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OpenVMS Programming Concepts Manual

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Only the condition-value argument must be specified; other arguments are optional. The condition-value argument is an OpenVMS 32-bit condition value. The condition-value argument is an unsigned longword that contains this condition value.

The number-of-arguments argument, if specified, contains the number of FAO arguments that are associated with condition-value. The optional number-of-arguments argument is a signed longword integer that contains this number. If omitted or specified as zero, no FAO arguments follow.

The FAO-argument argument is an optional FAO (formatted ASCII output) argument that is associated with the specified condition value.

The condition-value argument indicates the condition that is being signaled. However, LIB$STOP always sets the severity of condition-value to SEVERE before proceeding with the stack-scanning operation.

The FAO arguments describe the details of the exception condition. These are the same arguments that are passed to the OpenVMS Condition Handling facility as part of the signal argument vector. The system default condition handlers pass them to SYS$PUTMSG, which uses them to issue a system message.

Unlike most routines, LIB$SIGNAL and LIB$STOP preserve R0 and R1 as well as the other registers. Therefore, a call to LIB$SIGNAL allows the debugger to display the entire state of the process at the time of the exception condition. This is useful for debugging checks and gathering statistics.

The behavior of LIB$SIGNAL is the same as that of the exception dispatcher that performs the stack scan after hardware detects an exception condition. That is, the system scans the stack in the same way, and the same arguments are passed to each condition handler. This allows a user to write a single condition handler to detect both hardware and software conditions.

For more information about the RTL routines LIB$SIGNAL and LIB$STOP, see OpenVMS RTL Library (LIB$) Manual.

9.8.2 Signal Argument Vector

Signaling a condition value causes both VAX and Alpha systems to pass control to a special subprogram called a condition handler. The operating system invokes a default condition handler unless you have established your own. The default condition handler displays the associated error message and continues or, if the error is a severe error, terminates program execution.

The signal argument vector contains information describing the nature of the hardware or software condition. Figure 9-6 illustrates the open-ended structure of the signal argument vector, which can be from 3 to 257 longwords in length.

The format of the signal argument array and the data it returns is the same on VAX systems and Alpha systems, with the exception of the processor status (PS) returned on Alpha systems and the processor status longword (PSL) returned on VAX systems. On Alpha systems, it is the low-order 32 bits of the PS.

On Alpha systems, CHF$IS_SIG_ARGS and CHF$IS_SIG_NAME are aliases for CHF$L_SIG_ARGS and CHF$L_SIG_NAME, as shown in Figure 9-6, and the PSL field for VAX systems is the processor status (PS) field for Alpha systems.

Figure 9-6 Format of the Signal Argument Vector

Fields of the Signal Argument Vector


An unsigned integer (n) designating the number of longwords that follow in the vector, not counting the first, including PC and PSL. (On Alpha systems, the value used for the PSL is the low-order half of the Alpha processor status [PS] register.) For example, the first entry of a 4-longword vector would contain a 3.


On both VAX systems and Alpha systems, this argument is a 32-bit value that uniquely identifies a hardware or software exception condition. The format of the condition code, which is the same for both VAX systems and Alpha systems, is shown and described in Figure 9-3. However, Alpha systems do not support every condition returned on VAX systems, and Alpha systems define several new conditions that cannot be returned on VAX systems. Table 9-2 lists VAX system condition codes that cannot be returned on Alpha systems.

If more than one message is associated with the error, this is the condition value of the first message. Handlers should always check whether the condition is the one that they expect by examining the STS$V_COND_ID field of the condition value (bits <27:3>). Bits <2:0> are the severity field. Bits <31:28> are control bits; they may have been changed by an intervening handler and so should not be included in the comparison. You can use the RTL routine LIB$MATCH_COND to match the correct fields. If the condition is not expected, the handler should resignal by returning false (bit <0> = 0). The possible exception conditions and their symbolic definitions are listed in Table 9-1.

SIGARGS(3 to n --1)

Optional arguments that provide additional information about the condition. These arguments consist of one or more message sequences. The format of the message description varies depending on the type of message being signaled. For more information, see the SYS$PUTMSG description in the OpenVMS System Services Reference Manual. The format of a message sequence is described in Section 9.11.


The program counter (PC) of the next instruction to be executed if any handler (including the system-supplied handlers) returns with the status SS$_CONTINUE. For hardware faults, the PC is that of the instruction that caused the fault. For hardware traps, the PC is that of the instruction following the one that caused the trap. The error generated by LIB$SIGNAL is a trap. For conditions signaled by calling LIB$SIGNAL or LIB$STOP, the PC is the location following the CALLS or CALLG instruction. See the VAX Architecture Reference Manual or Alpha Architecture Reference Manual for a detailed description of faults and traps.


