4.3BSD - /usr/man/cat2/dup.0

DUP(2)                                                                  DUP(2)


NAME
       dup, dup2 - duplicate a descriptor

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

       dup2(oldd, newd)
       int oldd, newd;

DESCRIPTION
       Dup  duplicates  an existing object descriptor.  The argument oldd 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,  newd,  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  oldd and newd in any way.  Thus if newd and oldd 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 refer‐
       ences.   If  a  separate  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 the second form of the call, the value of newd desired is specified.
       If  this  descriptor is already in use, the descriptor is first deallo‐
       cated as if a close(2) call had been done first.

RETURN VALUE
       The value -1 is returned if an error occurs in either call.  The exter‐
       nal variable errno indicates the cause of the error.

ERRORS
       Dup and dup2 fail if:

       [EBADF]        Oldd or newd 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), socket‐
       pair(2), getdtablesize(2)


4th Berkeley Distribution        May 13, 1986                           DUP(2)