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
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
|
.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
.\" and Copyright (C) 2005, 2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.\" Modified 1993-07-21, Rik Faith <faith@cs.unc.edu>
.\" Modified 1994-08-21, Michael Chastain <mec@shell.portal.com>:
.\" Fixed typoes.
.\" Modified 1997-01-31, Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2002-09-28, aeb
.\" 2009-01-12, mtk, reordered text in DESCRIPTION and added some
.\" details for dup2().
.\" 2008-10-09, mtk: add description of dup3()
.\"
.TH DUP 2 2014-07-08 "Linux" "Linux Programmer's Manual"
.SH NAME
dup, dup2, dup3 \- duplicate a file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.sp
.BI "int dup(int " oldfd );
.BI "int dup2(int " oldfd ", int " newfd );
.sp
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
.BR "#include <fcntl.h>" " /* Obtain O_* constant definitions */
.B #include <unistd.h>
.sp
.BI "int dup3(int " oldfd ", int " newfd ", int " flags );
.fi
.SH DESCRIPTION
The
.BR dup ()
system call creates a copy of the file descriptor
.IR oldfd ,
using the lowest-numbered unused descriptor for the new descriptor.
After a successful return,
the old and new file descriptors may be used interchangeably.
They refer to the same open file description (see
.BR open (2))
and thus share file offset and file status flags;
for example, if the file offset is modified by using
.BR lseek (2)
on one of the descriptors, the offset is also changed for the other.
The two descriptors do not share file descriptor flags
(the close-on-exec flag).
The close-on-exec flag
.RB ( FD_CLOEXEC ;
see
.BR fcntl (2))
for the duplicate descriptor is off.
.\"
.SS dup2()
The
.BR dup2 ()
system call performs the same task as
.BR dup (),
but instead of using the lowest-numbered unused file descriptor,
it uses the descriptor number specified in
.IR newfd .
If the descriptor
.IR newfd
was previously open, it is silently closed before being reused.
The steps of closing and reusing the file descriptor
.IR newfd
are performed
.IR atomically .
This is important, because trying to implement equivalent functionality using
.BR close (2)
and
.BR dup ()
would be
subject to race conditions, whereby
.I newfd
might be reused between the two steps.
Such reuse could happen because the main program is interrupted
by a signal handler that allocates a file descriptor,
or because a parallel thread allocates a file descriptor.
Note the following points:
.IP * 3
If
.I oldfd
is not a valid file descriptor, then the call fails, and
.I newfd
is not closed.
.IP *
If
.I oldfd
is a valid file descriptor, and
.I newfd
has the same value as
.IR oldfd ,
then
.BR dup2 ()
does nothing, and returns
.IR newfd .
.\"
.SS dup3()
.BR dup3 ()
is the same as
.BR dup2 (),
except that:
.IP * 3
The caller can force the close-on-exec flag to be set
for the new file descriptor by specifying
.BR O_CLOEXEC
in
.IR flags .
See the description of the same flag in
.BR open (2)
for reasons why this may be useful.
.IP *
.\" FIXME . To confirm with Al Viro that this was intended, and its rationale
If
.IR oldfd
equals
.IR newfd ,
then
.BR dup3 ()
fails with the error
.BR EINVAL .
.SH RETURN VALUE
On success, these system calls
return the new descriptor.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EBADF
.I oldfd
isn't an open file descriptor, or
.I newfd
is out of the allowed range for file descriptors.
.TP
.B EBUSY
(Linux only) This may be returned by
.BR dup2 ()
or
.BR dup3 ()
during a race condition with
.BR open (2)
and
.BR dup ().
.TP
.B EINTR
The
.BR dup2 ()
or
.BR dup3 ()
call was interrupted by a signal; see
.BR signal (7).
.TP
.B EINVAL
.RB ( dup3 ())
.I flags
contain an invalid value.
.\" FIXME . To confirm with Al Viro that this was intended, and its rationale
Or,
.I oldfd
was equal to
.IR newfd .
.TP
.B EMFILE
The process already has the maximum number of file
descriptors open and tried to open a new one.
.SH VERSIONS
.BR dup3 ()
was added to Linux in version 2.6.27;
glibc support is available starting with
version 2.9.
.SH CONFORMING TO
.BR dup (),
.BR dup2 ():
SVr4, 4.3BSD, POSIX.1-2001.
.BR dup3 ()
is Linux-specific.
.\" SVr4 documents additional
.\" EINTR and ENOLINK error conditions. POSIX.1 adds EINTR.
.\" The EBUSY return is Linux-specific.
.SH NOTES
The error returned by
.BR dup2 ()
is different from that returned by
.BR fcntl( "..., " F_DUPFD ", ..." )
when
.I newfd
is out of range.
On some systems,
.BR dup2 ()
also sometimes returns
.B EINVAL
like
.BR F_DUPFD .
If
.I newfd
was open, any errors that would have been reported at
.BR close (2)
time are lost.
If this is of concern,
then\(emunless the program is single-threaded and does not allocate
file descriptors in signal handlers\(emthe correct approach is
.I not
to close
.I newfd
before calling
.BR dup2 (),
because of the race condition described above.
Instead, code something like the following could be used:
.nf
/* Obtain a duplicate of 'newfd' that can subsequently
be used to check for close() errors; an EBADF error
means that 'newfd' was not open. */
tmpfd = dup(newfd);
if (tmpfd == \-1 && errno != EBADF) {
/* Handle unexpected dup() error */
}
/* Atomically duplicate 'oldfd' on 'newfd' */
if (dup2(oldfd, newfd) == \-1) {
/* Handle dup2() error */
}
/* Now check for close() errors on the file originally
referred to by 'newfd' */
if (tmpfd != \-1) {
if (close(tmpfd) == \-1) {
/* Handle errors from close */
}
}
.fi
.SH SEE ALSO
.BR close (2),
.BR fcntl (2),
.BR open (2)
|
