1DUP(2)                     Linux Programmer's Manual                    DUP(2)
2
3
4

NAME

6       dup, dup2, dup3 - duplicate a file descriptor
7

SYNOPSIS

9       #include <unistd.h>
10
11       int dup(int oldfd);
12       int dup2(int oldfd, int newfd);
13
14       #define _GNU_SOURCE             /* See feature_test_macros(7) */
15       #include <fcntl.h>              /* Definition of O_* constants */
16       #include <unistd.h>
17
18       int dup3(int oldfd, int newfd, int flags);
19

DESCRIPTION

21       The  dup()  system  call allocates a new file descriptor that refers to
22       the same open file description as the descriptor oldfd.  (For an expla‐
23       nation  of open file descriptions, see open(2).)  The new file descrip‐
24       tor number is guaranteed to be the lowest-numbered file descriptor that
25       was unused in the calling process.
26
27       After a successful return, the old and new file descriptors may be used
28       interchangeably.  Since the two file descriptors refer to the same open
29       file description, they share file offset and file status flags; for ex‐
30       ample, if the file offset is modified by using lseek(2) on one  of  the
31       file  descriptors,  the  offset  is also changed for the other file de‐
32       scriptor.
33
34       The two file descriptors do not share file descriptor flags (the close-
35       on-exec  flag).   The close-on-exec flag (FD_CLOEXEC; see fcntl(2)) for
36       the duplicate descriptor is off.
37
38   dup2()
39       The dup2() system call performs the same task as dup(), but instead  of
40       using  the lowest-numbered unused file descriptor, it uses the file de‐
41       scriptor number specified in newfd.  In other words, the file  descrip‐
42       tor  newfd  is adjusted so that it now refers to the same open file de‐
43       scription as oldfd.
44
45       If the file descriptor newfd was previously open, it is  closed  before
46       being  reused; the close is performed silently (i.e., any errors during
47       the close are not reported by dup2()).
48
49       The steps of closing and reusing the file  descriptor  newfd  are  per‐
50       formed  atomically.   This  is  important,  because trying to implement
51       equivalent functionality using close(2) and dup() would be  subject  to
52       race  conditions,  whereby newfd might be reused between the two steps.
53       Such reuse could happen because the main program is  interrupted  by  a
54       signal  handler that allocates a file descriptor, or because a parallel
55       thread allocates a file descriptor.
56
57       Note the following points:
58
59       *  If oldfd is not a valid file descriptor, then the  call  fails,  and
60          newfd is not closed.
61
62       *  If oldfd is a valid file descriptor, and newfd has the same value as
63          oldfd, then dup2() does nothing, and returns newfd.
64
65   dup3()
66       dup3() is the same as dup2(), except that:
67
68       *  The caller can force the close-on-exec flag to be set  for  the  new
69          file  descriptor by specifying O_CLOEXEC in flags.  See the descrip‐
70          tion of the same flag in open(2) for reasons why this may be useful.
71
72       *  If oldfd equals newfd, then dup3() fails with the error EINVAL.
73

RETURN VALUE

75       On success, these system calls return the new file descriptor.  On  er‐
76       ror, -1 is returned, and errno is set to indicate the error.
77

ERRORS

79       EBADF  oldfd isn't an open file descriptor.
80
81       EBADF  newfd  is out of the allowed range for file descriptors (see the
82              discussion of RLIMIT_NOFILE in getrlimit(2)).
83
84       EBUSY  (Linux only) This may be returned by dup2() or dup3()  during  a
85              race condition with open(2) and dup().
86
87       EINTR  The  dup2() or dup3() call was interrupted by a signal; see sig‐
88              nal(7).
89
90       EINVAL (dup3()) flags contain an invalid value.
91
92       EINVAL (dup3()) oldfd was equal to newfd.
93
94       EMFILE The per-process limit on the number of open file descriptors has
95              been  reached  (see  the  discussion  of  RLIMIT_NOFILE in getr‐
96              limit(2)).
97

VERSIONS

99       dup3() was added to Linux in version 2.6.27; glibc support is available
100       starting with version 2.9.
101

CONFORMING TO

103       dup(), dup2(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
104
105       dup3() is Linux-specific.
106

NOTES

108       The  error  returned  by  dup2() is different from that returned by fc‐
109       ntl(..., F_DUPFD, ...)  when newfd is out of range.  On  some  systems,
110       dup2() also sometimes returns EINVAL like F_DUPFD.
111
112       If newfd was open, any errors that would have been reported at close(2)
113       time are lost.  If this is of concern, then—unless the program is  sin‐
114       gle-threaded and does not allocate file descriptors in signal handlers—
115       the correct approach is not to close newfd before calling  dup2(),  be‐
116       cause  of  the race condition described above.  Instead, code something
117       like the following could be used:
118
119           /* Obtain a duplicate of 'newfd' that can subsequently
120              be used to check for close() errors; an EBADF error
121              means that 'newfd' was not open. */
122
123           tmpfd = dup(newfd);
124           if (tmpfd == -1 && errno != EBADF) {
125               /* Handle unexpected dup() error. */
126           }
127
128           /* Atomically duplicate 'oldfd' on 'newfd'. */
129
130           if (dup2(oldfd, newfd) == -1) {
131               /* Handle dup2() error. */
132           }
133
134           /* Now check for close() errors on the file originally
135              referred to by 'newfd'. */
136
137           if (tmpfd != -1) {
138               if (close(tmpfd) == -1) {
139                   /* Handle errors from close. */
140               }
141           }
142

SEE ALSO

144       close(2), fcntl(2), open(2), pidfd_getfd(2)
145

COLOPHON

147       This page is part of release 5.13 of the Linux  man-pages  project.   A
148       description  of  the project, information about reporting bugs, and the
149       latest    version    of    this    page,    can     be     found     at
150       https://www.kernel.org/doc/man-pages/.
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154Linux                             2021-03-22                            DUP(2)
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