DUP(3P) POSIX Programmer's Manual DUP(3P)


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.


dup, dup2 - duplicate an open file descriptor


#include <unistd.h>
int dup(int fildes );
int dup2(int fildes , int fildes2 );


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:


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.


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.


The dup() function shall fail if:
The fildes argument is not a valid open file descriptor.
The number of file descriptors in use by this process would exceed {OPEN_MAX}.

The dup2() function shall fail if:

The fildes argument is not a valid open file descriptor or the argument fildes2 is negative or greater than or equal to {OPEN_MAX}.
The dup2() function was interrupted by a signal.

The following sections are informative.


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;

Redirecting Error Messages

The following example redirects messages from stderr to stdout.

#include <unistd.h>
dup2(1, 2);




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.




close(), fcntl(), open(), the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>


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 .
2003 IEEE/The Open Group