Creating a daemon in Linux

Question

In Linux I want to add a daemon that cannot be stopped and which monitors filesystem changes  If any changes are detected  it should write the path to the console where it was started plus a newline   I already have the filesystem changing code almost ready but I cannot figure out how to create a daemon   My code is from here  http   www yolinux com TUTORIALS ForkExecProcesses html  What to do after the fork   int main  int argc  char   argv       pid t pID   fork      if  pID    0                     child              Code only executed by child process           sIdentifier    Child Process               else if  pID  lt  0            cerr  lt  lt   Failed to fork   lt  lt  endl          exit 1             Throw exception           else                                      parent                Code only executed by parent process        sIdentifier    Parent Process                      return 0

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man 7 daemon describes how to create daemon in great detail  My answer is just excerpt from this manual   There are at least two types of daemons    traditional SysV daemons  old-style   systemd daemons  new-style       SysV Daemons  If you are interested in traditional SysV daemon  you should implement the following steps         Close all open file descriptors except standard input  output  and error  i e  the first three file descriptors 0  1  2   This ensures that no accidentally passed file descriptor stays around in the daemon process  On Linux  this is best implemented by iterating through  proc self fd  with a fallback of iterating from file descriptor 3 to the value returned by getrlimit   for RLIMIT NOFILE    Reset all signal handlers to their default  This is best done by iterating through the available signals up to the limit of  NSIG and resetting them to SIG DFL    Reset the signal mask using sigprocmask      Sanitize the environment block  removing or resetting environment variables that might negatively impact daemon runtime    Call fork    to create a background process    In the child  call setsid   to detach from any terminal and create an independent session    In the child  call fork   again  to ensure that the daemon can never re-acquire a terminal again    Call exit   in the first child  so that only the second child  the actual daemon process  stays around  This ensures that the daemon process is re-parented to init PID 1  as all daemons should be    In the daemon process  connect  dev null to standard input  output  and error    In the daemon process  reset the umask to 0  so that the file modes passed to open    mkdir   and suchlike directly control the access mode of the created files and directories    In the daemon process  change the current directory to the root directory      in order to avoid that the daemon involuntarily blocks mount points from being unmounted    In the daemon process  write the daemon PID  as returned by getpid    to a PID file  for example  run foobar pid  for a hypothetical daemon  foobar   to ensure that the daemon cannot be started more than once  This must be implemented in race-free fashion so that the PID file is only updated when it is verified at the same time that the PID previously stored in the PID file no longer exists or belongs to a foreign process    In the daemon process  drop privileges  if possible and applicable    From the daemon process  notify the original process started that initialization is complete  This can be implemented via an unnamed pipe or similar communication channel that is created before the first fork   and hence available in both the original and the daemon process    Call exit   in the original process  The process that invoked the daemon must be able to rely on that this exit   happens after initialization is complete and all external communication channels are established and accessible       Note this warning      The BSD daemon   function should not be used  as it implements only a subset of these steps       A daemon that needs to provide compatibility with SysV systems should implement the scheme pointed out above  However  it is recommended to make this behavior optional and configurable via a command line argument to ease debugging as well as to simplify integration into systems using systemd    Note that daemon   is not POSIX compliant     New-Style Daemons  For new-style daemons the following steps are recommended         If SIGTERM is received  shut down the daemon and exit cleanly    If SIGHUP is received  reload the configuration files  if this applies    Provide a correct exit code from the main daemon process  as this is used by the init system to detect service errors and problems  It is recommended to follow the exit code scheme as defined in the LSB recommendations for SysV init scripts    If possible and applicable  expose the daemon s control interface via the D-Bus IPC system and grab a bus name as last step of initialization    For integration in systemd  provide a  service unit file that carries information about starting  stopping and otherwise maintaining the daemon  See systemd service 5  for details    As much as possible  rely on the init system s functionality to limit the access of the daemon to files  services and other resources  i e  in the case of systemd  rely on systemd s resource limit control instead of implementing your own  rely on systemd s privilege dropping code instead of implementing it in the daemon  and similar  See systemd exec 5  for the available controls    If D-Bus is used  make your daemon bus-activatable by supplying a D-Bus service activation configuration file  This has multiple advantages  your daemon may be started lazily on-demand  it may be started in parallel to other daemons requiring it     which maximizes parallelization and boot-up speed  your daemon can be restarted on failure without losing any bus requests  as the bus queues requests for activatable services  See below for details    If your daemon provides services to other local processes or remote clients via a socket  it should be made socket-activatable following the scheme pointed out below  Like D-Bus activation  this enables on-demand starting of services as well as it allows improved parallelization of service start-up  Also  for state-less protocols  such as syslog  DNS   a daemon implementing socket-based activation can be restarted without losing a single request  See below for details    If applicable  a daemon should notify the init system about startup completion or status updates via the sd notify 3  interface    Instead of using the syslog   call to log directly to the system syslog service  a new-style daemon may choose to simply log to standard error via fprintf    which is then forwarded to syslog by the init system  If log levels are necessary  these can be encoded by prefixing individual log lines with strings like   lt 4    for log level 4  WARNING  in the syslog priority scheme   following a similar style as the Linux kernel s printk   level system  For details  see sd-daemon 3  and systemd exec 5          To learn more read whole man 7 daemon

