mirror of https://github.com/mkerrisk/man-pages
295 lines
7.3 KiB
Groff
295 lines
7.3 KiB
Groff
.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
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.\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
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.\" and Copyright (C) 2005, 2008 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" and Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" Modified 1993-07-21, Rik Faith <faith@cs.unc.edu>
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.\" Modified 1994-08-21, Michael Chastain <mec@shell.portal.com>:
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.\" Fixed typos.
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.\" Modified 1997-01-31, Eric S. Raymond <esr@thyrsus.com>
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.\" Modified 2002-09-28, aeb
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.\" 2009-01-12, mtk, reordered text in DESCRIPTION and added some
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.\" details for dup2().
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.\" 2008-10-09, mtk: add description of dup3()
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.\"
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.TH DUP 2 2021-03-22 "Linux" "Linux Programmer's Manual"
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.SH NAME
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dup, dup2, dup3 \- duplicate a file descriptor
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.SH SYNOPSIS
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.nf
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.B #include <unistd.h>
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.PP
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.BI "int dup(int " oldfd );
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.BI "int dup2(int " oldfd ", int " newfd );
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.PP
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.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
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.BR "#include <fcntl.h>" " /* Definition of " O_* " constants */"
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.B #include <unistd.h>
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.PP
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.BI "int dup3(int " oldfd ", int " newfd ", int " flags );
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.fi
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.SH DESCRIPTION
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The
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.BR dup ()
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system call allocates a new file descriptor that refers to the same
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open file description as the descriptor
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.IR oldfd .
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(For an explanation of open file descriptions, see
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.BR open (2).)
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The new file descriptor number is guaranteed to be the lowest-numbered
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file descriptor that was unused in the calling process.
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.PP
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After a successful return,
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the old and new file descriptors may be used interchangeably.
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Since the two file descriptors refer to the same open file description,
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they share file offset and file status flags;
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for example, if the file offset is modified by using
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.BR lseek (2)
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on one of the file descriptors,
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the offset is also changed for the other file descriptor.
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.PP
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The two file descriptors do not share file descriptor flags
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(the close-on-exec flag).
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The close-on-exec flag
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.RB ( FD_CLOEXEC ;
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see
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.BR fcntl (2))
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for the duplicate descriptor is off.
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.\"
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.SS dup2()
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The
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.BR dup2 ()
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system call performs the same task as
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.BR dup (),
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but instead of using the lowest-numbered unused file descriptor,
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it uses the file descriptor number specified in
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.IR newfd .
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In other words,
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the file descriptor
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.I newfd
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is adjusted so that it now refers to the same open file description as
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.IR oldfd .
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.PP
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If the file descriptor
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.IR newfd
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was previously open, it is closed before being reused;
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the close is performed silently
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(i.e., any errors during the close are not reported by
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.BR dup2 ()).
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.PP
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The steps of closing and reusing the file descriptor
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.IR newfd
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are performed
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.IR atomically .
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This is important, because trying to implement equivalent functionality using
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.BR close (2)
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and
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.BR dup ()
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would be
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subject to race conditions, whereby
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.I newfd
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might be reused between the two steps.
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Such reuse could happen because the main program is interrupted
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by a signal handler that allocates a file descriptor,
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or because a parallel thread allocates a file descriptor.
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.PP
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Note the following points:
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.IP * 3
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If
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.I oldfd
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is not a valid file descriptor, then the call fails, and
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.I newfd
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is not closed.
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.IP *
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If
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.I oldfd
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is a valid file descriptor, and
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.I newfd
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has the same value as
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.IR oldfd ,
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then
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.BR dup2 ()
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does nothing, and returns
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.IR newfd .
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.\"
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.SS dup3()
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.BR dup3 ()
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is the same as
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.BR dup2 (),
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except that:
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.IP * 3
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The caller can force the close-on-exec flag to be set
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for the new file descriptor by specifying
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.BR O_CLOEXEC
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in
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.IR flags .
