DAEMON(3)                 (2017-11-26)                  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, stan-
          dard 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 par-
          ent 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).  allbox; lb lb lb l l l.
          Interface Attribute Value T{ daemon() T}   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

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          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 ter-
          minal for the daemon.

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

     COLOPHON
          This page is part of release 5.10 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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