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.\" and Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
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+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified 1993-07-21, Rik Faith <faith@cs.unc.edu>
.\" Modified 1994-08-21, Michael Chastain <mec@shell.portal.com>:
.\" details for dup2().
.\" 2008-10-09, mtk: add description of dup3()
.\"
-.TH DUP 2 2020-04-11 "Linux" "Linux Programmer's Manual"
+.TH dup 2 (date) "Linux man-pages (unreleased)"
.SH NAME
dup, dup2, dup3 \- duplicate a file descriptor
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int dup(int " oldfd );
.BI "int dup2(int " oldfd ", int " newfd );
-
+.PP
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
-.BR "#include <fcntl.h>" " /* Obtain O_* constant definitions */
+.BR "#include <fcntl.h>" " /* Definition of " O_* " constants */"
.B #include <unistd.h>
.PP
.BI "int dup3(int " oldfd ", int " newfd ", int " flags );
.SH DESCRIPTION
The
.BR dup ()
-system call creates a copy of the file descriptor
-.IR oldfd ,
-using the lowest-numbered unused file descriptor for the new descriptor.
+system call allocates a new file descriptor that refers to the same
+open file description as the descriptor
+.IR oldfd .
+(For an explanation of open file descriptions, see
+.BR open (2).)
+The new file descriptor number is guaranteed to be the lowest-numbered
+file descriptor that was unused in the calling process.
.PP
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;
+Since the two file descriptors refer to the same open file description,
+they share file offset and file status flags;
for example, if the file offset is modified by using
.BR lseek (2)
-on one of the file descriptors, the offset is also changed for the other.
+on one of the file descriptors,
+the offset is also changed for the other file descriptor.
.PP
The two file descriptors do not share file descriptor flags
(the close-on-exec flag).
but instead of using the lowest-numbered unused file descriptor,
it uses the file descriptor number specified in
.IR newfd .
+In other words,
+the file descriptor
+.I newfd
+is adjusted so that it now refers to the same open file description as
+.IR oldfd .
+.PP
If the file descriptor
-.IR newfd
-was previously open, it is silently closed before being reused.
+.I newfd
+was previously open, it is closed before being reused;
+the close is performed silently
+(i.e., any errors during the close are not reported by
+.BR dup2 ()).
.PP
The steps of closing and reusing the file descriptor
-.IR newfd
+.I newfd
are performed
.IR atomically .
This is important, because trying to implement equivalent functionality using
or because a parallel thread allocates a file descriptor.
.PP
Note the following points:
-.IP * 3
+.IP \(bu 3
If
.I oldfd
is not a valid file descriptor, then the call fails, and
.I newfd
is not closed.
-.IP *
+.IP \(bu
If
.I oldfd
is a valid file descriptor, and
is the same as
.BR dup2 (),
except that:
-.IP * 3
+.IP \(bu 3
The caller can force the close-on-exec flag to be set
for the new file descriptor by specifying
-.BR O_CLOEXEC
+.B O_CLOEXEC
in
.IR flags .
See the description of the same flag in
.BR open (2)
for reasons why this may be useful.
-.IP *
+.IP \(bu
.\" Ulrich Drepper, LKML, 2008-10-09:
.\" We deliberately decided on this change. Otherwise, what is the
.\" result of dup3(fd, fd, O_CLOEXEC)?
If
-.IR oldfd
+.I oldfd
equals
.IR newfd ,
then
return the new file descriptor.
On error, \-1 is returned, and
.I errno
-is set appropriately.
+is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.B EBADF
.I newfd
is out of the allowed range for file descriptors (see the discussion of
-.BR RLIMIT_NOFILE
+.B RLIMIT_NOFILE
in
.BR getrlimit (2)).
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached
(see the discussion of
-.BR RLIMIT_NOFILE
+.B RLIMIT_NOFILE
in
.BR getrlimit (2)).
.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
+was added in Linux 2.6.27;
+glibc support is available since glibc 2.9.
+.SH STANDARDS
.BR dup (),
.BR dup2 ():
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
.PP
.in +4n
.EX
-/* Obtain a duplicate of 'newfd' that can subsequently
+/* Obtain a duplicate of \(aqnewfd\(aq that can subsequently
be used to check for close() errors; an EBADF error
- means that 'newfd' was not open. */
+ means that \(aqnewfd\(aq was not open. */
tmpfd = dup(newfd);
if (tmpfd == \-1 && errno != EBADF) {
- /* Handle unexpected dup() error */
+ /* Handle unexpected dup() error. */
}
-/* Atomically duplicate 'oldfd' on 'newfd' */
+/* Atomically duplicate \(aqoldfd\(aq on \(aqnewfd\(aq. */
if (dup2(oldfd, newfd) == \-1) {
- /* Handle dup2() error */
+ /* Handle dup2() error. */
}
/* Now check for close() errors on the file originally
- referred to by 'newfd' */
+ referred to by \(aqnewfd\(aq. */
if (tmpfd != \-1) {
if (close(tmpfd) == \-1) {
- /* Handle errors from close */
+ /* Handle errors from close. */
}
}
.EE
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