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-.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
-.\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
-.\" and Copyright (C) 2005, 2008 Michael Kerrisk <mtk.manpages@gmail.com>
-.\" and Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
-.\"
-.\" 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>:
-.\"   Fixed typos.
-.\" Modified 1997-01-31, Eric S. Raymond <esr@thyrsus.com>
-.\" Modified 2002-09-28, aeb
-.\" 2009-01-12, mtk, reordered text in DESCRIPTION and added some
-.\"     details for dup2().
-.\" 2008-10-09, mtk: add description of dup3()
-.\"
-.TH dup 2 2023-10-31 "Linux man-pages 6.7"
-.SH NAME
-dup, dup2, dup3 \- duplicate a file descriptor
-.SH LIBRARY
-Standard C library
-.RI ( libc ", " \-lc )
-.SH SYNOPSIS
-.nf
-.B #include <unistd.h>
-.P
-.BI "int dup(int " oldfd );
-.BI "int dup2(int " oldfd ", int " newfd );
-.P
-.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
-.BR "#include <fcntl.h>" "              /* Definition of " O_* " constants */"
-.B #include <unistd.h>
-.P
-.BI "int dup3(int " oldfd ", int " newfd ", int " flags );
-.fi
-.SH DESCRIPTION
-The
-.BR dup ()
-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.
-.P
-After a successful return,
-the old and new file descriptors may be used interchangeably.
-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 file descriptor.
-.P
-The two file descriptors do not share file descriptor flags
-(the close-on-exec flag).
-The close-on-exec flag
-.RB ( FD_CLOEXEC ;
-see
-.BR fcntl (2))
-for the duplicate descriptor is off.
-.\"
-.SS dup2()
-The
-.BR dup2 ()
-system call performs the same task as
-.BR dup (),
-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 .
-.P
-If the file descriptor
-.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 ()).
-.P
-The steps of closing and reusing the file descriptor
-.I newfd
-are performed
-.IR atomically .
-This is important, because trying to implement equivalent functionality using
-.BR close (2)
-and
-.BR dup ()
-would be
-subject to race conditions, whereby
-.I newfd
-might be reused between the two steps.
-Such reuse could happen because the main program is interrupted
-by a signal handler that allocates a file descriptor,
-or because a parallel thread allocates a file descriptor.
-.P
-Note the following points:
-.IP \[bu] 3
-If
-.I oldfd
-is not a valid file descriptor, then the call fails, and
-.I newfd
-is not closed.
-.IP \[bu]
-If
-.I oldfd
-is a valid file descriptor, and
-.I newfd
-has the same value as
-.IR oldfd ,
-then
-.BR dup2 ()
-does nothing, and returns
-.IR newfd .
-.\"
-.SS dup3()
-.BR dup3 ()
-is the same as
-.BR dup2 (),
-except that:
-.IP \[bu] 3
-The caller can force the close-on-exec flag to be set
-for the new file descriptor by specifying
-.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 \[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
-.I oldfd
-equals
-.IR newfd ,
-then
-.BR dup3 ()
-fails with the error
-.BR EINVAL .
-.SH RETURN VALUE
-On success, these system calls
-return the new file descriptor.
-On error, \-1 is returned, and
-.I errno
-is set to indicate the error.
-.SH ERRORS
-.TP
-.B EBADF
-.I oldfd
-isn't an open file descriptor.
-.TP
-.B EBADF
-.I newfd
-is out of the allowed range for file descriptors (see the discussion of
-.B RLIMIT_NOFILE
-in
-.BR getrlimit (2)).
-.TP
-.B EBUSY
-(Linux only) This may be returned by
-.BR dup2 ()
-or
-.BR dup3 ()
-during a race condition with
-.BR open (2)
-and
-.BR dup ().
-.TP
-.B EINTR
-The
-.BR dup2 ()
-or
-.BR dup3 ()
-call was interrupted by a signal; see
-.BR signal (7).
-.TP
-.B EINVAL
-.RB ( dup3 ())
-.I flags
-contain an invalid value.
-.TP
-.B EINVAL
-.RB ( dup3 ())
-.I oldfd
-was equal to
-.IR newfd .
-.TP
-.B EMFILE
-The per-process limit on the number of open file descriptors has been reached
-(see the discussion of
-.B RLIMIT_NOFILE
-in
-.BR getrlimit (2)).
-.SH STANDARDS
-.TP
-.BR dup ()
-.TQ
-.BR dup2 ()
-POSIX.1-2008.
-.TP
-.BR dup3 ()
-Linux.
-.SH HISTORY
-.TP
-.BR dup ()
-.TQ
-.BR dup2 ()
-POSIX.1-2001, SVr4, 4.3BSD.
-.\" SVr4 documents additional
-.\" EINTR and ENOLINK error conditions.  POSIX.1 adds EINTR.
-.\" The EBUSY return is Linux-specific.
-.TP
-.BR dup3 ()
-Linux 2.6.27,
-glibc 2.9.
-.SH NOTES
-The error returned by
-.BR dup2 ()
-is different from that returned by
-.BR fcntl( "..., " F_DUPFD ", ..." )
-when
-.I newfd
-is out of range.
-On some systems,
-.BR dup2 ()
-also sometimes returns
-.B EINVAL
-like
-.BR F_DUPFD .
-.P
-If
-.I newfd
-was open, any errors that would have been reported at
-.BR close (2)
-time are lost.
-If this is of concern,
-then\[em]unless the program is single-threaded and does not allocate
-file descriptors in signal handlers\[em]the correct approach is
-.I not
-to close
-.I newfd
-before calling
-.BR dup2 (),
-because of the race condition described above.
-Instead, code something like the following could be used:
-.P
-.in +4n
-.EX
-/* Obtain a duplicate of \[aq]newfd\[aq] that can subsequently
-   be used to check for close() errors; an EBADF error
-   means that \[aq]newfd\[aq] was not open. */
-\&
-tmpfd = dup(newfd);
-if (tmpfd == \-1 && errno != EBADF) {
-    /* Handle unexpected dup() error. */
-}
-\&
-/* Atomically duplicate \[aq]oldfd\[aq] on \[aq]newfd\[aq]. */
-\&
-if (dup2(oldfd, newfd) == \-1) {
-    /* Handle dup2() error. */
-}
-\&
-/* Now check for close() errors on the file originally
-   referred to by \[aq]newfd\[aq]. */
-\&
-if (tmpfd != \-1) {
-    if (close(tmpfd) == \-1) {
-        /* Handle errors from close. */
-    }
-}
-.EE
-.in
-.SH SEE ALSO
-.BR close (2),
-.BR fcntl (2),
-.BR open (2),
-.BR pidfd_getfd (2)