idle.c   [plain text]


   idle.c -- code for interruptible delays without sleep(3).

   interruptible_idle() -- delay for some time, interruptible by signal.

   Sometimes you need more than one time delay per program, so alarm(3)
won't cut it.  This code illustrates time delays with select(2).

   Eric S. Raymond <>, 1997.  This source code example
is part of fetchmail and the Unix Cookbook, and are released under the
MIT license.  Compile with -DMAIN to build the demonstrator.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <errno.h>
#include <sys/time.h>
#include <fetchmail.h>	/* for ROOT_UID */

#ifndef TRUE
#define TRUE 1
#define FALSE 0

volatile int lastsig;		/* last signal received */

 * The function of this variable is to remove the window during which a
 * SIGALRM can hose the code (ALARM is triggered *before* pause() is called).
 * This is a bit of a kluge; the real right thing would use sigprocmask(),
 * sigsuspend().  This workaround lets the interval timer trigger the first
 * alarm after the required interval and will then generate alarms
 * seconds until it is certain that the critical section (ie., the window)
 * is exited.
static sig_atomic_t	alarm_latch = FALSE;

void gotsigalrm(int sig)
    signal(sig, gotsigalrm);
    lastsig = sig;
    alarm_latch = TRUE;
#endif /* SLEEP_WITH_ALARM */

#ifdef __EMX__
/* Various EMX-specific definitions */
static int itimerflag;

void itimerthread(void* dummy)
    if (outlevel >= O_VERBOSE)
	       GT_("fetchmail: thread sleeping for %d sec.\n"), poll_interval);
	kill((getpid()), SIGALRM);

int interruptible_idle(int seconds)
/* time for a pause in the action; return TRUE if awakened by signal */
    int awoken = FALSE;

#ifndef __EMX__
#ifdef SLEEP_WITH_ALARM		/* not normally on */
     * We can't use sleep(3) here because we need an alarm(3)
     * equivalent in order to implement server nonresponse timeout.
     * We'll just assume setitimer(2) is available since fetchmail
     * has to have a BSDoid socket layer to work at all.
     * This code stopped working under glibc-2, apparently due
     * to the change in signal(2) semantics.  (The siginterrupt
     * line, added later, should fix this problem.) John Stracke
     * <> wrote:
     * The problem seems to be that, after hitting the interval
     * timer while talking to the server, the process no longer
     * responds to SIGALRM.  I put in printf()s to see when it
     * reached the pause() for the poll interval, and I checked
     * the return from setitimer(), and everything seemed to be
     * working fine, except that the pause() just ignored SIGALRM.
     * I thought maybe the itimer wasn't being fired, so I hit
     * it with a SIGALRM from the command line, and it ignored
     * that, too.  SIGUSR1 woke it up just fine, and it proceeded
     * to repoll--but, when the dummy server didn't respond, it
     * never timed out, and SIGALRM wouldn't make it.
     * (continued below...)
    struct itimerval ntimeout;

    ntimeout.it_interval.tv_sec = 5; /* repeat alarm every 5 secs */
    ntimeout.it_interval.tv_usec = 0;
    ntimeout.it_value.tv_sec  = seconds;
    ntimeout.it_value.tv_usec = 0;

    siginterrupt(SIGALRM, 1);
    alarm_latch = FALSE;
    signal(SIGALRM, gotsigalrm);	/* first trap signals */
    setitimer(ITIMER_REAL,&ntimeout,NULL);	/* then start timer */
    /* there is a very small window between the next two lines */
    /* which could result in a deadlock.  But this will now be  */
    /* caught by periodic alarms (see it_interval) */
    if (!alarm_latch)
    /* stop timer */
    ntimeout.it_interval.tv_sec = ntimeout.it_interval.tv_usec = 0;
    ntimeout.it_value.tv_sec  = ntimeout.it_value.tv_usec = 0;
    setitimer(ITIMER_REAL,&ntimeout,NULL);	/* now stop timer */
    signal(SIGALRM, SIG_IGN);
     * So the workaround I used is to make it sleep by using
     * select() instead of setitimer()/pause().  select() is
     * perfectly happy being called with a timeout and
     * no file descriptors; it just sleeps until it hits the
     * timeout.  The only concern I had was that it might
     * implement its timeout with SIGALRM--there are some
     * Unices where this is done, because select() is a library
     * function--but apparently not.
    struct timeval timeout;

    timeout.tv_sec = seconds;
    timeout.tv_usec = 0;
    do {
	lastsig = 0;
	select(0,0,0,0, &timeout);
    } while (lastsig == SIGCHLD);
#else /* EMX */
    alarm_latch = FALSE;
    signal(SIGALRM, gotsigalrm);
    _beginthread(itimerthread, NULL, 32768, NULL);
    /* see similar code above */
    if (!alarm_latch)
    signal(SIGALRM, SIG_IGN);
#endif /* ! EMX */
    if (lastsig == SIGUSR1 || ((seconds && getuid() == ROOT_UID)
	&& lastsig == SIGHUP))
       awoken = TRUE;

    /* now lock out interrupts again */
    signal(SIGUSR1, SIG_IGN);
    if (getuid() == ROOT_UID)
	signal(SIGHUP, SIG_IGN);

    return(awoken ? lastsig : 0);

#ifdef MAIN
int main(int argc, char **argv)
    for (;;)
	printf("How may I serve you, master?\n");
#endif /* MAIN */

/* idle.c ends here */