Guardian Procedure Calls Reference Manual (G06.25+)

Guardian Procedure Calls (A-B)

Guardian Procedure Calls Reference Manual—522629-013

2-40

ARMTRAP Procedure

(Superseded by SIGACTION_INIT_ Procedure )

or trap bit of the trap ENV variable at ‘L’[-1]. Such resumption is valid only for trap

2 (arithmetic overflow) or trap 4 (loop timeout). An attempt to resume at the point

of any other trap typically causes the same trap to occur again on a TNS system;

on a native system, such an attempt causes the process to abend.

•

Resuming at another point in the program

To resume execution at some other point in the program, you need to change the P

space bit in ENV at ‘L’[-1] to reflect the new location within your program. You also

need to set appropriate values for the S register at ‘L’[-3] and the L register at ‘L’[0]

and the appropriate environment state in ENV at ‘L’[-1]: The RP field

(ENV.<13:15>) should be set to 7 if the resumption point is the beginning of a

statement; ENV.<0> should be set to 0.

•

Traps in protected code

If the trap occurs in system code or system library and the trap handler is in user

code or user library, or if the trap occurs in a licensed user library and the trap

handler is in user code, the reported program location and process state (space ID,

P, ENV, and L) indicate the point of the user call to one of these protected code

regions and S is reported as -1.

•

How to avoid writing over the application’s data stack

If ‘L’[-3] (the value of ‘S’ at the time of the trap) is -1, the trap handler should not

resume from the trap handler without first changing ‘L’[-3] to a more appropriate

value. Otherwise, ‘G’[0] through ‘G’[10] of the application’s data stack are

overwritten.

•

When the trap handler is not invoked

Under some circumstances (for example, if system resources that are necessary to

initiate trap handling are not available), the trap handler specified though

ARMTRAP might not execute. In such a case, the process abends.

Additional Considerations for Native Systems

Special restrictions apply to trap handlers that execute on native systems. The

following rules should be observed:

•

Trap P variable

The TNS trap P variable is only approximate for a process running in accelerated

mode. You should not use it to inspect the code area and determine the failing

instruction.

You should not increment the trap P variable and resume execution; doing so

causes undefined results. However, you can change the trap P variable to a valid

TNS restart point. The restart point would typically be a label in your program.

See “Resuming at another point in the program” in “Considerations.”