trusty (3) dup.3posix.gz

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 deallocate 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,
       Standard 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 .