WiringPi/wiringPiD/daemonise.c

/*
 * daemonise.c:
 *	Fairly generic "Turn the current process into a daemon" code.
 *
 *	Copyright (c) 2016-2017 Gordon Henderson.
 *********************************************************************************
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <syslog.h>
#include <signal.h>
#include <sys/stat.h>

#include "daemonise.h"

void daemonise (const char *pidFile)
{
  pid_t pid;
  int i;
  FILE *fd;

  syslog (LOG_DAEMON | LOG_INFO, "Becoming daemon");

  // Fork from the parent
  if ((pid = fork()) < 0)
  {
    syslog (LOG_DAEMON | LOG_ALERT, "Fork no. 1 failed: %m");
    exit (EXIT_FAILURE);
  }

  if (pid > 0) // Parent - terminate
  {
    exit (EXIT_SUCCESS);
  }

  // Now running on the child - become session leader
  if (setsid() < 0)
  {
    syslog (LOG_DAEMON | LOG_ALERT, "setsid failed: %m");
    exit (EXIT_FAILURE);
  }

  // Ignore a few signals
  signal (SIGCHLD, SIG_IGN);
  signal (SIGHUP,  SIG_IGN);

  // Fork again
  if ((pid = fork()) < 0)
  {
    syslog (LOG_DAEMON | LOG_ALERT, "Fork no. 2 failed: %m");
    exit (EXIT_FAILURE);
  }

  if (pid > 0) // parent - terminate
  {
    exit (EXIT_SUCCESS);
  }

  // Tidying up - reset umask, change to / and close all files
  umask (0);
  chdir ("/");

  for (i = 0; i < sysconf (_SC_OPEN_MAX); ++i)
  {
    close (i);
  }

  // Write PID into /var/run
  if (pidFile != NULL)
  {
    if ((fd = fopen (pidFile, "w")) == NULL)
    {
      syslog (LOG_DAEMON | LOG_ALERT, "Unable to write PID file: %m");
      exit (EXIT_FAILURE);
    }

    fprintf (fd, "%d\n", getpid());
    fclose (fd);
  }
}