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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.1‐2017 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.1‐2017, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
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 .