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DAEMON(3) Linux Programmer's Manual DAEMON(3)


     daemon - run in the background


     #include <unistd.h>
     int daemon(int nochdir, int noclose);
 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
         Since glibc 2.21:
         In glibc 2.19 and 2.20:
             _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
         Up to and including glibc 2.19:
             _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)


     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  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.


     (This function forks, and if the fork(2)  succeeds,  the  parent  calls
     _exit(2),  so that further errors are seen by the child only.)  On suc-
     cess daemon() returns zero.  If an error occurs,  daemon()  returns  -1
     and  sets errno to any of the errors specified for the fork(2) and set-


     For  an  explanation  of  the  terms  used   in   this   section,   see
     |Interface | Attribute     | Value   |
     |daemon()  | Thread safety | MT-Safe |


     Not  in POSIX.1.  A similar function appears on the BSDs.  The daemon()
     function first appeared in 4.4BSD.


     The glibc implementation can also return -1 when /dev/null  exists  but
     is  not  a  character device with the expected major and minor numbers.
     In this case, errno need not be set.


     The GNU C library implementation of this function was taken  from  BSD,
     and  does  not  employ  the  double-fork technique (i.e., fork(2), set-
     sid(2), fork(2)) that is necessary to ensure that the resulting  daemon
     process  is  not  a session leader.  Instead, the resulting daemon is a
     session leader.  On systems  that  follow  System  V  semantics  (e.g.,
     Linux),  this  means  that  if  the daemon opens a terminal that is not
     already a controlling terminal for another session, then that  terminal
     will inadvertently become the controlling terminal for the daemon.


     fork(2), setsid(2), daemon(7), logrotate(8)


     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at

GNU 2017-11-26 DAEMON(3)

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