OpenVMS Programming Concepts Manual
8.8.3 Resource Waits and Page Faults
When a process is executing an image, the system can place the process
in a wait state until a required resource becomes available, or until a
page in its virtual address space is paged into memory. These waits,
which are generally transparent to the process, can also be interrupted
for the delivery of an AST.
8.9 Examples of Using AST Services
The following is an example of a Compaq Fortran program that finds the
process identification (PID) number of any user working on a particular
disk and delivers an AST to a local routine that notifies the user that
the disk is coming down:
! Implicit none
! Status variable
RECORD /ITMLST/ DVILIST(2),
! Information for GETDVI call
! Information for GETJPI call
! AST routine and flag
PARAMETER (AST_FLAG = 2)
! Set up for SYS$GETDVI
DVILIST(1).BUFLEN = 4
DVILIST(1).CODE = %LOC(DVI$_PID)
DVILIST(1).BUFADR = %LOC(PID_BUF)
DVILIST(1).RETLENADR = %LOC(PID_LEN)
DVILIST(2).END_LIST = 0
! Find PID number of process using SYS$DRIVE0
STATUS = SYS$GETDVIW (,
2 '_MTA0:', ! device
2 DVILIST, ! item list
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS))
! Get terminal name and fire AST
JPILIST(1).CODE = %LOC(JPI$_TERMINAL)
JPILIST(1).BUFLEN = 7
JPILIST(1).BUFADR = %LOC(TERM_NAME)
JPILIST(1).RETLENADR = %LOC(TERM_LEN)
JPILIST(2).END_LIST = 0
STATUS = SYS$GETJPI (,
2 PID_BUF, !process id
2 JPILIST, !itemlist
2 NOTIFY_USER, !AST
2 TERM_NAME) !AST arg
IF (.NOT. STATUS) CALL LIB$SIGNAL(%VAL(STATUS))
! Ensure that AST was executed
STATUS = SYS$WAITFR(%VAL(AST_FLAG))
IF (.NOT. STATUS) CALL LIB$SIGNAL(%VAL(STATUS))
SUBROUTINE NOTIFY_USER (TERM_STR)
! AST routine that broadcasts a message to TERMINAL
! Dummy argument
! Status variable
PARAMETER (MESSAGE =
2 'SYS$TAPE going down in 10 minutes')
! Flag to indicate AST executed
! Declare system routines
! Add underscore to device name
LENGTH = LEN (TERM_STR)
TERMINAL(2:LENGTH+1) = TERM_STR
TERMINAL(1:1) = '_'
! Send message
STATUS = SYS$BRDCST(MESSAGE,
IF (.NOT. STATUS) CALL LIB$SIGNAL(%VAL(STATUS))
! Set event flag
STATUS = SYS$SETEF (%VAL(AST_FLAG))
IF (.NOT. STATUS) CALL LIB$SIGNAL(%VAL(STATUS))
The following is an example of a C program setting up an AST:
#module SETAST "SRH X1.0-000"
