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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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 aa File SS
       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 .



IEEE/The Open Group                  2017                               DUP(3)
