HP OpenVMS Systems Documentation
OpenVMS Programming Concepts Manual
9.12.1 Continuing Execution
To continue execution from the instruction following the signal, with no error messages or traceback, the handler returns with the function value SS$_CONTINUE (bit <0> = 1). If, however, the condition was signaled with a call to LIB$STOP, the SS$_CONTINUE return status causes an error message (Attempt To Continue From Stop), and the image exits. The only way to continue from a call to LIB$STOP is for the condition handler to request a stack unwind.
If execution is to continue after a hardware fault (such as a reserved operand fault), the condition handler must correct the cause of the condition before returning the function value SS$_CONTINUE or requesting a stack unwind. Otherwise, the instruction that caused the fault executed again.
Condition handlers check for specific errors. If the signaled condition is not one of the expected errors, the handler resignals. That is, it returns control to the OpenVMS Condition Handling facility with the function value SS$_RESIGNAL (with bit <0> clear). To alter the severity of the signal, the handler modifies the low-order 3 bits of the condition value and resignals.
A condition handler can dismiss the signal by calling the system
service SYS$UNWIND. The stack unwind is initiated when a condition
handler that has called SYS$UNWIND returns to OpenVMS Condition
Handling facility. For an explanation of unwinding, see Section 9.10.1;
for an example of using SYS$UNWIND to return control to the program,
see Section 220.127.116.11.
The operating system passes two arrays to a condition handler. Any condition handler that you write should declare two dummy arguments as variable-length arrays, as in the following:
18.104.22.168 Signal Array
The first dummy argument, the signal array, describes the signaled
condition codes that indicate which error occurred and the state of the
process when the condition code was signaled. For the structure of the
signal array, see Section 9.8.2.
The second dummy argument, the mechanism array, describes the state of the process when the condition code was signaled. Typically, a condition handler references only the call depth and the saved function value. Currently, the mechanism array contains exactly five elements; however, because its length is subject to change, you should declare the dummy argument as a variable-length array. For the structure of the mechanism array, see Section 9.8.3.
Usually you write a condition handler in anticipation of a particular
set of condition values. Because a handler is invoked in response to
any signaled condition code, begin your handler by comparing the
condition code passed to the handler (element 2 of the signal array)
against the condition codes expected by the handler. If the signaled
condition code is not one of the expected codes, resignal the condition
code by equating the function value of the handler to the global symbol
You can use the RTL routine LIB$MATCH_COND to compare the signaled condition code to a list of expected condition values. The first argument passed to LIB$MATCH_COND is the signaled condition code, the second element of the signal array. The rest of the arguments passed to LIB$MATCH_COND are the expected condition values. LIB$MATCH_COND compares the first argument with each of the remaining arguments and returns the number of the argument that matches the first one. For example, if the second argument matches the first argument, LIB$MATCH_COND returns a value of 1. If the first argument does not match any of the other arguments, LIB$MATCH_COND returns 0.
The following condition handler determines whether the signaled condition code is one of four Compaq Fortran I/O errors. If it is not, the condition handler resignals the condition code. Note that, when a Compaq Fortran I/O error is signaled, the signal array describes operating system's condition code, not the Compaq Fortran error code.
22.214.171.124 Exiting from the Condition Handler
126.96.36.199 Returning Control to the Program
Your handlers should return control either to the program unit that established the handler or to the program unit that invoked the program unit that established the handler.
To return control to the program unit that established the handler, invoke SYS$UNWIND and pass the call depth (third element of the mechanism array) as the first argument with no second argument.
To return control to the caller of the program unit that established the handler, invoke SYS$UNWIND without passing any arguments.
The first argument SYS$UNWIND specifies the number of program units to unwind (remove from the stack). If you specify this argument at all, you should do so as shown in the previous example. (MECHARGS(3) contains the number of program units that must be unwound to reach the program unit that established the handler that invoked SYS$UNWIND.) The second argument SYS$UNWIND contains the location of the next statement to be executed. Typically, you omit the second argument to indicate that the program should resume execution at the statement following the last statement executed in the program unit that is regaining control.
Each time SYS$UNWIND removes a program unit from the stack, it invokes any condition handler established by that program unit and passes the condition handler the SS$_UNWIND condition code. To prevent the condition handler from resignaling the SS$_UNWIND condition code (and so complicating the unwind operation), include SS$_UNWIND as an expected condition code when you invoke LIB$MATCH_COND. When the condition code is SS$_UNWIND, your condition handler might perform necessary cleanup operations or do nothing.
