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.

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

     Unless the argument noclose is non-zero, daemon() will redirect 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
     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 13.1-RELEASE-p6        December 23, 2017       FreeBSD 13.1-RELEASE-p6