** COPYRIGHT (c) 1992 BY
** COMPAQ COMPUTER CORPORATION, MAYNARD, MASSACHUSETTS.
** ALL RIGHTS RESERVED.
** THIS SOFTWARE IS FURNISHED UNDER A LICENSE AND MAY BE USED AND COPIED
** ONLY IN ACCORDANCE WITH THE TERMS OF SUCH LICENSE AND WITH THE
** INCLUSION OF THE ABOVE COPYRIGHT NOTICE. THIS SOFTWARE OR ANY OTHER
** COPIES THEREOF MAY NOT BE PROVIDED OR OTHERWISE MADE AVAILABLE TO ANY
** OTHER PERSON. NO TITLE TO AND OWNERSHIP OF THE SOFTWARE IS HEREBY
** THE INFORMATION IN THIS SOFTWARE IS SUBJECT TO CHANGE WITHOUT NOTICE
** AND SHOULD NOT BE CONSTRUED AS A COMMITMENT BY COMPAQ COMPUTER
** COMPAQ ASSUMES NO RESPONSIBILITY FOR THE USE OR RELIABILITY OF ITS
** SOFTWARE ON EQUIPMENT THAT IS NOT SUPPLIED BY COMPAQ.
** Version: V1.0
** Example of working with the $SETAST call and ASTs.
** Steve Hoffman
** Creation Date: 1-Jan-1990
** Modification History:
* AST and $SETAST demo
* raise the AST shields
* request an AST, parameter is 1
* request an AST, parameter is 2
* lower the shields
int retstat = 0;
* $SETAST() returns SS$_WASSET and SS$_WASCLR depending
* on the previous setting of the AST shield. Watch out,
* SS$_WASSET looks like a SUCCESSFUL SS$_ACCVIO. (ie:
* a debug EXAMINE/COND shows SS$_WASSET as the error
* %SYSTEM-S-ACCVIO. *Real* ACCVIO's never have the "-S-"
retstat = SYS$SETAST( 0 );
printf("\n disable/ was: %d\n", retstat );
retstat = SYS$DCLAST( bogus, 1, 0 );
retstat = SYS$DCLAST( bogus, 2, 0 );
printf("\ndclast %x\n", retstat );
retstat = SYS$SETAST( 1 );
* and, since we just lowered the shields, the ASTs should hit
* in here somewhere....
printf("\n enable/ was: %d\n", retstat );
return( 1 );
* and, here's the entire, sophisticated, twisted AST code...
bogus( astprm )
printf("\nAST tripped. ast parameter was 0x%x\n\n", astprm);
return( 1 );
Condition-Handling Routines and Services
This chapter describes the OpenVMS Condition Handling facility and
contains the following sections:
Section 9.1 gives an overview of run-time errors.
Section 9.2 gives an overview of the OpenVMS Condition Handling
facility, presenting condition-handling terminology and functionality.
Section 9.3 describes VAX system and Alpha system exceptions,
arithmetic exceptions, and unaligned access traps on Alpha systems.
Section 9.4 describes how run-time library routines handle exceptions.
Section 9.5 describes the condition value field and the testing and
modifying of values.
Section 9.6 describes the exception dispatcher.
Section 9.7 describes the argument list that is passed to a condition
Section 9.8 describes signaling.
Section 9.9 describes types of condition handlers.
Section 9.10 describes types of actions performed by condition
Section 9.11 describes messages and how to use them.
Section 9.12 describes how to write a condition handler.
Section 9.13 describes how to debug a condition handler.
Section 9.14 describes several run-time library routines that can be
established as condition handlers.
Section 9.15 describes how to establish, write, and debug an exit
9.1 Overview of Run-Time Errors
Run-time errors are hardware- or software-detected events, usually
errors, that alter normal program execution. Examples of run-time
errors are as follows:
- System errors---for example, specifying an invalid argument to a
- Language-specific errors---for example, in Fortran, a data type
conversion error during an I/O operation
- Application-specific errors---for example, attempting to use
When an error occurs, the operating system either returns a condition
code or value identifying the error to your program or signals the
condition code. If the operating system signals the condition code, an
error message is displayed and program execution continues or
terminates, depending on the severity of the error. See Section 9.5
for details about condition values.
When unexpected errors occur, your program should display a message
identifying the error and then either continue or stop, depending on
the severity of the error. If you know that certain run-time errors
might occur, you should provide special actions in your program to
handle those errors.
Both an error message and its associated condition code identify an
error by the name of the facility that generated it and an abbreviation
of the message text. Therefore, if your program displays an error
message, you can identify the condition code that was signaled. For
example, if your program displays the following error message, you know
that the condition code SS$_NOPRIV was signaled:
%SYSTEM-F-NOPRIV, no privilege for attempted operation
9.2 Overview of the OpenVMS Condition Handling Facility
The operating system provides a set of signaling and condition-handling
routines and related system services to handle exception conditions.
