Re: [patch] fsck.minix.c, mkfs.minix.c: use bsd_signal()

["H. Peter Anvin" <hpa@xxxxxxxxx>]

Karel Zak wrote:
> On Wed, Jul 30, 2008 at 03:55:13PM +0200, maximilian attems wrote:
> > s/signal/bsd_signal/ to make it evident which  semantics
> > are expected.
>
> (guy, be happy that Ulrich Drepper is not around:-)
>
> why not sigaction(2) ?
>
> man bsd_signal:
>     On modern Linux systems, bsd_signal() and signal(2)
>     are equivalent. 
>
> Do you mean that klibc signal() is different from glibc?

I dropped signal() from klibc entirely, because I didn't like the 
ambiguity between SysV and BSD semantics.  "On modern Linux systems" 
refer to the unilateral decision of glibc to switch from signal() 
meaning sysv_signal() to signal() meaning bsd_signal().

This *did* turn out to be a reasonable thing to do, as most code out 
there that uses unadorned signal() appears to be BSD-derived.

I specifically stated in the README for klibc that "if you want signal, 
compile with -Dsignal=bsd_signal or -Dsignal=sysv_signal".

At this point, sysv_signal() seems to largely have disappeared from the 
Linux universe, even though the system call still implements it. 
However, stylistically speaking, I still think signal() is quite the 
time bomb to have lying around, and that bsd_signal() [which *IS* in 
POSIX!] is a better choice, alternatively the following function:

void (*xsignal(int sig, void (*func)(int), int flags))(int)
{
	struct sigaction act, oact;

	act.sa_handler = func;
	act.sa_flags   = flags;
	sigemptyset(&act.sa_mask);
	sigaddset(&act.sa_mask, sig);
	if (sigaction(sig, &act, &oact) == -1)
		return SIG_ERR;
	return oact.sa_handler;
}

... which is equivalent to bsd_signal(foo, bar) if called as
xsignal(foo, bar, SA_RESTART) or sysv_signal(foo, bar) if called as
xsignal(foo, bar, SA_RESETHAND).

Now, for simple "crap out and die" signal handler, classical signal() 
does work, but for the vast majority of all cases, it's simply too 
ambiguous, and is a portability timebomb.

	-hpa
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