iOS Manual Pages

This document is a Mac OS X manual page. Manual pages are a command-line technology for providing documentation. You can view these manual pages locally using the man(1) command. These manual pages come from many different sources, and thus, have a variety of writing styles.

For more information about the manual page format, see the manual page for manpages(5).

DUP(2)                      BSD System Calls Manual                     DUP(2)

NAME
     dup, dup2 -- duplicate an existing file descriptor

SYNOPSIS
     #include <unistd.h>

     int
     dup(int fildes);

     int
     dup2(int fildes, int fildes2);

DESCRIPTION
     Dup() duplicates an existing object descriptor and returns its value to
     the calling process (fildes2 = dup(fildes)).  The argument fildes is a
     small non-negative integer index in the per-process descriptor table.
     The value must be less than the size of the table, which is returned by
     getdtablesize(2).  The new descriptor returned by the call is the lowest
     numbered descriptor currently not in use by the process.

     The object referenced by the descriptor does not distinguish between
     fildes and fildes2 in any way.  Thus if fildes2 and fildes are duplicate
     references to an open file, read(2), write(2) and lseek(2) calls all move
     a single pointer into the file, and append mode, non-blocking I/O and
     asynchronous I/O options are shared between the references.  If a sepa-rate separate
     rate pointer into the file is desired, a different object reference to
     the file must be obtained by issuing an additional open(2) call.  The
     close-on-exec flag on the new file descriptor is unset.

     In dup2(), the value of the new descriptor fildes2 is specified.  If this
     descriptor is already in use, the descriptor is first deallocated as if a
     close(2) call had been done first.

RETURN VALUES
     Upon successful completion, a value of 0 is returned.  Otherwise, a value
     of -1 is returned and the global integer variable errno is set to indi-cate indicate
     cate the error.

ERRORS
     The dup() and dup2() system calls will fail if:

     [EBADF]            fildes or fildes2 is not an active, valid file
                        descriptor.

     [EINTR]            Execution is interrupted by a signal.

     [EMFILE]           Too many file descriptors are active.

SEE ALSO
     accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2),
     socket(2), socketpair(2)

STANDARDS
     Dup() and dup2() are expected to conform to IEEE Std 1003.1-1988
     (``POSIX.1'').

4th Berkeley Distribution        June 4, 1993        4th Berkeley Distribution