This set of services is called the OpenVMS Condition Handling facility
(CHF). The OpenVMS Condition Handling Facility is a part of the common
run-time environment of OpenVMS, which includes run-time library (RTL)
routines and other components of the operating system.
The OpenVMS Condition Handling facility provides a single, unified
method to enable condition handlers, signal conditions, print error
messages, change the error behavior from the system default, and enable
or disable detection of certain hardware errors. The RTL and all
layered products of the operating system use the CHF for condition
See the OpenVMS Calling Standard for a detailed description of OpenVMS condition
9.2.1 Condition-Handling Terminology
This section defines the terms used to describe condition handling.
An event detected by the hardware or software that changes the normal
flow of instruction execution. An exception is a synchronous event
caused by the execution of an instruction and often means something
generated by hardware. When an exception occurs, the processor
transfers control by forcing a change in the flow of control from that
explicitly indicated in the currently executing process.
Some exceptions are relevant primarily to the current process and
normally invoke software in the context of the current process. An
integer overflow exception detected by the hardware is an example of an
event that is reported to the process. Other exceptions, such as page
faults, are handled by the operating system and are transparent to the
An exception may also be signaled by a routine (software signaling) by
calling the RTL routines LIB$SIGNAL or LIB$STOP.
An informational state that exists when an exception occurs.
Condition is a more general term than exception; a
condition implies either a hardware exception or a software-raised
condition. Often, the term condition is preferred because the term
exception implies an error. Section 9.3.1 further defines the
differences between exceptions and conditions.
When a condition is detected during the execution of a routine, a
signal can be raised by the routine. The routine is then permitted to
respond to the condition. The routine's response is called
handling the condition.
On VAX systems, an address of 0 in the first longword of a procedure
call frame or in an exception vector indicates that a condition handler
does not exist for that call frame or vector.
On Alpha systems, the handler valid flag bit in the procedure
descriptor is cleared to indicate that a condition handler does not
The condition handlers are themselves routines; they have their own
call frames. Because they are routines, condition handlers can have
condition handlers of their own. This allows condition handlers to
field exceptions that might occur within themselves in a modular
On VAX systems, a routine can enable a condition handler by placing the
address of the condition handler in the first longword of its stack
On Alpha systems, the association of a handler with a procedure is
static and must be specified at the time a procedure is compiled (or
assembled). Some languages that lack their own exception-handling
syntax, however, may support emulation of dynamic specified handlers by
means of built-in routines.
If you determine that a program needs to be informed of particular
exceptions so it can take corrective action, you can write and specify
a condition handler. This condition handler, which receives control
when any exception occurs, can test for specific exceptions.
If an exception occurs and you have not specified a condition handler,
the default condition handler established by the operating system is
given control. If the exception is a fatal error, the default condition
handler issues a descriptive message and causes the image that incurred
the exception to exit.
To declare or enable a condition handler, use the following system
- Set Exception Vector (SYS$SETEXV)
- Unwind from Condition Handler Frame (SYS$UNWIND)
- Declare Change Mode or Compatibility Mode Handler (SYS$DCLCMH)
Parallel mechanisms exist for uniform dispatching of hardware and
software exception conditions. Exceptions that are detected and
signaled by hardware transfer control to an exception service routine
in the executive. Software-detected exception conditions are generated
by calling the run-time library routines LIB$SIGNAL or LIB$STOP.
Hardware- and software-detected exceptions eventually execute the same
exception dispatching code. Therefore, a condition handler may handle
an exception condition generated by hardware or by software identically.
The Set Exception Vector (SYS$SETEXV) system service allows you to
specify addresses for a primary exception handler, a secondary
exception handler, and a last-chance exception handler. You can specify
handlers for each access mode. The primary exception vector is reserved
for the debugger. In general, you should avoid using these vectored
handlers unless absolutely necessary. If you use a vectored handler, it
must be prepared for all exceptions occurring in that access mode.
