4.4BSD/usr/share/man/cat2/dup2.0

DUP(2)                      BSD Programmer's Manual                     DUP(2)

NNAAMMEE
     dduupp, dduupp22 - duplicate an existing file descriptor

SSYYNNOOPPSSIISS
     ##iinncclluuddee <<uunniissttdd..hh>>

     _i_n_t
     dduupp(_i_n_t _o_l_d_d);

     _i_n_t
     dduupp22(_i_n_t _o_l_d_d, _i_n_t _n_e_w_d);

DDEESSCCRRIIPPTTIIOONN
     DDuupp() duplicates an existing object descriptor and returns its value to
     the calling process (_n_e_w_d = dduupp(_o_l_d_d)). The argument _o_l_d_d 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 _o_l_d_d
     and _n_e_w_d in any way.  Thus if _n_e_w_d and _o_l_d_d 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 separate pointer in-
     to 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 dduupp22(), the value of the new descriptor _n_e_w_d is specified.  If this
     descriptor is already in use, the descriptor is first deallocated as if a
     close(2) call had been done first.

RREETTUURRNN VVAALLUUEESS
     The value -1 is returned if an error occurs in either call.  The external
     variable _e_r_r_n_o indicates the cause of the error.

EERRRROORRSS
     DDuupp() and dduupp22() fail if:

     [EBADF]       _O_l_d_d or _n_e_w_d is not a valid active descriptor

     [EMFILE]      Too many descriptors are active.

SSEEEE AALLSSOO
     accept(2),  open(2),  close(2),  fcntl(2),  pipe(2),  socket(2),
     socketpair(2),  getdtablesize(2)

SSTTAANNDDAARRDDSS
     DDuupp() and dduupp22() are expected to conform to IEEE Std 1003.1-1988
     (``POSIX'').

4th Berkeley Distribution        June 4, 1993                                1