mirror of https://github.com/mkerrisk/man-pages
137 lines
3.6 KiB
Groff
137 lines
3.6 KiB
Groff
.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
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.\" 1993 Michael Haardt, Ian Jackson.
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.\"
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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.\" manual page may be incorrect or out-of-date.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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.\"
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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 typoes.
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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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.\"
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.TH DUP 2 1994-08-21 "Linux 1.1.46" "Linux Programmer's Manual"
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.SH NAME
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dup, dup2 \- 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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.sp
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.BI "int dup(int " oldfd );
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.BI "int dup2(int " oldfd ", int " newfd );
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.fi
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.SH DESCRIPTION
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.BR dup ()
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and
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.BR dup2 ()
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create a copy of the file descriptor
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.IR oldfd .
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After a successful return from \fBdup\fR() or \fBdup2\fR(),
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the old and new file descriptors may be used interchangeably.
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They refer to the same open file description (see
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.BR open (2))
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and thus 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 descriptors, the offset is also changed for the other.
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The two 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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.BR dup ()
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uses the lowest-numbered unused descriptor for the new descriptor.
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.BR dup2 ()
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.RI "makes " newfd " be the copy of " oldfd ", closing " newfd
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first if necessary.
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.SH "RETURN VALUE"
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.BR dup ()
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and
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.BR dup2 ()
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return the new descriptor, or \-1 if an error occurred (in which case,
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.I errno
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is set appropriately).
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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, or
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.I newfd
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is out of the allowed range for file descriptors.
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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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during a race condition with
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.BR open ()
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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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call was interrupted by a signal.
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.TP
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.B EMFILE
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The process already has the maximum number of file
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descriptors open and tried to open a new one.
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.SH WARNINGS
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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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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 ()
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time, are lost.
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A careful programmer will not use
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.BR dup2 ()
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without closing
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.I newfd
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first.
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.SH "CONFORMING TO"
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SVr4, 4.3BSD, POSIX.1-2001.
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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 "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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