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.1‐2008 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.1‐2008, <unistd.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2013 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX),  The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical  Corrigendum  1  applied.)  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.unix.org/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 .