Name

dup, dup2 — duplicate a file descriptor

Synopsis

#include <unistd.h>
int dup( int   oldfd);
int dup2( int   oldfd,
  int   newfd);

DESCRIPTION

dup() and dup2() create a copy of the file descriptor oldfd.

dup() uses the lowest-numbered unused descriptor for the new descriptor.

dup2() makes newfd be the copy of oldfd, closing newfd first if necessary, but note the following:

  • If olfd is not a valid file descriptor, then the call fails, and newfd is not closed.

  • If oldfd is a valid file descriptor, and newfd has the same value as oldfd, then dup2() does nothing, and returns newfd.

After a successful return from dup() or dup2(), the old and new file descriptors may be used interchangeably. They refer to the same open file description (see open(2)) and thus share file offset and file status flags; for example, if the file offset is modified by using lseek(2) on one of the descriptors, the offset is also changed for the other.

The two descriptors do not share file descriptor flags (the close-on-exec flag). The close-on-exec flag (FD_CLOEXEC; see fcntl(2)) for the duplicate descriptor is off.

RETURN VALUE

dup() and dup2() return the new descriptor, or −1 if an error occurred (in which case, errno is set appropriately).

ERRORS

EBADF

oldfd isn't an open file descriptor, or newfd is out of the allowed range for file descriptors.

EBUSY

(Linux only) This may be returned by dup2() during a race condition with open(2) and dup().

EINTR

The dup2() call was interrupted by a signal.

EMFILE

The process already has the maximum number of file descriptors open and tried to open a new one.

CONFORMING TO

SVr4, 4.3BSD, POSIX.1-2001.

NOTES

The error returned by dup2() is different from that returned by fcntl(...,F_DUPFD, ...) when newfd is out of range. On some systems dup2() also sometimes returns EINVAL like F_DUPFD.

If newfd was open, any errors that would have been reported at close(2) time are lost. A careful programmer will not use dup2() without closing newfd first.

SEE ALSO

close(2), fcntl(2), open(2)

COLOPHON

This page is part of release 2.79 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


This manpage is Copyright (C) 1992 Drew Eckhardt;
and Copyright (C) 1993 Michael Haardt, Ian Jackson.
and Copyright (C) 2005, 2008 Michael Kerrisk <mtk.manpagesgmail.com>

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
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Since the Linux kernel and libraries are constantly changing, this
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responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
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Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

Modified 1993-07-21, Rik Faith <faithcs.unc.edu>
Modified 1994-08-21, Michael Chastain <mecshell.portal.com>:
  Fixed typoes.
Modified 1997-01-31, Eric S. Raymond <esrthyrsus.com>
Modified 2002-09-28, aeb
2009-01-12, mtk, reordered text in DESCRIPTION and added some
    details for dup2().