DUP(P) POSIX Programmer's Manual 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 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 suc‐
cessful 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 vol‐
ume 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 descrip‐
tors. 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 invalid‐
ity 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 ALSOclose() , 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 .
IEEE/The Open Group 2003 DUP(P)
