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daemon(3)


NAME

   daemon - run in the background

SYNOPSIS

   #include <unistd.h>

   int daemon(int nochdir, int noclose);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   daemon():
       Since glibc 2.21:
           _DEFAULT_SOURCE
       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)

DESCRIPTION

   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.

RETURN VALUE

   (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
   success daemon() returns zero.  If an error occurs, daemon() returns -1
   and  sets  errno  to  any  of  the errors specified for the fork(2) and
   setsid(2).

ATTRIBUTES

   For  an  explanation  of  the  terms  used   in   this   section,   see
   attributes(7).

   
   Interface  Attribute      Value   
   
   daemon()   Thread safety  MT-Safe

CONFORMING TO

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

NOTES

   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.

BUGS

   The GNU C library implementation of this function was taken  from  BSD,
   and   does   not  employ  the  double-fork  technique  (i.e.,  fork(2),
   setsid(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.

SEE ALSO

   fork(2), setsid(2)

COLOPHON

   This  page  is  part of release 4.09 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
   https://www.kernel.org/doc/man-pages/.



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