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
12 dup, dup2 — duplicate an open file descriptor
13
15 #include <unistd.h>
16 int dup(int fildes);
18 int dup2(int fildes, int fildes2);
19
21 The dup() function provides an alternative interface to the service
22 provided by fcntl() using the F_DUPFD command. The call dup(fildes)
23 shall be equivalent to:
24 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>
83 ...
84 int pfd;
85 ...
86 close(1);
87 dup(pfd);
88 close(pfd);
89 ...
90 Redirecting Error Messages
92 The following example redirects messages from stderr to stdout.
93 #include <unistd.h>
96 ...
97 dup2(1, 2);
98 ...
99
101 Implementations may use file descriptors that must be inherited into
102 child processes for the child process to remain conforming, such as for
103 message catalog or tracing purposes. Therefore, an application that
104 calls dup2() with an arbitrary integer for fildes2 risks non-conforming
105 behavior, and dup2() can only portably be used to overwrite file
106 descriptor values that the application has obtained through explicit
107 actions, or for the three file descriptors corresponding to the stan‐
108 dard file streams. In order to avoid a race condition of leaking an
109 unintended file descriptor into a child process, an application should
110 consider opening all file descriptors with the FD_CLOEXEC bit set
111 unless the file descriptor is intended to be inherited across exec.
112
114 The dup() function is redundant. Its services are also provided by the
115 fcntl() function. It has been included in this volume of POSIX.1‐2017
116 primarily for historical reasons, since many existing applications use
117 it. On the other hand, the dup2() function provides unique services, as
118 no other interface is able to atomically replace an existing file
119 descriptor.
120 The dup2() function is not marked obsolescent because it presents a
122 type-safe version of functionality provided in a type-unsafe version by
123 fcntl(). It is used in the POSIX Ada binding.
124 The dup2() function is not intended for use in critical regions as a
126 synchronization mechanism.
127 In the description of [EBADF], the case of fildes being out of range is
129 covered by the given case of fildes not being valid. The descriptions
130 for fildes and fildes2 are different because the only kind of invalid‐
131 ity that is relevant for fildes2 is whether it is out of range; that
132 is, it does not matter whether fildes2 refers to an open file when the
133 dup2() call is made.
134
136 None.
137
139 close(), fcntl(), open()
140 The Base Definitions volume of POSIX.1‐2017, <unistd.h>
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144 Portions of this text are reprinted and reproduced in electronic form
145 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
146 table Operating System Interface (POSIX), The Open Group Base Specifi‐
147 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
148 Electrical and Electronics Engineers, Inc and The Open Group. In the
149 event of any discrepancy between this version and the original IEEE and
150 The Open Group Standard, the original IEEE and The Open Group Standard
151 is the referee document. The original Standard can be obtained online
152 at http://www.opengroup.org/unix/online.html .
153 Any typographical or formatting errors that appear in this page are
155 most likely to have been introduced during the conversion of the source
156 files to man page format. To report such errors, see https://www.ker‐
157 nel.org/doc/man-pages/reporting_bugs.html .
158IEEE/The Open Group 2017 DUP(3P)