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You cannot create a process in linux that cannot be killed   The root user  uid 0  can send a signal to a process  and there are two signals which cannot be caught  SIGKILL 9  SIGSTOP 19   And other signals  when uncaught  can also result in process termination   You may want a more general daemonize function  where you can specify a name for your program daemon  and a path to run your program  perhaps     or   tmp     You may also want to provide file s  for stderr and stdout  and possibly a control path using stdin    Here are the necessary includes    include  lt stdio h gt       printf 3   include  lt stdlib h gt      exit 3   include  lt unistd h gt      fork 3   chdir 3   sysconf 3   include  lt signal h gt      signal 3   include  lt sys stat h gt    umask 3   include  lt syslog h gt      syslog 3   openlog 3   closelog 3    And here is a more general function   int daemonize char  name  char  path  char  outfile  char  errfile  char  infile         if  path    path            if  name    name  medaemon         if  infile    infile   dev null         if  outfile    outfile   dev null         if  errfile    errfile   dev null           printf   s  s  s  s n  name path outfile infile       pid t child        fork  detach from process group leader     if   child fork    lt 0       failed fork         fprintf stderr  error  failed fork n            exit EXIT FAILURE             if  child gt 0      parent         exit EXIT SUCCESS             if  setsid   lt 0       failed to become session leader         fprintf stderr  error  failed setsid n            exit EXIT FAILURE                catch ignore signals     signal SIGCHLD SIG IGN       signal SIGHUP SIG IGN          fork second time     if    child fork    lt 0      failed fork         fprintf stderr  error  failed fork n            exit EXIT FAILURE             if  child gt 0       parent         exit EXIT SUCCESS                new file permissions     umask 0         change to path directory     chdir path          Close all open file descriptors     int fd      for  fd sysconf  SC OPEN MAX   fd gt 0  --fd                 close fd                reopen stdin  stdout  stderr     stdin fopen infile  r        fd 0     stdout fopen outfile  w        fd 1     stderr fopen errfile  w        fd 2        open syslog     openlog name LOG PID LOG DAEMON       return 0       Here is a sample program  which becomes a daemon  hangs around  and then leaves   int main         int res      int ttl 120      int delay 5      if   res daemonize  mydaemon    tmp  NULL NULL NULL      0             fprintf stderr  error  daemonize failed n            exit EXIT FAILURE             while  ttl gt 0               daemon code here         syslog LOG NOTICE  daemon ttl  d  ttl           sleep delay           ttl- delay            syslog LOG NOTICE  daemon ttl expired        closelog        return EXIT SUCCESS       Note that SIG IGN indicates to catch and ignore the signal   You could build a signal handler that can log signal receipt  and set flags  such as a flag to indicate graceful shutdown

