On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-12-02 08:05 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)
DUP(P) DUP(P)
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 pro-
vided 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 com-
pletion, 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 applica-
tions use them.
While the brief code segment shown is very similar in behavior to dup2(), a con-
forming 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 allo-
cate or deallocate file descriptors. This could be avoided by blocking signals.
The dup2() function is not marked obsolescent because it presents a type-safe ver-
sion 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 synchroniza-
tion 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 .
POSIX 2003 DUP(P)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-12-02 08:05 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)