dup(3p) — Linux manual page


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

PROLOG         top

       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         top

       dup, dup2 — duplicate an open file descriptor

SYNOPSIS         top

       #include <unistd.h>

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

DESCRIPTION         top

       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         top

       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         top

       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

       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

       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         top

   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;

   Redirecting Error Messages
       The following example redirects messages from stderr to stdout.

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


       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         top

       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.



SEE ALSO         top

       close(3p), fcntl(3p), open(3p)

       The Base Definitions volume of POSIX.1‐2008, unistd.h(0p)

COPYRIGHT         top

       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 .

IEEE/The Open Group                 2013                             DUP(3P)

Pages that refer to this page: unistd.h(0p)sh(1p)fstatvfs(3p)open(3p)posix_spawn(3p)posix_spawn_file_actions_addclose(3p)posix_spawn_file_actions_adddup2(3p)posix_typed_mem_open(3p)shm_open(3p)write(3p)