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
     code 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 15.1-STABLE-HBSD         June 1, 2026                        DAEMON(3)