9.2.2 Functions of the Condition Handling Facility
The OpenVMS Condition Handling facility and the related run-time
library routines and system services perform the following functions:
- Establish and call condition-handler routines
You can establish condition handlers to receive control in the
event of an exception in one of the following ways:
- On VAX systems, by specifying the address of a condition handler in
the first longword of a procedure call frame.
On Alpha systems, the
method for establishing a dynamic (that is, nonvectored) condition
handler is specified by the language.
- By establishing exception handlers with the Set Exception Vector
(SYS$SETEXV) system service.
The first of these methods is the preferred way to specify a
condition handler for a particular image. The use of dynamic handlers
is also the most efficient way in terms of declaration. You should use
vectored handlers for special purposes, such as writing debuggers.
The VAX MACRO programmer can use the following single-move address
instruction to place the address of the condition handler in the
longword pointed to by the current frame pointer (FP):
You can associate a condition handler for the currently executing
routine by specifying an address pointing to the handler, either in the
routine's stack frame on VAX systems or in one of the exception
vectors. (The MACRO-32 compiler for OpenVMS Alpha systems generates the
appropriate Alpha code from this VAX instruction to establish a dynamic
On VAX systems, the high-level language programmer can call the
common run-time library routine LIB$ESTABLISH (see the OpenVMS RTL Library (LIB$) Manual),
using the name of the handler as an argument. LIB$ESTABLISH returns as
a function value either the address of the former handler established
for the routine or 0 if no handler existed.
On VAX systems, the new condition handler remains in effect for your
routine until you call LIB$REVERT or control returns to the caller of
the caller of LIB$ESTABLISH. Once this happens, you must call
LIB$ESTABLISH again if the same (or a new) condition handler is to be
associated with the caller of LIB$ESTABLISH.
On VAX systems, some
languages provide access to condition handling as part of the language.
You can use the ON ERROR GOTO statement in BASIC and the ON statement
in PL/I to define condition handlers. If you are using a language that
does provide access to condition handling, use its language mechanism
rather than LIB$ESTABLISH. Each procedure can declare a condition
When the routine signals an exception, the OpenVMS
Condition Handling facility calls the condition handler associated with
the routine. See Section 9.8 for more information about exception
vectors. Figure 9-5 shows a sample stack scan for a condition
The following Compaq Fortran program segment establishes
the condition handler ERRLOG. Because the condition handler is used as
an actual argument, it must be declared in an EXTERNAL statement.
OLD_HANDLER = LIB$ESTABLISH (ERRLOG)
LIB$ESTABLISH returns the address of the previous handler as its
function value. If only part of a program unit requires a special
condition handler, you can reestablish the original handler by invoking
LIB$ESTABLISH and specifying the saved handler address as follows:
CALL LIB$ESTABLISH (OLD_HANDLER)
The run-time library provides several condition handlers and
routines that a condition handler can call. These routines take care of
several common exception conditions. Section 9.14 describes these
On Alpha systems, LIB$ESTABLISH and LIB$REVERT are not
supported, though a high-level language may support them for
compatibility. (Table 9-4 lists other run-time library routines
supported and not supported on Alpha systems.)
- On VAX systems, remove an established condition-handler routine
On VAX systems using LIB$REVERT, you can remove a condition handler
from a routine's stack frame by setting the frame's handler address to
0. If your high-level language provides condition-handling statements,
you should use them rather than LIB$REVERT.
- On VAX systems, enable or disable the detection of arithmetic
On VAX systems, using run-time library
routines, you can enable or disable the signaling of floating point
underflow, integer overflow, and decimal overflow, which are detected
by the VAX hardware.
