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FreeBSD 11.0 - man page for daemon (freebsd section 3)

DAEMON(3)						   BSD 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);
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.
RETURN VALUES
The daemon() function returns 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() function may fail and set errno for any of the errors specified for the library functions fork(2) and setsid(2).
SEE ALSO
fork(2), setsid(2), sigaction(2)
HISTORY
The daemon() function first appeared in 4.4BSD.
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 out- put, 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.
BSD
June 9, 1993 BSD