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In Linux i want to add a daemon that cannot be stopped and which monitors filesystem changes  If any changes would be detected it should write the path to the console where it was started   a newline    Daemons work in the background and  usually     don t belong to a TTY that s why you can t use stdout stderr in the way you probably want  Usually a syslog daemon  syslogd  is used for logging messages to files  debug  error        Besides that  there are a few required steps to daemonize a process     If I remember correctly these steps are    fork off the parent process  amp  let it terminate if forking was successful  -  Because the parent process has terminated  the child process now runs in the background  setsid - Create a new session  The calling process becomes the leader of the new session and the process group leader of the new process group  The process is now detached from its controlling terminal  CTTY   Catch signals - Ignore and or handle signals  fork again  amp  let the parent process terminate to ensure that you get rid of the session leading process   Only session leaders may get a TTY again   chdir - Change the working directory of the daemon  umask - Change the file mode mask according to the needs of the daemon  close - Close all open file descriptors that may be inherited from the parent process      To give you a starting point  Look at this skeleton code that shows the basic steps  This code can now also be forked on GitHub  Basic skeleton of a linux daemon        daemonize c    This example daemonizes a process  writes a few log messages     sleeps 20 seconds and terminates afterwards        include  lt stdio h gt   include  lt stdlib h gt   include  lt unistd h gt   include  lt signal h gt   include  lt sys types h gt   include  lt sys stat h gt   include  lt syslog h gt   static void skeleton daemon         pid t pid          Fork off the parent process        pid   fork            An error occurred        if  pid  lt  0          exit EXIT FAILURE           Success  Let the parent terminate        if  pid  gt  0          exit EXIT SUCCESS           On success  The child process becomes session leader        if  setsid    lt  0          exit EXIT FAILURE           Catch  ignore and handle signals          TODO  Implement a working signal handler        signal SIGCHLD  SIG IGN       signal SIGHUP  SIG IGN           Fork off for the second time       pid   fork            An error occurred        if  pid  lt  0          exit EXIT FAILURE           Success  Let the parent terminate        if  pid  gt  0          exit EXIT SUCCESS           Set new file permissions        umask 0           Change the working directory to the root directory           or another appropriated directory        chdir               Close all open file descriptors        int x      for  x   sysconf  SC OPEN MAX   x gt  0  x--                close  x                 Open the log file        openlog   firstdaemon   LOG PID  LOG DAEMON        int main         skeleton daemon         while  1                  TODO  Insert daemon code here          syslog  LOG NOTICE   First daemon started             sleep  20           break             syslog  LOG NOTICE   First daemon terminated         closelog         return EXIT SUCCESS         Compile the code  gcc -o firstdaemon daemonize c Start the daemon    firstdaemon Check if everything is working properly  ps -xj   grep firstdaemon The output should be similar to this one      ------ ------ ------ ------ ----- ------- ------ ------ ------ -----    PPID   PID    PGID   SID    TTY   TPGID   STAT   UID    TIME   CMD    ------ ------ ------ ------ ----- ------- ------ ------ ------ -----       1   3387   3386   3386            -1   S      1000   0 00          ------ ------ ------ ------ ----- ------- ------ ------ ------ -----    What you should see here is    The daemon has no controlling terminal  TTY      The parent process ID  PPID  is 1  The init process  The PID    SID which means that our process is NOT the session leader  because of the second fork    Because PID    SID our process can t take control of a TTY again   Reading the syslog    Locate your syslog file  Mine is here   var log syslog Do a  grep firstdaemon  var log syslog The output should be similar to this one       firstdaemon 3387   First daemon started    firstdaemon 3387   First daemon terminated     A note  In reality you would also want to implement a signal handler and set up the logging properly  Files  log levels       Further reading    Linux-UNIX-Programmierung - German Unix Daemon Server Programming