- Signal a condition
When the hardware detects an exception, such as an integer
overflow, a signal is raised at that instruction. A routine may also
raise a signal by calling LIB$SIGNAL or LIB$STOP. Signals raised by
LIB$SIGNAL allow the condition handler either to terminate or to resume
the normal flow of the routine. Signals raised by LIB$STOP require
termination of the operation that raises the condition. The condition
handler will not be allowed to continue from the point of call to
- Display an informational message
The system establishes default condition handlers before it calls
the main program. Because these default condition handlers provide
access to the system's standard error messages, the standard method for
displaying a message is by signaling the severity of the condition:
informational, warning, or error. See Section 9.5 for the definition
of the severity field of a condition vector. The system default
condition handlers resume execution of the instruction after displaying
the messages associated with the signal. If the condition value
indicates a severe condition, then the image exits after the message is
- Display a stack traceback on errors
The default operations of the LINK and RUN commands provide a
system-supplied handler (the traceback handler) to print a symbolic
stack traceback. The traceback shows the state of the routine stack at
the point where the condition occurred. The traceback information is
displayed along with the messages associated with the signaled
- Compile customer-defined messages
The Message utility allows you to define your own exception
conditions and the associated messages. Message source files contain
the condition values and their associated messages. See Section 9.11.3
for a complete description of how to define your own messages.
- Unwind the stack
A condition handler can cause a signal to be dismissed and the
stack to be unwound to the establisher or caller of the establisher of
the condition handler when it returns control to the OpenVMS Condition
Handling facility (CHF). During the unwinding operation, the CHF scans
the stack. If a condition handler is associated with a frame, the
system calls that handler before removing the frame. Calling the
condition handlers during the unwind allows a routine to perform
cleanup operations specific to a particular application, such as
recovering from noncontinuable errors or deallocating resources that
were allocated by the routine (such as virtual memory, event flags, and
so forth). See Section 9.12.3 for a description of the SYS$UNWIND system
- Log error messages to a file
The Put Message (SYS$PUTMSG) system service permits any
user-written handler to include a message in a listing file. Such
message logging can be separate from the default messages the user
receives. See Section 9.11 for a detailed description of the
SYS$PUTMSG system service.
9.3 Exception Conditions
Exceptions can be generated by any of the following:
Hardware-generated exceptions always result in conditions that require
special action if program execution is to continue.
Software-generated exceptions may result in error or warning
conditions. These conditions and their message descriptions are
documented in the online Help Message utility and in the OpenVMS system messages documentation.
To access online message descriptions, use the HELP/MESSAGE command.
More information on using the Help Message utility is available in
OpenVMS System Messages: Companion Guide for Help Message Users. That document describes only those messages that occur
when the system is not fully operational and you cannot access Help
Some examples of exception conditions are as follows:
- Arithmetic exception condition in a user-written program detected
and signaled by hardware (for example, floating-point overflow)
- Error in a user argument to a run-time library routine detected by
software and signaled by calling LIB$STOP (for example, a negative
- Error in a run-time library language-support routine, such as an
I/O error or an error in a data-type conversion
- RMS success condition stating that the record is already locked
- RMS success condition stating that the created file superseded an
There are two standard methods for a Compaq- or user-written routine to
indicate that an exception condition has occurred:
- Return a completion code to the calling program using the function
Most general-purpose run-time library routines
indicate exception conditions by returning a condition value in R0. The
calling program then tests bit 0 of R0 for success or failure. This
method allows better programming structure, because the flow of control
can be changed explicitly after the return from each call. If the
actual function value returned is greater than 32 bits, then use both
R0 and R1.
On Alpha systems, if the actual function returned is a
floating-point value, the floating-point value is returned in F0, or F0
- Signal the exception condition
A condition can be signaled by calling the RTL routine LIB$SIGNAL
or LIB$STOP. Any condition handlers that were enabled are then called
by the CHF. See Figure 9-5 for the order in which CHF invokes
Exception conditions raised by hardware or
software are signaled to the routine identically.
For more details,
see Section 9.8 and Section 9.8.1.