gen/FreeBSD/daemon.3

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  28 .\"     @(#)daemon.3    8.1 (Berkeley) 6/9/93
  29 .\" $FreeBSD: src/lib/libc/gen/daemon.3,v 1.15 2007/01/09 00:27:53 imp Exp $
  30 .\"
  31 .Dd June 9, 1993
  32 .Dt DAEMON 3
  33 .Os
  34 .Sh NAME
  35 .Nm daemon
  36 .Nd run in the background
  37 .Sh LIBRARY
  38 .Lb libc
  39 .Sh SYNOPSIS
  40 .In stdlib.h
  41 .Ft int
  42 .Fn daemon "int nochdir" "int noclose"
  43 .Sh DESCRIPTION
  44 The
  45 .Fn daemon
  46 function is for programs wishing to detach themselves from the
  47 controlling terminal and run in the background as system daemons.
  48 .Pp
  49 Unless the argument
  50 .Fa nochdir
  51 is non-zero,
  52 .Fn daemon
  53 changes the current working directory to the root
  54 .Pq Pa / .
  55 .Pp
  56 Unless the argument
  57 .Fa noclose
  58 is non-zero,
  59 .Fn daemon
  60 will redirect standard input, standard output, and standard error to
  61 .Pa /dev/null .
  62 .Sh RETURN VALUES
  63 .Rv -std daemon
  64 .Sh ERRORS
  65 The
  66 .Fn daemon
  67 function may fail and set
  68 .Va errno
  69 for any of the errors specified for the library functions
  70 .Xr fork 2
  71 and
  72 .Xr setsid 2 .
  73 .Sh SEE ALSO
  74 .Xr fork 2 ,
  75 .Xr setsid 2 ,
  76 .Xr sigaction 2
  77 .Sh HISTORY
  78 The
  79 .Fn daemon
  80 function first appeared in
  81 .Bx 4.4 .
  82 .Sh CAVEATS
  83 Unless the
  84 .Fa noclose
  85 argument is non-zero,
  86 .Fn daemon
  87 will close the first three file descriptors and redirect them to
  88 .Pa /dev/null .
  89 Normally, these correspond to standard input, standard output, and
  90 standard error.
  91 However, if any of those file descriptors refer to something else, they
  92 will still be closed, resulting in incorrect behavior of the calling program.
  93 This can happen if any of standard input, standard output, or standard
  94 error have been closed before the program was run.
  95 Programs using
  96 .Fn daemon
  97 should therefore either call
  98 .Fn daemon
  99 before opening any files or sockets, or verify that any file
 100 descriptors obtained have values greater than 2.
 101 .Pp
 102 The
 103 .Fn daemon
 104 function temporarily ignores
 105 .Dv SIGHUP
 106 while calling
 107 .Xr setsid 2
 108 to prevent a parent session group leader's calls to
 109 .Xr fork 2
 110 and then
 111 .Xr _exit 2
 112 from prematurely terminating the child process.