In the following example, if the condition code is SS$_UNWIND, no action is performed. If the condition code is another of the expected codes, the handler displays the message and then returns control to the program unit that called the program unit that established the condition handler.
9.12.5 Example of Condition-Handling Routines
The following example shows two procedures, A and B, that have declared condition handlers. The notes describe the sequence of events that would occur if a call to a system service failed during the execution of procedure B.
9.13 Debugging a Condition Handler
You can debug a condition handler as you would any subprogram, except that you cannot use the DEBUG command STEP/INTO to enter a condition handler. You must set a breakpoint in the handler and wait for the debugger to invoke the handler.
Typically, to trace execution of a condition handler, you set breakpoints at the statement in your program that should signal the condition code, at the statement following the one that should signal, and at the first executable statement in your condition handler.
The SET BREAK debugger command with the /HANDLER qualifier causes the
debugger to scan the call stack and attempt to set a breakpoint on
every established frame-based handler whenever the program being
debugged has an exception. The debugger does not discriminate between
standard RTL handlers and user-defined handlers.
The following sections present information about Alpha systems RTL
jacket handlers, and RTL routines that can be either established as
condition handlers or called from a condition handler to handle
signaled exception conditions.
On Alpha systems, many RTLs establish a jacket RTL handler on a frame where the user program has defined its own handler. This RTL jacket does some setup and argument manipulation before actually calling the handler defined by the user. When processing the exception, the debugger sets the breakpoint on the jacket RTL jacket handler, because that is the address on the call stack. If the debugger suspends program execution at a jacket RTL handler, it is usually possible to reach the user-defined handler by entering a STEP/CALL command followed by a STEP/INTO command. Some cases might require that additional sequences of STEP/CALL and STEP/INTO commands be entered. For more information on frame-based handlers, see OpenVMS Calling Standard.
If the jacket RTL handler is part of an installed shared image such as ALPHA LIBOTS, the debugger cannot set a breakpoint on it. In this case, activate the RTL as a private image by defining the RTL as a logical name, as in the following example:
9.14.2 Converting a Floating-Point Fault to a Floating-Point Trap (VAX Only)
On VAX systems, a trap is an exception condition that is signaled after the instruction that caused it has finished executing. A fault is an exception condition that is signaled during the execution of the instruction. When a trap is signaled, the program counter (PC) in the signal argument vector points to the next instruction after the one that caused the exception condition. When a fault is signaled, the PC in the signal argument vector points to the instruction that caused the exception condition. See the VAX Architecture Reference Manual for more information about faults and traps.
LIB$SIM_TRAP can be established as a condition handler or be called
from a condition handler to convert a floating-point fault to a
floating-point trap. After LIB$SIM_TRAP is called, the PC points to the
instruction after the one that caused the exception condition. Thus,
your program can continue execution without fixing up the original
condition. LIB$SIM_TRAP intercepts only floating-point overflow,
floating-point underflow, and divide-by-zero faults.
When it is preferable to detect errors by signaling but the calling routine expects a returned status, LIB$SIG_TO_RET can be used by the routine that signals. For instance, if you expect a particular condition code to be signaled, you can prevent the operating system from invoking the default condition handler by establishing a different condition handler. LIB$SIG_TO_RET is a condition handler that converts any signaled condition to a return status. The status is returned to the caller of the routine that established LIB$SIG_TO_RET. You may establish LIB$SIG_TO_RET as a condition handler by specifying it in a call to LIB$ESTABLISH.
On Alpha systems, LIB$ESTABLISH is not supported, though high-level languages may support it for compatibility.
LIB$SIG_TO_RET can also be called from another condition handler. If LIB$SIG_TO_RET is called from a condition handler, the signaled condition is returned as a function value to the caller of the establisher of that handler when the handler returns to the OpenVMS Condition Handling facility. When a signaled exception condition occurs, LIB$SIG_TO_RET routine does the following:
To change a signal to a return status, you must put any code that might signal a condition code into a function where the function value is a return status. The function containing the code must perform the following operations:
If the program unit GET_1_STAT in the following function signals a condition code, LIB$SIG_TO_RET changes the signal to the return status of the INTERCEPT_SIGNAL function and returns control to the program unit that invoked INTERCEPT_SIGNAL. (If GET_1_STAT has a condition handler established, the operating system invokes that handler before invoking LIB$SIG_TO_RET.)
When the program unit that invoked INTERCEPT_SIGNAL regains control, it should check the return status (as shown in Section 9.5.1) to determine which condition code, if any, was signaled during execution of INTERCEPT_SIGNAL.