Man Pages

dup(3p) - phpMan dup(3p) - phpMan

Command: man perldoc info search(apropos)  


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 dif-
       fer (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()  and  dup2() functions provide an alternative interface to the service provided by fcntl() using the
       F_DUPFD command. The call:


              fid = dup(fildes);

       shall be equivalent to:


              fid = fcntl(fildes, F_DUPFD, 0);

       The call:


              fid = dup2(fildes, fildes2);

       shall be equivalent to:


              close(fildes2);
              fid = fcntl(fildes, F_DUPFD, fildes2);

       except for the following:

        * If fildes2 is less than 0 or greater than or equal to {OPEN_MAX}, dup2() shall return -1 with errno  set  to
          [EBADF].


        * If  fildes  is  a valid file descriptor and is equal to fildes2, dup2() shall return fildes2 without closing
          it.


        * If fildes is not a valid file descriptor, dup2() shall return -1 and shall not close fildes2.


        * The value returned shall be equal to the value of fildes2 upon successful completion, or -1 upon failure.


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 The number of file descriptors in use by this process would exceed {OPEN_MAX}.


       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 following sections are informative.

EXAMPLES
   Redirecting Standard Output to a File
       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
       None.

RATIONALE
       The  dup()  and  dup2() functions are redundant. Their services are also provided by the fcntl() function. They
       have been included in this volume of IEEE Std 1003.1-2001 primarily for historical reasons, since many existing
       applications use them.

       While  the brief code segment shown is very similar in behavior to dup2(), a conforming implementation based on
       other functions defined in this volume of IEEE Std 1003.1-2001 is significantly more complex. Least obvious  is
       the  possible  effect of a signal-catching function that could be invoked between steps and allocate or deallo-
       cate file descriptors. This could be avoided by blocking signals.

       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 IEEE Std 1003.1-2001, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard  for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 6, Copyright (C) 2001-2003 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 .



IEEE/The Open Group                  2003                              DUP(3P)