DAEMON(3)               FreeBSD Library Functions Manual               DAEMON(3)

NAME
     daemon – run in the background

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     int
     daemon(int nochdir, int noclose);

     int
     daemonfd(int chdirfd, int nullfd);

DESCRIPTION
     The daemon() function is for programs wishing to detach themselves from the
     controlling terminal and run in the background as system daemons.  The
     fork(2) system call is used; see CAVEATS below about the environment after
     a fork() (without a corresponding call to one of the exec routines).

     If the argument nochdir is zero, daemon() changes the current working
     directory to the root (/).

     If the argument noclose is zero, daemon() redirects standard input,
     standard output, and standard error to /dev/null.

     The daemonfd() function is equivalent to the daemon() function except that
     arguments are the descriptors for the current working directory and to the
     descriptor to /dev/null.

     If chdirfd is equal to (-1) the current working directory is not changed.

     If nullfd is equals to (-1) the redirection of standard input, standard
     output, and standard error is not closed.

RETURN VALUES
     The daemon() and daemonfd() functions return the value 0 if successful;
     otherwise the value -1 is returned and the global variable errno is set to
     indicate the error.

ERRORS
     The daemon() and daemonfd() function may fail and set errno for any of the
     errors specified for the library functions fork(2), open(2), and setsid(2).

SEE ALSO
     fork(2), setsid(2), sigaction(2)

HISTORY
     The daemon() function first appeared in 4.4BSD.  The daemonfd() function
     first appeared in FreeBSD 12.0.

CAVEATS
     In multithreaded programs, the child process after fork() inherits copies
     of all mutexes and other synchronization state, but only the calling thread
     survives.  Any locks held by other threads at the time of the call will
     remain permanently acquired in the child, causing deadlocks in any library
     that attempts to acquire them.  Until one of the exec(3) functions is
     called, the child should restrict itself to async-signal safe operations
     (see sigaction(2)).

     Unless the noclose argument is non-zero, daemon() will close the first
     three file descriptors and redirect them to /dev/null.  Normally, these
     correspond to standard input, standard output, and standard error.
     However, if any of those file descriptors refer to something else, they
     will still be closed, resulting in incorrect behavior of the calling
     program.  This can happen if any of standard input, standard output, or
     standard error have been closed before the program was run.  Programs using
     daemon() should therefore either call daemon() before opening any files or
     sockets, or verify that any file descriptors obtained have values greater
     than 2.

     The daemon() function temporarily ignores SIGHUP while calling setsid(2) to
     prevent a parent session group leader's calls to fork(2) and then _exit(2)
     from prematurely terminating the child process.

FreeBSD 16.0-CURRENT              June 1, 2026                         DAEMON(3)