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.\"     @(#)dup.2	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt DUP 2
.Os BSD 4
.Sh NAME
.Nm dup ,
.Nm dup2
.Nd duplicate an existing file descriptor
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn dup "int oldd"
.Ft int
.Fn dup2 "int oldd" "int newd"
.Sh DESCRIPTION
.Fn Dup
duplicates an existing object descriptor and returns its value to
the calling process
.Fa ( newd
=
.Fn dup oldd ) .
The argument
.Fa 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
.Xr getdtablesize 2 .
The new descriptor returned by the call
is the lowest numbered descriptor
currently not in use by the process.
.Pp
The object referenced by the descriptor does not distinguish
between
.Fa oldd
and
.Fa newd
in any way.
Thus if
.Fa newd
and
.Fa oldd
are duplicate references to an open
file,
.Xr read 2 ,
.Xr write 2
and
.Xr 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 into the file is desired, a different
object reference to the file must be obtained by issuing an
additional
.Xr open 2
call.
The close-on-exec flag on the new file descriptor is unset.
.Pp
In 
.Fn dup2 ,
the value of the new descriptor
.Fa newd
is specified.  If this descriptor is already
in use, the descriptor is first deallocated as if a
.Xr close 2
call had been done first.
.Sh IMPLEMENTATION NOTES
.Pp
In the non-threaded library
.Fn dup
is implemented as the
.Va dup
syscall.
.Pp
In the threaded library, the
.Va dup
syscall is assembled to
.Fn _thread_sys_dup
and
.Fn dup
is implemented as a function which locks
.Va oldd
for read and write, then calls
.Fn _thread_sys_dup .
Before returning,
.Fn dup
unlocks
.Va oldd .
.Pp
In the non-threaded library
.Fn dup2
is implemented as the
.Va dup2
syscall.
.Pp
In the threaded library, the
.Va dup2
syscall is assembled to
.Fn _thread_sys_dup2
and
.Fn dup2
is implemented as a function which locks both
.Va oldd
and
.Va newd
for read and write, then calls
.Fn _thread_sys_dup2 .
Before returning,
.Fn dup2
unlocks
.Va oldd .
and
.Va newd .
.Sh RETURN VALUES
The value -1 is returned if an error occurs in either call.
The external variable
.Va errno
indicates the cause of the error.
.Sh ERRORS
.Fn Dup
and
.Fn dup2
fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa Oldd
or
.Fa newd
is not a valid active descriptor
.It Bq Er EMFILE
Too many descriptors are active.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr open 2 ,
.Xr close 2 ,
.Xr fcntl 2 ,
.Xr pipe 2 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr getdtablesize 2
.Sh STANDARDS
.Fn Dup
and
.Fn dup2
are expected to conform
to IEEE Std 1003.1-1988
.Pq Dq Tn POSIX .