line(s) in the variable of your choice. Be sure the action
routine returns a "false" (low bit clear) function value so that
SYS$PUTMSG
doesn't then try to display the message (unless you want it to.) See
the description of $PUTMSG in the System Services Reference Manual for
an example of using an action routine.
10.5 How do I link against SYS$SYSTEM:SYS.STB on an Alpha system?
LINK/SYSEXE is the OpenVMS Alpha equivalent of linking against SYS.STB. This links against the base image: SYS$BASE_IMAGE.EXE
Also see Section 10.11, particularly for pointers to the details on
shareable images and shareable image creation, and see Section 10.22
for details of inner-mode floating point requirements, of data
alignment, requirements for use of /NOSYSLIB, and other related
inner-mode programming details, and see Section 10.11 for image-related
information.
10.6 How do I do a SET DEFAULT from inside a program?
The problem is that SYS$SETDDIR only changes the default directory -
NOT the default disk. The default disk is determined by the logical
SYS$DISK. If you want to change the default disk within a program, then
call LIB$SET_LOGICAL to change the logical SYS$DISK. You will need to
call both LIB$SET_LOGICAL and SYS$SETDDIR to change both default disk
and the default directory!
10.7 How do I turn my Fortran COMMON into a shareable image on Alpha?
You need to add SYMBOL_VECTOR=(<common-name>=PSECT) to your options file. On OpenVMS VAX all OVR/REL/GBL psects were automatically exported into the shareable image's Global Symbol Table. On OpenVMS Alpha you have to tell the linker that you want this done by means of the PSECT keyword in the SYMBOL_VECTOR options file statement.
This has several advantages over OpenVMS VAX. First, you don't have to worry about the address of the psect when you try to create a new, upwardly compatible version of the shareable image. Second, you can control which psects, if any, are made visible outside the shareable image.
By default, COMMON PSECTs in HP Fortran for OpenVMS Alpha (as well as most other OpenVMS Alpha compilers) are NOSHR. On VAX, the default was SHR which required you to change the attribute to NOSHR if you wanted your COMMON to be in a shareable image but not write-shared by all processes on the system. If you do want write-sharing, use:
CDEC$ PSECT common-name=SHR |
in the Fortran source code (the CDEC$ must be begin in column 1) or a linker options file PSECT_ATTR statement to set the COMMON PSECT attribute to SHR.
For further information, see the Linker manual.
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:
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 DSPP (formerly CSA) members---see Section 2.8.3 for general details on the DSPP offerings, and on the DSPP-provided Freeware PAKGEN PAK that is available for those that are interested in details. (Sorry, no; you can't use the Freeware PAKGEN PAK to generate any PAK you might want. Folks at HP also thought of that one, too.)
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. ATW has been superceded (for new questions) by the ITRC
discussion forums; the area remains available for reference.
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. ATW has been superceded (for new questions) by the ITRC
discussion forums; the area remains available for reference.
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 (!$VMS_STATUS_SUCCESS( RetStat ))
return RetStat;
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Static descriptors reference storage entirely under application program control, and the contents of the descriptor data structure can be modified as required (by the application). OpenVMS routines do not modify the contents of a static descriptor, nor do they alter the address or length values stored in the static descriptor. (The term "static" refers to the descriptor data structure, and not necessarily to the storage referenced by the descriptor.)
Dynamic descriptors reference storage under the control of the run-time library, and the contents of a dynamic descriptor data structure---once initialized---can only be modified under control of run-time library routines. The dynamic storage referenced by the dynamic descriptor is allocated and maintained by the run-time library routines. Various OpenVMS routines do alter the contents of the descriptor data structure, changing the value for the amount and the address of the storage associated with the dynamic descriptor, as required. Routines can obviously access and alter the contents of the storage referenced by the descriptor.
OpenVMS languages that include support for strings or arrays are expected to use descriptors for the particular structure. Most OpenVMS languages, such as Fortran and BASIC, use descriptors entirely transparently. Some, like DEC C, require the programmer to explicitly create and maintain the descriptor.
For further information on string descriptors, see the OpenVMS Programming Concepts manual, part of the OpenVMS documentation set.
Fortran defaults to passing integers by reference and characters by descriptor. The following sites discuss mixing Fortran and C source code in the same application:
Many server processes can operate within the context of the target user using privileges, using calls such as sys$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:
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.
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