4.3BSD-UWisc/man/cat2/dup.2

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



NAME
     dup, dup2 - duplicate a descriptor

SYNOPSIS
     newd = dup(oldd)
     int newd, oldd;

     dup2(oldd, newd)
     int oldd, newd;

DESCRIPTION
     _D_u_p duplicates an existing object descriptor.  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 _g_e_t_d_t_a_b_l_e_s_i_z_e(2).
     The new descriptor returned by the call, _n_e_w_d, is the lowest
     numbered descriptor that is not currently in use by the pro-
     cess.

     The object referenced by the descriptor does not distinguish
     between references using _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,
     _r_e_a_d(2), _w_r_i_t_e(2) and _l_s_e_e_k(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 into the file is desired, a different
     object reference to the file must be obtained by issuing an
     additional _o_p_e_n(2) call.  The close-on-exec flag on the new
     file descriptor is unset.

     In the second form of the call, the value of _n_e_w_d desired is
     specified.  If this descriptor is already in use, the
     descriptor is first deallocated as if a _c_l_o_s_e(2) call had
     been done first.

RETURN VALUE
     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.

ERRORS
     _D_u_p and _d_u_p_2 fail if:

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

     [EMFILE]       Too many descriptors are active.

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





Printed 12/27/86          May 13, 1986                          1