comp.sys.acorn (1647/1650)
Path: cix!slxsys!uknet!acorn!enevill
From: enevill@acorn.co.uk (Edward Nevill)
Newsgroups: comp.sys.acorn
Subject: Re: Trap handling in C
Message-ID: <12372@acorn.co.uk>
Date: 24 Jan 92 12:26:00 GMT
References: <1992Jan20.090419.17124@rdg.dec.com>
Sender: enevill@acorn.co.uk
Distribution: comp
Organization: Acorn Computers Ltd, Cambridge, England
Lines: 77

In article <1992Jan20.090419.17124@rdg.dec.com> goodwin@system.enet.dec.com (Pete Goodwin) writes:

>I wanted to use this to catch faults in my 3d front end to PVray (!Model) and
>exit gracefully without wiping the current picture I've spent ages building up.
>Trouble is, it didn't seem to work. I got the usual "Application !Model has a
>fatal error (type=2) and must exit immediately". I lost a fiddly picture this
>way (sob!).
>
>Why won't:
>
>signal(SIGFPE, trap_routine);
>signal(SIGSEGV, trap_routine);
>signal(SIGILL, trap_routine);
>
>call my trap_routine? All I want to do is dump the edited picture and exit.

The message "Application ... and must exit immediately" is generated by a
signal handler which is installed by 'wimpt_init'. If you wish to replace
this signal handler with one of your own you must install your signal
handler AFTER the call to wimpt_init. The complete list of signals which you
will probably need to catch is

Name      Type

SIGFPE      2   Aritmetic exception (eg divide by 0)
SIGILL      3   Illegal instruction executed
SIGSEGV     5   Data abort or address exception
SIGSTAK     6   Stack overflow
SIGOSERROR 10   Error generated by non X form SWI call

Note, if you trap SIGSTAK your signal handler must be compiled with stack
checking disabled, otherwise a second SIGSTACK will be generated immediately
on entry to your signal handler and the error "Trap while in trap handler"
will be generated. To disable stack checking put

#pragma -s1

before your signal handler and 

#pragma -s0

after your signal handler to re-enable stack checking.

The is a limited amount of stack available to your SIGSTAK handler so you
should not do anything which may require stack space. The best way around
this is to 'longjmp' out of your signal handler back to 'main'. This will
unwind the stack and free up enough stack space for you to save your
picture. So, the code looks something like.

static jmp_buf env;

#pragma -s1
static void my_handler(sig : integer)
{
    longjmp(env, sig);
}
#pragma -s0

int main(void)
{
    ...
    wimpt_init(...);
    signal(SIGFPE, my_handler);
    signal(SIGILL, my_handler);
    signal(SIGSEGV, my_handler);
    signal(SIGSTAK, my_handler);
    signal(SIGOSERROR, my_handler);
    if (setjmp(env)) {
        signal(SIGFPE, SIG_DFL);        /* disable signal handlers in case */
        signal(SIGILL, SIG_DFL);        /* something nasty happens while */
        signal(SIGSEGV, SIG_DFL);       /* trying to save picture */
        signal(SIGSTAK, SIG_DFL);
        signal(SIGOSERROR, SIG_DFL);
        ...                             /* save picture */
        exit(1);
    }
}