dup(3p) — Linux manual page

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

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

       The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)

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(3P)

Pages that refer to this page: unistd.h(0p), sh(1p), close(3p), 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)

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