daemon(3)                  Library Functions Manual                  daemon(3)

NAME
       daemon - run in the background

LIBRARY
       Standard C library (libc, -lc)

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  di-
       rectory to the root directory ("/"); otherwise, the current working di-
       rectory 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 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-
       sid(2).

ATTRIBUTES
       For an  explanation  of  the  terms  used  in  this  section,  see  at-
       tributes(7).

       +--------------------------------------------+---------------+---------+
       |Interface                                   | Attribute     | Value   |
       +--------------------------------------------+---------------+---------+
       |daemon()                                    | Thread safety | MT-Safe |
       +--------------------------------------------+---------------+---------+

STANDARDS
       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), 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.,  Lin-
       ux), this means that if the daemon opens a terminal that is not already
       a controlling terminal for another session, then that terminal will in-
       advertently become the controlling terminal for the daemon.

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

Linux man-pages 6.03              2022-12-15                         daemon(3)

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