DUP
Section: POSIX Programmer's Manual (3P)
Updated: 2013
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PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
dup,
dup2
--- duplicate an open file descriptor
SYNOPSIS
#include <unistd.h>
int dup(int fildes);
int dup2(int fildes, int fildes2);
DESCRIPTION
The
dup()
function provides an alternative interface to the service provided by
fcntl()
using the F_DUPFD command. The call
dup(
fildes)
shall be equivalent to:
-
fcntl(fildes, F_DUPFD, 0);
The
dup2()
function shall cause the file descriptor
fildes2
to refer to the same open file description as the file descriptor
fildes
and to share any locks, and shall return
fildes2.
If
fildes2
is already a valid open file descriptor, it shall be closed first, unless
fildes
is equal to
fildes2
in which case
dup2()
shall return
fildes2
without closing it. If the close operation fails to close
fildes2,
dup2()
shall return -1 without changing the open file description to which
fildes2
refers. If
fildes
is not a valid file descriptor,
dup2()
shall return -1 and shall not close
fildes2.
If
fildes2
is less than 0 or greater than or equal to
{OPEN_MAX},
dup2()
shall return -1 with
errno
set to
[EBADF].
Upon successful completion, if
fildes
is not equal to
fildes2,
the FD_CLOEXEC flag associated with
fildes2
shall be cleared. If
fildes
is equal to
fildes2,
the FD_CLOEXEC flag associated with
fildes2
shall not be changed.
If
fildes
refers to a typed memory object, the result of the
dup2()
function is unspecified.
RETURN VALUE
Upon successful completion a non-negative integer, namely the file
descriptor, shall be returned; otherwise, -1 shall be returned
and
errno
set to indicate the error.
ERRORS
The
dup()
function shall fail if:
- EBADF
-
The
fildes
argument is not a valid open file descriptor.
- EMFILE
-
All file descriptors available to the process are currently open.
The
dup2()
function shall fail if:
- EBADF
-
The
fildes
argument is not a valid open file descriptor or the argument
fildes2
is negative or greater than or equal to
{OPEN_MAX}.
- EINTR
-
The
dup2()
function was interrupted by a signal.
The
dup2()
function may fail if:
- EIO
-
An I/O error occurred while attempting to close
fildes2.
The following sections are informative.
EXAMPLES
Redirecting Standard Output to a File S
The following example closes standard output for the current processes,
re-assigns standard output to go to the file referenced by
pfd,
and closes the original file descriptor to clean up.
-
#include <unistd.h>
...
int pfd;
...
close(1);
dup(pfd);
close(pfd);
...
Redirecting Error Messages
The following example redirects messages from
stderr
to
stdout.
-
#include <unistd.h>
...
dup2(1, 2);
...
APPLICATION USAGE
Implementations may use file descriptors that must be inherited into
child processes for the child process to remain conforming, such as for
message catalog or tracing purposes. Therefore, an application that calls
dup2()
with an arbitrary integer for
fildes2
risks non-conforming behavior, and
dup2()
can only portably be used to overwrite file descriptor values that the
application has obtained through explicit actions, or for the three file
descriptors corresponding to the standard file streams. In order to avoid
a race condition of leaking an unintended file descriptor into a child
process, an application should consider opening all file descriptors
with the FD_CLOEXEC bit set unless the file descriptor is intended to
be inherited across
exec.
RATIONALE
The
dup()
function is redundant. Its services are also provided by the
fcntl()
function. It has been included in this volume of POSIX.1-2008 primarily for historical reasons,
since many existing applications use it. On the other hand, the
dup2()
function provides unique services, as no other interface is able to
atomically replace an existing file descriptor.
The
dup2()
function is not marked obsolescent because it presents a type-safe
version of functionality provided in a type-unsafe version by
fcntl().
It is used in the POSIX Ada binding.
The
dup2()
function is not intended for use in critical regions as a
synchronization mechanism.
In the description of
[EBADF],
the case of
fildes
being out of range is covered by the given case of
fildes
not being valid. The descriptions for
fildes
and
fildes2
are different because the only kind of invalidity that is relevant for
fildes2
is whether it is out of range; that is, it does not matter whether
fildes2
refers to an open file when the
dup2()
call is made.
FUTURE DIRECTIONS
None.
SEE ALSO
close(),
fcntl(),
open()
The Base Definitions volume of POSIX.1-2008,
<unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.unix.org/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- Redirecting Standard Output to a File S
-
- Redirecting Error Messages
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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