Interrupt Handler in 'C'

[David Dymm]

    I POSTED THIS THE OTHER DAY --- FROM THE MAIL RECEIVED (THANK
    YOU VERY MUCH), I REALIZE THAT I DID A POOR JOB OF EXPLAINING MYSELF.
    SO I WILL POST THIS AGAIN WITH A DIFFERENT EXPLANATION:

I would like to write an interrupt handling mechanism
in 'C' (on BSD 4.2 Unix) to work in the following way:

1) Set up the signal facility:

	signal (SIGALRM, alarm_handler)

2) Set the alarm:

	ualarm (100000, 0)  <== This will send a signal after
				100,0000 microseconds.

3) The interrupt handler "alarm_handler" (a separate function)
    is called with the signal mechanism when the alarm timer times out.
    Let's call "location A" the place where we were in
    the code when we were interrupted by the timer.

    When "alarm_handler" has finished its local processing,
    it resets the alarm.  At this point, I do NOT want to
    return control back to where the interrupt occurred!!!!
    Instead, I want to "longjmp" to a separate "function B"
    When "function B" has  completed its work, I want to
    return to "location A".

4)  However while function "B" is doing its thing, the alarm
    may again signal an interrupt, causing the above actions
    to happen again.  The longjmp mechanism prevents continually
    pushing the call to function "B" on the system stack.

The question is:  How do I return to "location A" ???

    I have looked at the "sigstack" mechanism, and also at the
    definitions for "sigcontext" and "sigstack" in "signal.h".
    But I do not see how to put this all together to accomplish
    my task.  The system saves the state of the process on the
    signal stack when "signal" causes control to jump to 
    "alarm_handler".  How do I get at that information, AND more
    importantly, how do I use that information to accomplish
    my task.

---------------------------------------------------------------
The following questions were raised by people who sent me mail:

A) Why do a longjump to function "B" --- just call the function ???

    If I "call" function "B", the function call gets pushed onto
    the system stack.  The problem is that the above actions
    ( alarm timeout --> "alarm_handler" --> longjmp to "B" )
    can occur a number of times.  If I continually call function "B",
    then the stack will continue to grow.

    However whether I call "B" or longjmp, this does not help
    me in returning to location "A" !!!



David Dymm			Software Engineer

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