This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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.

       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(3POSIX)

       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.

       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.

       Thefollowingsectionsareinformative.

RedirectingStandardOutputtoaFileS
       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);
           ...

   RedirectingErrorMessages
       The following example redirects messages from stderr to stdout.

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

       None.

       dup, dup2 — duplicate an open file descriptor

       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.

       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.

       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.

close(), fcntl(), open()

       The Base Definitions volume of POSIX.1‐2017, <unistd.h>

       #include <unistd.h>

       int dup(int fildes);
       int dup2(int fildes, int fildes2);