1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
|
.\" Copyright (c) 1980, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" @(#)dup.2 8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd June 4, 1993
.Dt DUP 2
.Os BSD 4
.Sh NAME
.Nm dup ,
.Nm dup2
.Nd duplicate an existing file descriptor
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn dup "int oldd"
.Ft int
.Fn dup2 "int oldd" "int newd"
.Sh DESCRIPTION
.Fn Dup
duplicates an existing object descriptor and returns its value to
the calling process
.Fa ( newd
=
.Fn dup oldd ) .
The argument
.Fa oldd
is a small non-negative integer index in
the per-process descriptor table. The value must be less
than the size of the table, which is returned by
.Xr getdtablesize 2 .
The new descriptor returned by the call
is the lowest numbered descriptor
currently not in use by the process.
.Pp
The object referenced by the descriptor does not distinguish
between
.Fa oldd
and
.Fa newd
in any way.
Thus if
.Fa newd
and
.Fa oldd
are duplicate references to an open
file,
.Xr read 2 ,
.Xr write 2
and
.Xr lseek 2
calls all move a single pointer into the file,
and append mode, non-blocking I/O and asynchronous I/O options
are shared between the references.
If a separate pointer into the file is desired, a different
object reference to the file must be obtained by issuing an
additional
.Xr open 2
call.
The close-on-exec flag on the new file descriptor is unset.
.Pp
In
.Fn dup2 ,
the value of the new descriptor
.Fa newd
is specified. If this descriptor is already
in use, the descriptor is first deallocated as if a
.Xr close 2
call had been done first.
.Sh IMPLEMENTATION NOTES
.Pp
In the non-threaded library
.Fn dup
is implemented as the
.Va dup
syscall.
.Pp
In the threaded library, the
.Va dup
syscall is assembled to
.Fn _thread_sys_dup
and
.Fn dup
is implemented as a function which locks
.Va oldd
for read and write, then calls
.Fn _thread_sys_dup .
Before returning,
.Fn dup
unlocks
.Va oldd .
.Pp
In the non-threaded library
.Fn dup2
is implemented as the
.Va dup2
syscall.
.Pp
In the threaded library, the
.Va dup2
syscall is assembled to
.Fn _thread_sys_dup2
and
.Fn dup2
is implemented as a function which locks both
.Va oldd
and
.Va newd
for read and write, then calls
.Fn _thread_sys_dup2 .
Before returning,
.Fn dup2
unlocks
.Va oldd .
and
.Va newd .
.Sh RETURN VALUES
The value -1 is returned if an error occurs in either call.
The external variable
.Va errno
indicates the cause of the error.
.Sh ERRORS
.Fn Dup
and
.Fn dup2
fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa Oldd
or
.Fa newd
is not a valid active descriptor
.It Bq Er EMFILE
Too many descriptors are active.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr open 2 ,
.Xr close 2 ,
.Xr fcntl 2 ,
.Xr pipe 2 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr getdtablesize 2
.Sh STANDARDS
The
.Fn dup
and
.Fn dup2
function calls are expected to conform to
.St -p1003.1-90 .
.Sh HISTORY
A
.Fn dup
and a
.Fn dup2
function call appeared in
.At v7 .
|
