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30 .\" @(#)daemon.3 8.1 (Berkeley) 6/9/93
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32 .Dd September 3, 1999
33 .Dt DAEMON 3
34 .Os
35 .Sh NAME
36 .Nm daemon
37 .Nd run in the background
38 .Sh LIBRARY
39 .Lb libc
40 .Sh SYNOPSIS
41 .In stdlib.h
42 .Ft int
43 .Fn daemon "int nochdir" "int noclose"
44 .Sh DESCRIPTION
45 The
46 .Fn daemon
47 function is for programs wishing to detach themselves from the
48 controlling terminal and run in the background as system daemons.
49 .Pp
50 Unless the argument
51 .Fa nochdir
52 is non-zero,
53 .Fn daemon
54 changes the current working directory to the root
55 .Pq Pa / .
56 .Pp
57 Unless the argument
58 .Fa noclose
59 is non-zero,
60 .Fn daemon
61 will redirect standard input, standard output and standard error
62 to
63 .Pa /dev/null .
64 .Sh RETURN VALUES
65 On return 0 indicates success with \-1 indicating error.
66 .Sh ERRORS
67 The function
68 .Fn daemon
69 may fail and set
70 .Va errno
71 for any of the errors specified for the library functions
72 .Xr fork 2
73 and
74 .Xr setsid 2 .
75 .Sh SEE ALSO
76 .Xr fork 2 ,
77 .Xr setsid 2
78 .Sh HISTORY
79 The
80 .Fn daemon
81 function first appeared in
82 .Bx 4.4 .
83 .Sh CAVEATS
84 Unless the
85 .Ar noclose
86 argument is non-zero,
87 .Fn daemon
88 will close the first three file descriptors and redirect them to
89 .Pa /dev/null .
90 Normally, these correspond to standard input, standard output and
91 standard error.
92 However, if any of those file descriptors refer to something else they
93 will still be closed, resulting in incorrect behavior of the calling program.
94 This can happen if any of standard input, standard output or standard error
95 have been closed before the program was run.
96 Programs using
97 .Fn daemon
98 should therefore make sure to either call
99 .Fn daemon
100 before opening any files or sockets or, alternately, verifying that
101 any file descriptors obtained have a value greater than 2.
102 .Sh BUGS
103 .Fn daemon
104 uses
105 .Fn fork
106 as part of its tty detachment mechanism.
107 Consequently the process id changes when
108 .Fn daemon
109 is invoked.
110 Processes employing
111 .Fn daemon
112 can not be reliably waited upon until
113 .Fn daemon
114 has been invoked.