Specifies the action
to take when program execution is interrupted.
Syntax
ON interrupt-condition action |
- interrupt-condition
is the interrupt to be handled, either an arithmetic
error or a keyboard interrupt.
- action
is one of the following:
where:
trap-routine
is an external subroutine name.
Description
The ON statement is an HP extension. It is an executable
statement that specifies the action to be taken after the occurrence
of an exception that interrupts program execution.
For each interrupt-condition, you can specify one of the following actions:
CALL: specifies a subroutine to be called.
ABORT: causes the program to abort.
IGNORE: causes the interrupt to be ignored.
Table 10-32 “Exceptions handled by the ON statement” lists the range
of values for interrupt-condition. The first column identifies the type of trap; the
second gives the keywords that must appear on the ON statement, immediately following the word
ON; and the third column gives equivalent keywords
you can specify instead of those in the second column. For example,
the following ON statement causes the program to trap an attempt
to divide by zero with 8-byte floating-point operands, passing control
to a user-written trap handler called div_zero_trap:
ON REAL(8) DIV 0 CALL trap_div_by_zero |
The following ON statement does the same thing, but it specifies
the equivalent keywords from the third column of the table:
ON DOUBLE PRECISION DIV 0 CALL trap_div_by_zero |
Table 10-32 Exceptions handled by the ON statement
Exceptions | Exception
keywords | Alternate
keywords |
|---|
Division
by zero | REAL(4) DIV 0 | REAL DIV 0 |
| REAL(8) DIV 0 | DOUBLE PRECISION DIV 0 |
| REAL(16) DIV 0 | (none) |
| INTEGER(2) DIV 0 | INTEGER*2 DIV 0 |
| INTEGER(4) DIV 0 | INTEGER DIV 0 |
Overflow | REAL(4) OVERFLOW | REAL OVERFLOW |
| REAL(8) OVERFLOW | DOUBLE PRECISION OVERFLOW |
| REAL(16) OVERFLOW | (none) |
| INTEGER(2) OVERFLOW | INTEGER*2 OVERFLOW |
| INTEGER(4) OVERFLOW | INTEGER OVERFLOW |
Underflow | REAL(4) UNDERFLOW | REAL UNDERFLOW |
| REAL(8) UNDERFLOW | DOUBLE PRECISION UNDERFLOW |
| | REAL(16) UNDERFLOW | (none) |
Invalid
(illegal) operation | REAL(4) ILLEGAL | REAL ILLEGAL |
| REAL(8) ILLEGAL | DOUBLE PRECISION ILLEGAL |
| REAL(16) ILLEGAL | (none) |
Inexact
result | REAL(16) INEXACT | (none) |
| REAL(4) INEXACT | REAL INEXACT |
| REAL(8) INEXACT | DOUBLE PRECISION INEXACT |
Control-C | CONTROLC | (none) |
To use the ON statement to trap for integer overflow, you must
also include the $HP$ CHECK_OVERFLOW directive. This is described in the HP
Fortran Programmer’s Guide.
Using the ON statement at optimization levels 2 and above
is restricted. When compiling at optimization level 2 or above,
the optimizer makes assumptions about the program that do not take
into account the behavior of procedures called by the ON statement.
Such procedures must therefore be “well-behaved”—in
particular, they must meet the following criteria:
The ON procedure must not assume that
any variable in the interrupted procedure or in its caller has its
current value. (The optimizer may have placed the variable in a
register to be stored there until after the call to the interrupted
procedure is complete.)
The ON procedure must not change the value of any
variable in the interrupted procedure or in its caller if the effect
of the ON procedure is to return program control to the point of interrupt.
 |
| |
|
 | NOTE: If you include the ON statement in a program that is compiled at optimization level
2 or higher and the program takes an exception, the results may
vary from those you would get from the unoptimized program or from
the same program without the ON statement. |
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| |
|
Examples
The following example uses the ON statement to call the procedure trap_div_by_zero if the function do_div is passed 0 in argument y. If trap_div_by_zero is called, it prints an error message and assigns
0 to the result.
REAL FUNCTION do_div(x, y)
REAL :: x, y
ON REAL DIV 0 CALL trap
do_div = x/y ! causes an interrupt if y = 0
RETURN
END FUNCTION do_div
SUBROUTINE trap(res)
REAL :: res
PRINT *, "Don’t do that."
res = 0
END SUBROUTINE trap |
Related concepts
The HP Fortran Programmer’s Guide provides
detailed information about using the ON statement, including example programs that use
the ON statement.
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