1DUP(3P) POSIX Programmer's Manual DUP(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
13 dup, dup2 — duplicate an open file descriptor
14
16 #include <unistd.h>
17 int dup(int fildes);
19 int dup2(int fildes, int fildes2);
20
22 The dup() function provides an alternative interface to the service
23 provided by fcntl() using the F_DUPFD command. The call dup(fildes)
24 shall be equivalent to:
25 fcntl(fildes, F_DUPFD, 0);
27 The dup2() function shall cause the file descriptor fildes2 to refer to
29 the same open file description as the file descriptor fildes and to
30 share any locks, and shall return fildes2. If fildes2 is already a
31 valid open file descriptor, it shall be closed first, unless fildes is
32 equal to fildes2 in which case dup2() shall return fildes2 without
33 closing it. If the close operation fails to close fildes2, dup2() shall
34 return −1 without changing the open file description to which fildes2
35 refers. If fildes is not a valid file descriptor, dup2() shall return
36 −1 and shall not close fildes2. If fildes2 is less than 0 or greater
37 than or equal to {OPEN_MAX}, dup2() shall return −1 with errno set to
38 [EBADF].
39 Upon successful completion, if fildes is not equal to fildes2, the
41 FD_CLOEXEC flag associated with fildes2 shall be cleared. If fildes is
42 equal to fildes2, the FD_CLOEXEC flag associated with fildes2 shall not
43 be changed.
44 If fildes refers to a typed memory object, the result of the dup2()
46 function is unspecified.
47
49 Upon successful completion a non-negative integer, namely the file
50 descriptor, shall be returned; otherwise, −1 shall be returned and
51 errno set to indicate the error.
52
54 The dup() function shall fail if:
55 EBADF The fildes argument is not a valid open file descriptor.
57 EMFILE All file descriptors available to the process are currently
59 open.
60 The dup2() function shall fail if:
62 EBADF The fildes argument is not a valid open file descriptor or the
64 argument fildes2 is negative or greater than or equal to
65 {OPEN_MAX}.
66 EINTR The dup2() function was interrupted by a signal.
68 The dup2() function may fail if:
70 EIO An I/O error occurred while attempting to close fildes2.
72 The following sections are informative.
74
76 Redirecting Standard Output to a File S
77 The following example closes standard output for the current processes,
78 re-assigns standard output to go to the file referenced by pfd, and
79 closes the original file descriptor to clean up.
80 #include <unistd.h>
82 ...
83 int pfd;
84 ...
85 close(1);
86 dup(pfd);
87 close(pfd);
88 ...
89 Redirecting Error Messages
91 The following example redirects messages from stderr to stdout.
92 #include <unistd.h>
94 ...
95 dup2(1, 2);
96 ...
97
99 Implementations may use file descriptors that must be inherited into
100 child processes for the child process to remain conforming, such as for
101 message catalog or tracing purposes. Therefore, an application that
102 calls dup2() with an arbitrary integer for fildes2 risks non-conforming
103 behavior, and dup2() can only portably be used to overwrite file
104 descriptor values that the application has obtained through explicit
105 actions, or for the three file descriptors corresponding to the stan‐
106 dard file streams. In order to avoid a race condition of leaking an
107 unintended file descriptor into a child process, an application should
108 consider opening all file descriptors with the FD_CLOEXEC bit set
109 unless the file descriptor is intended to be inherited across exec.
110
112 The dup() function is redundant. Its services are also provided by the
113 fcntl() function. It has been included in this volume of POSIX.1‐2008
114 primarily for historical reasons, since many existing applications use
115 it. On the other hand, the dup2() function provides unique services, as
116 no other interface is able to atomically replace an existing file
117 descriptor.
118 The dup2() function is not marked obsolescent because it presents a
120 type-safe version of functionality provided in a type-unsafe version by
121 fcntl(). It is used in the POSIX Ada binding.
122 The dup2() function is not intended for use in critical regions as a
124 synchronization mechanism.
125 In the description of [EBADF], the case of fildes being out of range is
127 covered by the given case of fildes not being valid. The descriptions
128 for fildes and fildes2 are different because the only kind of invalid‐
129 ity that is relevant for fildes2 is whether it is out of range; that
130 is, it does not matter whether fildes2 refers to an open file when the
131 dup2() call is made.
132
134 None.
135
137 close(), fcntl(), open()
138 The Base Definitions volume of POSIX.1‐2008, <unistd.h>
140
142 Portions of this text are reprinted and reproduced in electronic form
143 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
144 -- Portable Operating System Interface (POSIX), The Open Group Base
145 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
146 cal and Electronics Engineers, Inc and The Open Group. (This is
147 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
148 event of any discrepancy between this version and the original IEEE and
149 The Open Group Standard, the original IEEE and The Open Group Standard
150 is the referee document. The original Standard can be obtained online
151 at http://www.unix.org/online.html .
152 Any typographical or formatting errors that appear in this page are
154 most likely to have been introduced during the conversion of the source
155 files to man page format. To report such errors, see https://www.ker‐
156 nel.org/doc/man-pages/reporting_bugs.html .
157IEEE/The Open Group 2013 DUP(3P)