From: "Alberto Chessa" <achessa AT fiar DOT finmeccanica DOT it>
Newsgroups: comp.os.msdos.djgpp
Subject: signal & seitimer
Date: 15 Apr 1999 10:16:48 GMT
Organization: TIN
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Message-ID: <01be8729$2599d500$92c809c0@chessa>
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com

Hello,

I'm writing a simple multi-threading C++ library and I'm trying to use
alarm/setitimer for preemptive rescheduing. It works only with "big"
interval timer value (>1sec). Lower values works for the first 1/4 timeout
expirations (depending on interval timer value - 10..90ms), then it simple
stop to call the signal handler (but the program - the last thread - still
running - the resched counter stop).

The basic scheme is as follows:

static volatile int resched;

void Handler(int s)
{
  resched++;
  setsignal(SIGALRM, Handler);	// It's really required ?
  if (!rescheduling)
    Reschedule();		// Will swap stacks
  else
    abort();	// Just for debug, but it never fall here!
}

Setup()
{
  setsignal(SIGALRM, Handler);
  seitime( 10ms );	// off course, I really feel both it_value and
it_interval
}

I try with two threads:
- First: simple write some box on a 800x600x256 SVGA screen (usign my
custom driver)
- Second: write a prompt (on the graphic screen) and check for user input
(kbhit(), then getch())

Note that:
- I change the stack point unsed the signal handler
- I lock all the critical kernel code (cotext switching, rescheduling
routine, signal handler)
- I lock all kernel data used by rescheduling routine and signal handler
- I'm using DJGPP 2.8.1
- I try under Win95B, both dos-box and command prompt only

Thre are problem using setitime/signal on small timeout or I have to look
in my program ?
The last thread activated is always the second one. It could depend on DOS
I/O (kbhit()) ?


Thanks in advance
-- 
Alberto Chessa achessa AT fiar DOT finmeccanica DOT it