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By calling fork   you ve created a child process   If the fork is successful  fork returned a non-zero PID  execution will continue from this point from within the child process   In this case we want to gracefully exit the parent process and then continue our work in the child process   Maybe this will help  http   www netzmafia de skripten unix linux-daemon-howto html

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Try using the daemon function    include  lt unistd h gt   int daemon int nochdir  int noclose     From the man page      The daemon   function is for programs wishing to detach themselves   from the controlling terminal and run in the background as system   daemons       If nochdir is zero  daemon   changes the calling process s current   working directory to the root directory        otherwise  the current   working directory is left unchanged       If noclose is zero  daemon   redirects standard input  standard   output and standard error to  dev null  otherwise  no changes are   made to these file descriptors

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I can stop at the first requirement  A daemon which cannot be stopped       Not possible my friend  however  you can achieve the same with a much better tool  a kernel module   http   www infoq com articles inotify-linux-file-system-event-monitoring  All daemons can be stopped   Some are more easily stopped than others   Even a daemon pair with the partner in hold down  respawning the partner if lost  can be stopped   You just have to work a little harder at it

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If your app is one of         sh    bash       py    python       rb    ruby       coffee     coffee       php    php       pl     perl       js     node      and you don t mind a NodeJS dependency then install NodeJS and then   npm install -g pm2  pm2 start yourapp yourext --name  fred    where  yourext is one of the above  pm2 start yourapp yourext -i 0 --name  fred    run your app on all cores  pm2 list   To keep all apps running on reboot  and daemonise pm2    pm2 startup  pm2 save   Now you can   service pm2 stop restart start status    also easily allows you to watch for code changes in your app directory and auto restart the app process when a code change happens

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Daemon Template  I wrote a daemon template following the new-style daemon  link  You can find the entire template code on GitHub  here  Main cpp     This function will be called when the daemon receive a SIGHUP signal  void reload         LOG INFO  Reload function called        int main int argc  char   argv           The Daemon class is a singleton to avoid be instantiate more than once     Daemon amp  daemon   Daemon  instance           Set the reload function to be called in case of receiving a SIGHUP signal     daemon setReloadFunction reload          Daemon main loop     int count   0      while daemon IsRunning              LOG DEBUG  Count     count             std  this thread  sleep for std  chrono  seconds 1              LOG INFO  The daemon process ended gracefully         Daemon hpp  class Daemon       public       static Daemon amp  instance             static Daemon instance          return instance             void setReloadFunction std  function lt void   gt  func        bool IsRunning         private       std  function lt void   gt  m reloadFunc      bool m isRunning      bool m reload       Daemon        Daemon Daemon const amp     delete      void operator  Daemon const amp     delete       void Reload         static void signalHandler int signal         Daemon cpp  Daemon  Daemon         m isRunning   true      m reload   false      signal SIGINT  Daemon  signalHandler       signal SIGTERM  Daemon  signalHandler       signal SIGHUP  Daemon  signalHandler      void Daemon  setReloadFunction std  function lt void   gt  func        m reloadFunc   func     bool Daemon  IsRunning         if  m reload            m reload   false          m reloadFunc              return m isRunning     void Daemon  signalHandler int signal        LOG INFO  Interrup signal number     signal    recived         switch signal            case SIGINT          case SIGTERM                Daemon  instance   m isRunning   false              break                    case SIGHUP                Daemon  instance   m reload   true              break                      daemon-template service   Unit  Description Simple daemon template After network taget   Service  Type simple ExecStart  usr bin daemon-template --conf file  etc daemon-template daemon-tenplate conf ExecReload  bin kill -HUP  MAINPID User root StandardOutput syslog StandardError syslog SyslogIdentifier daemon-template   Install  WantedBy multi-user target

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A daemon is just a process in the background   If you want to start your program when the OS boots  on linux  you add your start command to  etc rc d rc local  run after all other scripts  or  etc startup sh   On windows  you make a service  register the service  and then set it to start automatically at boot in administration -  services panel

[c] Creating a daemon in Linux

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