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>              /* Obtain O_* constant definitions */
16       #include <unistd.h>
17
18       int dup3(int oldfd, int newfd, int flags);
19

DESCRIPTION

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

RETURN VALUE

66       On  success, these system calls return the new file descriptor.  On er‐
67       ror, -1 is returned, and errno is set appropriately.
68

ERRORS

70       EBADF  oldfd isn't an open file descriptor.
71
72       EBADF  newfd is out of the allowed range for file descriptors (see  the
73              discussion of RLIMIT_NOFILE in getrlimit(2)).
74
75       EBUSY  (Linux  only)  This may be returned by dup2() or dup3() during a
76              race condition with open(2) and dup().
77
78       EINTR  The dup2() or dup3() call was interrupted by a signal; see  sig‐
79              nal(7).
80
81       EINVAL (dup3()) flags contain an invalid value.
82
83       EINVAL (dup3()) oldfd was equal to newfd.
84
85       EMFILE The per-process limit on the number of open file descriptors has
86              been reached (see  the  discussion  of  RLIMIT_NOFILE  in  getr‐
87              limit(2)).
88

VERSIONS

90       dup3() was added to Linux in version 2.6.27; glibc support is available
91       starting with version 2.9.
92

CONFORMING TO

94       dup(), dup2(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
95
96       dup3() is Linux-specific.
97

NOTES

99       The error returned by dup2() is different from  that  returned  by  fc‐
100       ntl(...,  F_DUPFD,  ...)  when newfd is out of range.  On some systems,
101       dup2() also sometimes returns EINVAL like F_DUPFD.
102
103       If newfd was open, any errors that would have been reported at close(2)
104       time  are lost.  If this is of concern, then—unless the program is sin‐
105       gle-threaded and does not allocate file descriptors in signal handlers—
106       the  correct  approach is not to close newfd before calling dup2(), be‐
107       cause of the race condition described above.  Instead,  code  something
108       like the following could be used:
109
110           /* Obtain a duplicate of 'newfd' that can subsequently
111              be used to check for close() errors; an EBADF error
112              means that 'newfd' was not open. */
113
114           tmpfd = dup(newfd);
115           if (tmpfd == -1 && errno != EBADF) {
116               /* Handle unexpected dup() error */
117           }
118
119           /* Atomically duplicate 'oldfd' on 'newfd' */
120
121           if (dup2(oldfd, newfd) == -1) {
122               /* Handle dup2() error */
123           }
124
125           /* Now check for close() errors on the file originally
126              referred to by 'newfd' */
127
128           if (tmpfd != -1) {
129               if (close(tmpfd) == -1) {
130                   /* Handle errors from close */
131               }
132           }
133

SEE ALSO

135       close(2), fcntl(2), open(2), pidfd_getfd(2)
136

COLOPHON

138       This  page  is  part of release 5.10 of the Linux man-pages project.  A
139       description of the project, information about reporting bugs,  and  the
140       latest     version     of     this    page,    can    be    found    at
141       https://www.kernel.org/doc/man-pages/.
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145Linux                             2020-11-01                            DUP(2)
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