daemon(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       daemon - run in the background

SYNOPSIS         top

       #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         top

       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         top

       (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         top

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

       ┌──────────────────────────────────────┬───────────────┬─────────┐
       │Interface                             Attribute     Value   │
       ├──────────────────────────────────────┼───────────────┼─────────┤
       │daemon()                              │ Thread safety │ MT-Safe │
       └──────────────────────────────────────┴───────────────┴─────────┘

CONFORMING TO         top

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

NOTES         top

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 5.11 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/.

GNU                            2021-03-22                      DAEMON(3)

Pages that refer to this page: fork(2)daemon(7)