from small one page howto to huge articles all in one place
poll results
Last additions:
May 25th. 2007:
April, 26th. 2006:
| You are here: manpages
DUP
Section: POSIX Programmer's Manual (3P)
Updated: 2003
Index
Return to Main Contents
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() and dup2() functions provide an alternative
interface to the service provided by fcntl() using the F_DUPFD
command. The call:
-
fid = dup(fildes);
shall be equivalent to:
-
fid = fcntl(fildes, F_DUPFD, 0);
The call:
-
fid = dup2(fildes, fildes2);
shall be equivalent to:
-
close(fildes2);
fid = fcntl(fildes, F_DUPFD, fildes2);
except for the following:
- *
-
If fildes2 is less than 0 or greater than or equal to {OPEN_MAX},
dup2() shall return -1 with errno set to
[EBADF].
- *
-
If fildes is a valid file descriptor and is equal to fildes2,
dup2() shall return fildes2 without
closing it.
- *
-
If fildes is not a valid file descriptor, dup2() shall
return -1 and shall not close fildes2.
- *
-
The value returned shall be equal to the value of fildes2 upon
successful completion, or -1 upon failure.
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
-
The number of file descriptors in use by this process would exceed
{OPEN_MAX}.
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 following sections are informative.
EXAMPLES
Redirecting Standard Output to a File
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
None.
RATIONALE
The dup() and dup2() functions are redundant. Their services
are also provided by the fcntl() function. They have been included
in this volume of IEEE Std 1003.1-2001
primarily for historical reasons, since many existing applications
use them.
While the brief code segment shown is very similar in behavior to
dup2(), a conforming implementation based on other
functions defined in this volume of IEEE Std 1003.1-2001 is significantly
more complex. Least obvious is the possible
effect of a signal-catching function that could be invoked between
steps and allocate or deallocate file descriptors. This could be
avoided by blocking signals.
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 IEEE Std 1003.1-2001, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- Redirecting Standard Output to a File
-
- Redirecting Error Messages
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
Please read "Why adblockers are badwww.cars2fast4u.de
|
Other free services .
.
| 
|
|
- Running on 
-
:
: 