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See the description of the same flag in
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.BR open (2)
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for reasons why this may be useful.
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.IP *
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.\" Ulrich Drepper, LKML, 2008-10-09:
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.\" We deliberately decided on this change. Otherwise, what is the
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.\" result of dup3(fd, fd, O_CLOEXEC)?
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If
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.IR oldfd
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equals
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.IR newfd ,
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then
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.BR dup3 ()
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fails with the error
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.BR EINVAL .
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.SH RETURN VALUE
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On success, these system calls
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return the new file descriptor.
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On error, \-1 is returned, and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EBADF
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.I oldfd
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isn't an open file descriptor.
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.TP
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.B EBADF
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.I newfd
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is out of the allowed range for file descriptors (see the discussion of
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.BR RLIMIT_NOFILE
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in
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.BR getrlimit (2)).
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.TP
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.B EBUSY
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(Linux only) This may be returned by
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.BR dup2 ()
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or
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.BR dup3 ()
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during a race condition with
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.BR open (2)
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and
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.BR dup ().
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.TP
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.B EINTR
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The
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.BR dup2 ()
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or
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.BR dup3 ()
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call was interrupted by a signal; see
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.BR signal (7).
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.TP
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.B EINVAL
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.RB ( dup3 ())
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.I flags
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contain an invalid value.
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.TP
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.B EINVAL
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.RB ( dup3 ())
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.I oldfd
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was equal to
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.IR newfd .
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.TP
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.B EMFILE
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The per-process limit on the number of open file descriptors has been reached
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(see the discussion of
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.BR RLIMIT_NOFILE
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in
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.BR getrlimit (2)).
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.SH VERSIONS
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.BR dup3 ()
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was added to Linux in version 2.6.27;
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glibc support is available starting with
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version 2.9.
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.SH CONFORMING TO
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.BR dup (),
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.BR dup2 ():
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POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
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.PP
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.BR dup3 ()
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is Linux-specific.
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.\" SVr4 documents additional
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.\" EINTR and ENOLINK error conditions. POSIX.1 adds EINTR.
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.\" The EBUSY return is Linux-specific.
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.SH NOTES
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The error returned by
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.BR dup2 ()
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is different from that returned by
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.BR fcntl( "..., " F_DUPFD ", ..." )
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when
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.I newfd
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is out of range.
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On some systems,
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.BR dup2 ()
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also sometimes returns
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.B EINVAL
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like
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.BR F_DUPFD .
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.PP
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If
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.I newfd
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was open, any errors that would have been reported at
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.BR close (2)
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time are lost.
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If this is of concern,
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then\(emunless the program is single-threaded and does not allocate
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file descriptors in signal handlers\(emthe correct approach is
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.I not
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to close
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.I newfd
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before calling
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.BR dup2 (),
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because of the race condition described above.
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Instead, code something like the following could be used:
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.PP
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.in +4n
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.EX
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/* Obtain a duplicate of \(aqnewfd\(aq that can subsequently
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be used to check for close() errors; an EBADF error
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means that \(aqnewfd\(aq was not open. */
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tmpfd = dup(newfd);
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if (tmpfd == \-1 && errno != EBADF) {
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/* Handle unexpected dup() error. */
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}
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/* Atomically duplicate \(aqoldfd\(aq on \(aqnewfd\(aq. */
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if (dup2(oldfd, newfd) == \-1) {
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/* Handle dup2() error. */
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}
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/* Now check for close() errors on the file originally
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referred to by \(aqnewfd\(aq. */
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if (tmpfd != \-1) {
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if (close(tmpfd) == \-1) {
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/* Handle errors from close. */
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}
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}
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.EE
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.in
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.SH SEE ALSO
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.BR close (2),
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.BR fcntl (2),
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.BR open (2),
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.BR pidfd_getfd (2)
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