Re: [lm-sensors] [PATCH 7/8] sensord: Use sigaction() for signal

[Andre Prendel <andre.prendel@gmx.de>]

On Fri, May 08, 2009 at 03:06:34PM +0200, Jean Delvare wrote:
> Hi Andre,

Hello Jean,

> 
> Sorry for the late review.
> 
> On Mon, 6 Apr 2009 09:57:58 +0200, Andre Prendel wrote:
> > Replace the deprecated signal() function by sigaction().
> > 
> > The signal() function has some disadvantages. This patch replaces
> > signal() by sigaction() to install the signal handlers.
> > ---
> > 
> >  sensord.c |   32 +++++++++++++++++++++++++-------
> >  1 file changed, 25 insertions(+), 7 deletions(-)
> > 
> > --- quilt-sensors.orig/prog/sensord/sensord.c	2009-04-04 18:25:22.000000000 +0200
> > +++ quilt-sensors/prog/sensord/sensord.c	2009-04-05 17:07:19.000000000 +0200
> > @@ -65,7 +65,6 @@
> >  
> >  static void signalHandler(int sig)
> >  {
> > -	signal(sig, signalHandler);
> >  	switch (sig) {
> >  	case SIGTERM:
> >  		done = 1;
> > @@ -147,6 +146,30 @@
> >  	logOpened = 1;
> >  }
> >  
> > +static void install_sighandler(void)
> > +{
> > +	struct sigaction new;
> > +	int ret;
> > +
> > +	new.sa_handler = signalHandler;
> 
> "new" is a structure on the stack, it is uninitialized, you must zero
> it using memset() before using it.
> 

never seen this in any example before (and never did it before), but
you're right.

Manpage says:

        The sigaction structure is defined as something like

           struct sigaction {
               void     (*sa_handler)(int);
               void     (*sa_sigaction)(int, siginfo_t *, void *);
               sigset_t   sa_mask;
               int        sa_flags;
               void     (*sa_restorer)(void);
           };

        On some architectures a union  is  involved:  do  not  assign  to  both
        sa_handler and sa_sigaction.

        The sa_restorer element is obsolete and should not be used.  POSIX does
        not specify a sa_restorer element.

So the first two elements needn't to be a union. If so, sa_action
should be initialized (to NULL). Sa_restorer is deprecated (not
specified in POSIX). sa_mask and sa_flags is already initialized.

> > +	sigemptyset(&new.sa_mask);
> > +	new.sa_flags = SA_RESTART;
> 
> Why do we need this?

System calls will be restarted automatically if they were interrupted
by a signal. That means, the syscall (if interrupted) doesn't return
EINTR. IMO useful feature.

> 
> > +
> > +	ret = sigaction(SIGTERM, &new, NULL);
> > +	if (ret = -1) {
> > +		fprintf(stderr, "Could not set sighandler for SIGTERM: %s\n",
> > +			strerror(errno));
> > +		exit(EXIT_FAILURE);
> > +	}
> > +
> > +	ret = sigaction(SIGHUP, &new, NULL);
> > +	if (ret = -1) {
> > +		fprintf(stderr, "Could not set sighandler for SIGHUP: %s\n",
> > +			strerror(errno));
> > +		exit(EXIT_FAILURE);
> > +	}
> > +}
> > +
> >  static void daemonize(void)
> >  {
> >  	int pid;
> > @@ -172,12 +195,7 @@
> >  		exit(EXIT_FAILURE);
> >  	}
> >  
> > -	/* I should use sigaction but... */
> > -	if (signal(SIGTERM, signalHandler) = SIG_ERR ||
> > -	    signal (SIGHUP, signalHandler) = SIG_ERR) {
> > -		perror("signal");
> > -		exit(EXIT_FAILURE);
> > -	}
> > +	install_sighandler();
> >  
> >  	if ((pid = fork()) = -1) {
> >  		perror("fork()");
> 
> Looks otherwise good.
> 
> -- 
> Jean Delvare

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