On VAX systems the processor status longword (PSL), or on Alpha systems the processor status (PS) register, of the program at the time that the condition was signaled.

For information about the PSL on VAX systems, and the PS on Alpha systems, see the VAX Architecture Reference Manual and the Alpha Architecture Reference Manual.


LIB$SIGNAL and LIB$STOP copy the variable-length argument list passed by the caller. Then, before calling a condition handler, they append the PC and PSL (or on Alpha systems the processor status (PS) register) entries to the end of the list.

The formats for some conditions signaled by the operating system and the run-time library are shown in Figure 9-7 and Figure 9-8. These formats are the same on VAX systems and Alpha systems, except for the PSL, or on Alpha systems, the PS register.

Figure 9-7 Signal Argument Vector for the Reserved Operand Error Conditions

Figure 9-8 Signal Argument Vector for RTL Mathematics Routine Errors

The caller's PC is the PC following the calling program's JSB or CALL to the mathematics routine that detected the error. The PC is that following the call to LIB$SIGNAL.

9.8.3 VAX Mechanism Argument Vector (VAX Only)

On VAX systems, the mechanism argument vector is a 5-longword vector that contains all of the information describing the state of the process at the time of the hardware or software signaled condition. Figure 9-9 illustrates a mechanism argument vector for VAX systems.

Figure 9-9 Format of a VAX Mechanism Argument Vector

Fields of the VAX Mechanism Argument Vector


An unsigned integer indicating the number of longwords that follow, not counting the first, in the vector. Currently, this number is always 4.


The address of the stack frame of the routine that established the handler being called. You can use this address as a base from which to reference the local stack-allocated storage of the establisher, as long as the restrictions on the handler's use of storage are observed. For example, if the call stack is as shown in Figure 9-4, this argument points to the call frame for procedure A.

You can use this value to display local variables in the procedure that established the condition handler if the variables are at known offsets from the frame pointer (FP) of the procedure.


The stack depth, which is the number of stack frames between the establisher of the condition handler and the frame in which the condition was signaled. To ensure that calls to LIB$SIGNAL and LIB$STOP appear as similar as possible to hardware exception conditions, the call to LIB$SIGNAL or LIB$STOP is not included in the depth.

If the routine that contained the hardware exception condition or that called LIB$SIGNAL or LIB$STOP also handled the exception condition, then the depth is zero; if the exception condition occurred in a called routine and its caller handled the exception condition, then the depth is 1. If a system service signals an exception condition, a handler established by the immediate caller is also entered with a depth of 1.

The following table shows the stack depths for the establishers of condition handlers:

Depth Meaning
--3 Condition handler was established in the last-chance exception vector.
--2 Condition handler was established in the primary exception vector.
--1 Condition handler was established in the secondary exception vector.
0 Condition handler was established by the frame that was active when the exception occurred.
1 Condition handler was established by the caller of the frame that was active when the exception occurred.
2 Condition handler was established by the caller of the caller of the frame that was active when the exception occurred.

For example, if the call stack is as shown in Figure 9-4, the depth argument passed to handler A would have a value of 2.

The condition handler can use this argument to determine whether to handle the condition. For example, the handler might not want to handle the condition if the exception that caused the condition did not occur in the establisher frame.


Copies of the contents of registers R0 and R1 at the time of the exception condition or the call to LIB$SIGNAL or LIB$STOP. When execution continues or a stack unwind occurs, these values are restored to R0 and R1. Thus, a handler can modify these values to change the function value returned to a caller.

9.8.4 Alpha Mechanism Argument Vector (Alpha Only)

On Alpha systems, the mechanism array returns much the same data as it does on VAX systems, though its format is changed. The mechanism array returned on Alpha systems preserves the contents of a larger set of integer scratch registers as well as the Alpha floating-point scratch registers. In addition, because Alpha registers are 64 bits long, the mechanism array is constructed of quadwords (64 bits), not longwords (32 bits) as it is on VAX systems. Figure 9-10 shows the format of the mechanism array on Alpha systems.

Figure 9-10 Mechanism Array on Alpha Systems

Table 9-11 describes the arguments in the mechanism array.

Table 9-11 Fields in the Alpha Mechanism Array
Argument Description
CHF$IS_MCH_ARGS Represents the number of quadwords in the mechanism array, not counting the argument count quadword. (The value contained in this argument is always 43.)
CHF$IS_MCH_FLAGS Flag bits <63:0> for related argument mechanism information defined as follows for CHF$V_FPREGS:

Bit <0>: When set, the process has already performed a floating-point operation and the floating-point registers stored in this structure are valid.

