daemon.c

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3 #include <string.h>
   4 #include <unistd.h>
   5 #include <sys/types.h>
   6 #include <sys/stat.h>
   7 #include <fcntl.h>
   8 #include <syslog.h>
   9 #include <errno.h>
  10 #include <pwd.h>
  11 #include <signal.h>
  12
  13 #define EXIT_SUCCESS 0
  14 #define EXIT_FAILURE 1
  15
  16
  17 static void child_handler(int signum)
  18 {
  19     switch (signum) {
  20     case SIGALRM:
  21             exit(EXIT_FAILURE);
  22             break;
  23     case SIGUSR1:
  24             exit(EXIT_SUCCESS);
  25             break;
  26     case SIGCHLD:
  27             exit(EXIT_FAILURE);
  28             break;
  29     }
  30 }
  31
  32 void daemonize(const char *lockfile, const char *username)
  33 {
  34     pid_t pid, sid, parent;
  35     int lfp = -1;
  36
  37     /* already a daemon */
  38     if (getppid() == 1) {
  39             return;
  40     }
  41
  42     /* Create the lock file as the current user */
  43     if (lockfile && lockfile[0]) {
  44         lfp = open(lockfile, O_RDWR|O_CREAT, 0640);
  45         if (lfp < 0) {
  46             syslog(LOG_ERR, "unable to create lock file %s, code=%d (%s)",
  47                     lockfile, errno, strerror(errno));
  48             exit(EXIT_FAILURE);
  49         }
  50     }
  51
  52     /* Drop user if there is one, and we were run as root */
  53     if (username != NULL && (getuid() == 0 || geteuid() == 0)) {
  54         struct passwd *pw = getpwnam(username);
  55         if (pw) {
  56             syslog(LOG_NOTICE, "setting user to %s", username);
  57             setuid(pw->pw_uid);
  58         }
  59     }
  60
  61     /* Trap signals that we expect to recieve */
  62     signal(SIGCHLD, child_handler);
  63     signal(SIGUSR1, child_handler);
  64     signal(SIGALRM, child_handler);
  65
  66     /* Fork off the parent process */
  67     pid = fork();
  68     if (pid < 0) {
  69         syslog(LOG_ERR, "unable to fork daemon, code=%d (%s)",
  70                 errno, strerror(errno));
  71         exit(EXIT_FAILURE);
  72     }
  73     /* If we got a good PID, then we can exit the parent process. */
  74     if (pid > 0) {
  75
  76         /* Wait for confirmation from the child via SIGTERM or SIGCHLD, or
  77            for two seconds to elapse (SIGALRM).  pause() should not return. */
  78         alarm(2);
  79         pause();
  80
  81         exit(EXIT_FAILURE);
  82     }
  83
  84     /* At this point we are executing as the child process */
  85     parent = getppid();
  86
  87     /* Cancel certain signals */
  88     signal(SIGCHLD, SIG_DFL); /* A child process dies */
  89     signal(SIGTSTP, SIG_IGN); /* Various TTY signals */
  90     signal(SIGTTOU, SIG_IGN);
  91     signal(SIGTTIN, SIG_IGN);
  92     signal(SIGHUP, SIG_IGN); /* Ignore hangup signal */
  93     signal(SIGTERM, SIG_DFL); /* Die on SIGTERM */
  94
  95     /* Change the file mode mask */
  96     umask(0);
  97
  98     /* Create a new SID for the child process */
  99     sid = setsid();
 100     if (sid < 0) {
 101         syslog(LOG_ERR, "unable to create a new session, code %d (%s)",
 102                 errno, strerror(errno));
 103         exit(EXIT_FAILURE);
 104     }
 105
 106     /* Change the current working directory.  This prevents the current
 107        directory from being locked; hence not being able to remove it. */
 108     if ((chdir("/")) < 0) {
 109         syslog(LOG_ERR, "unable to change directory to %s, code %d (%s)",
 110                 "/", errno, strerror(errno));
 111         exit(EXIT_FAILURE);
 112     }
 113
 114     /* Redirect standard files to /dev/null */
 115     freopen("/dev/null", "r", stdin);
 116     freopen("/dev/null", "w", stdout);
 117     freopen("/dev/null", "w", stderr);
 118
 119     /* Tell the parent process that we are A-okay */
 120     kill(parent, SIGUSR1);
 121 }
 122