Copyright (c) 1980, 1991, 1993
The Regents of the University of California. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. All advertising materials mentioning features or use of this software
must display the following acknowledgement:
This product includes software developed by the University of
California, Berkeley and its contributors.
4. Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
@(#)dup.2 8.1 (Berkeley) 6/4/93
.Dd June 4, 1993 .Dt DUP 2 .Os .Sh NAME .Nm dup , .Nm dup2 .Nd duplicate an existing file descriptor .Sh LIBRARY .Lb libc .Sh SYNOPSIS .Fd #include \*[Lt]unistd.h\*[Gt] .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 3 . The new descriptor returned by the call is the lowest numbered descriptor currently not in use by the process.
p 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.
p 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 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: l -tag -width Er t Bq Er EBADF .Fa oldd or .Fa newd is not a valid active descriptor t Bq Er EMFILE Too many descriptors are active. .El .Sh SEE ALSO .Xr accept 2 , .Xr close 2 , .Xr fcntl 2 , .Xr open 2 , .Xr pipe 2 , .Xr socket 2 , .Xr socketpair 2 , .Xr getdtablesize 3 .Sh STANDARDS The .Fn dup and .Fn dup2 functions conform to .St -p1003.1-90 .
Indexes created Sun Oct 12 09:09:55 GMT 2025