If this bit is clear, the process has not yet performed any floating-point operations, and the values in the floating-point register slots in this structure are unpredictable.

CHF$PH_MCH_FRAME The frame pointer (FP) in the procedure context of the establisher.
CHF$IS_MCH_DEPTH Positive count of the number of procedure activation stack frames between the frame in which the exception occurred and the frame depth that established the handler being called.
CHF$PS_MCH_DADDR Address of the handler data quadword if the exception handler data field is present (as indicated by PDSC.FLAGS.HANDLER_DATA_VALID); otherwise, contains zero.
CHF$PH_MCH_ESF_ADDR Address of the exception stack frame (see the Alpha Architecture Reference Manual).
CHF$PH_MCH_SIG_ADDR Address of the signal array. The signal array is a 32-bit (longword) array.
CHF$IH_MCH_SAVR nn A copy of the saved integer registers at the time of the exception. The following registers are saved: R0, R1, and R16--R28. Registers R2--R15 are implicitly saved in the call chain.
CHF$FM_MCH_SAVF nn A copy of the saved floating-point registers at the time of the exception or may have unpredictable data as described in CHF$IS_MCH_FLAGS. If the floating-point register fields are valid, the following registers are saved: F0, F1, and F10--F30. Registers F2--F9 are implicitly saved in the call chain.

For more information and recommendations about using the mechanism argument vector on Alpha systems, see Migrating to an OpenVMS AXP System: Recompiling and Relinking Applications. Note that this manual has been archived but is available on the OpenVMS Documentation CD-ROM.

9.8.5 Multiple Active Signals

A signal is said to be active until the routine that signaled regains control or until the stack is unwound or the image exits. A second signal can occur while a condition handler or a routine it has called is executing. This situation is called multiple active signals or multiple exception conditions. When this situation occurs, the stack scan is not performed in the usual way. Instead, the frames that were searched while processing all of the previous exception conditions are skipped when the current exception condition is processed. This is done in order to avoid recursively reentering a routine that is not reentrant. For example, Fortran code typically is not recursively reentrant. If a Fortran handler were called while another activation of that handler was still going, the results would be unpredictable.

A second exception may occur while a condition handler or a procedure that it has called is still executing. In this case, when the exception dispatcher searches for a condition handler, it skips the frames that were searched to locate the first handler.

The search for a second handler terminates in the same manner as the initial search, as described in Section 9.6.

If the SYS$UNWIND system service is issued by the second active condition handler, the depth of the unwind is determined according to the same rules followed in the exception dispatcher's search of the stack: all frames that were searched for the first condition handler are skipped.

Primary and secondary vectored handlers, on the other hand, are always entered when an exception occurs.

If an exception occurs during the execution of a handler that was established in the primary or secondary exception vector, that handler must handle the additional condition. Failure to do so correctly might result in a recursive exception loop in which the vectored handler is repeatedly called until the user stack is exhausted.

The modified search routine is best illustrated with an example. Assume the following calling sequence:

  1. Routine A calls routine B, which calls routine C.
  2. Routine C signals an exception condition (signal S), and the handler for routine C (CH) resignals.
  3. Control passes to BH, the handler for routine B. The call frame for handler BH is located on top of the signal and mechanism arrays for signal S. The saved frame pointer in the call frame for BH points to the frame for routine C.
  4. BH calls routine X; routine X calls routine Y.
  5. Routine Y signals a second exception condition (signal T). Figure 9-11 illustrates the stack contents after the second exception condition is signaled.

    Figure 9-11 Stack After Second Exception Condition Is Signaled

    Normally, the OpenVMS Condition Handling facility (CHF) searches all currently active frames for condition handlers, including B and C. If this happens, however, BH is called again. At this point, you skip the condition handlers that have already been called. Thus, the search for condition handlers should proceed in the following order:
    BHH (the handler for routine B's handler)
  6. The search now continues in its usual fashion. The CHF examines the primary and secondary exception vectors, then frames Y, X, and BH. Thus, handlers YH, XH, and BHH are called. Assume that these handlers resignal.
  7. The CHF now skips the frames that have already been searched and resumes the search for condition handlers in routine A's frame. The depths that are passed to handlers as a result of this modified search are 0 for YH, 1 for XH, 2 for BHH, and 3 for AH.

Because of the possibility of multiple active signals, you should be careful if you use an exception vector to establish a condition handler. Vectored handlers are called, not skipped, each time an exception occurs.

