daemon(3) — Linux manual page


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)):

           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)

DESCRIPTION         top

       The daemon() function is for programs wishing to detach themselves
       from the controlling terminal and run in the background as system

       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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │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 5.09 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

GNU                              2017-11-26                        DAEMON(3)

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