9.9 Types of Condition Handlers

On VAX systems, when a routine is activated, the first longword in its stack frame is set to 0. This longword is reserved to contain an address pointing to another routine called the condition handler. If an exception condition is signaled during the execution of the routine, the OpenVMS Condition Handling Facility uses the address in the first longword of the frame to call the associated condition handler.

On Alpha systems, each procedure, other than a null frame procedure, can have a condition handler potentially associated with it, identified by the HANDLER_VALID, STACK_HANDLER, or REG_HANDLER field of the associated procedure descriptor. You establish a handler by including the procedure value of the handler procedure in that field.

The arguments passed to the condition-handling routine are the signal and mechanism argument vectors, described in Section 9.8.2, Section 9.8.3, and Section 9.8.4.

Various types of condition handlers can be called for a given routine:

  • User-supplied condition handlers
    You can write your own condition handler and set up its address in the stack frame of your routine using the run-time library routine LIB$ESTABLISH or the mechanism supplied by your language.
    On Alpha systems, LIB$ESTABLISH is not supported, though high-level languages may support it for compatibility.
  • Language-supplied condition handlers
    Many high-level languages provide a means for setting up handlers that are global to a single routine. If your language provides a condition-handling mechanism, you should always use it. If you also try to establish a condition handler using LIB$ESTABLISH, the two methods of handling exception conditions conflict, and the results are unpredictable.
  • System default condition handlers
    The operating system provides a set of default condition handlers. These take over if there are no other condition handler addresses on the stack, or if all the previous condition handlers have passed on (resignaled) the indication of the exception condition.

9.9.1 Default Condition Handlers

The operating system establishes the following default condition handlers each time a new image is started. The default handlers are shown in the order they are encountered when the operating system processes a signal. These three handlers are the only handlers that output error messages.

  • Traceback handler
    The traceback handler is established on the stack after the catchall handler. This enables the traceback handler to get control first. This handler performs three functions in the following order:
    1. Outputs an error message using the Put Message (SYS$PUTMSG) system service. SYS$PUTMSG formats the message using the Formatted ASCII Output (SYS$FAO) system service and sends the message to the devices identified by the logical names SYS$ERROR and SYS$OUTPUT (if it differs from SYS$ERROR). That is, it displays the message associated with the signaled condition code, the traceback message, the program unit name and line number of the statement that signaled the condition code, and the relative and absolute program counter values. (On a warning or error, the number of the next statement to be executed is displayed.)
    2. Issues a symbolic traceback, which shows the state of the routine stack at the time of the exception condition. That is, it displays the names of the program units in the calling hierarchy and the line numbers of the invocation statements.

    3. Decides whether to continue executing the image or to force an exit based on the severity field of the condition value:
      Severity Error Type Action
      1 Success Continue
      3 Information Continue
      0 Warning Continue
      2 Error Continue
      4 Severe Exit

      The traceback handler is in effect if you link your program with the /TRACEBACK qualifier of the LINK command (the default). Once you have completed program development, you generally link your program with the /NOTRACEBACK qualifier and use the catchall handler.
  • Catchall handler
    The operating system establishes the catchall handler in the first stack frame and thus calls it last. This handler performs the same functions as the traceback handler except for the stack traceback. That is, it issues an error message and decides whether to continue execution. The catchall is called only if you link with the /NOTRACEBACK qualifier. It displays the message associated with the condition code and then continues program execution or, if the error is severe, terminates execution.
  • Last-chance handler
    The operating system establishes the last-chance handler with a system exception vector. In most cases, this vector contains the address of the catchall handler, so that these two handlers are actually the same. The last-chance handler is called only if the stack is invalid or all the handlers on the stack have resignaled. If the debugger is present, the debugger's own last-chance handler replaces the system last-chance handler.

Displays the message associated with the condition code and then continues program execution or, if the error is severe, terminates execution. The catchall handler is not invoked if the traceback handler is enabled.

In the following example, if the condition code INCOME_LINELOST is signaled at line 496 of GET_STATS, regardless of which default handler is in effect, the following message is displayed:

%INCOME-W-LINELOST, Statistics on last line lost due to CTRL/Z

If the traceback handler is in effect, the following text is also displayed:

%TRACE-W-TRACEBACK, symbolic stack dump follows
module name     routine name       line      rel PC    abs PC

GET_STATS       GET_STATS           497     00000306  00008DA2
INCOME          INCOME              148     0000015A  0000875A
                                            0000A5BC  0000A5BC
                                            00009BDB  00009BDB
                                            0000A599  0000A599

Because INCOME_LINELOST is a warning, the line number of the next statement to be executed (497), rather than the line number of the statement that signaled the condition code, is displayed. Line 148 of the program unit INCOME invoked GET_STATS.

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