.\" $NetBSD: dup.2,v 1.4 1995/02/27 12:32:21 cgd Exp $ .\" .\" 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 December 1, 2010 .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 .Fo dup .Fa "int fildes" .Fc .Ft int .Fo dup2 .Fa "int fildes" .Fa "int fildes2" .Fc .Sh DESCRIPTION .Fn Dup duplicates an existing object descriptor and returns its value to the calling process .Fa ( fildes2 = .Fn dup fildes ) . The argument .Fa fildes 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 fildes and .Fa fildes2 in any way. Thus if .Fa fildes2 and .Fa fildes 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 fildes2 is specified. If .Fa fildes and .Fa fildes2 are equal, then .Fn dup2 just returns .Fa fildes2 ; no other changes are made to the existing descriptor. Otherwise, if descriptor .Fa fildes2 is already in use, it is first deallocated as if a .Xr close 2 call had been done first. .Sh RETURN VALUES Upon successful completion, the new file descriptor is returned. Otherwise, a value of -1 is returned and the global integer variable .Va errno is set to indicate the error. .Sh ERRORS The .Fn dup and .Fn dup2 system calls will fail if: .Bl -tag -width Er .\" ========== .It Bq Er EBADF .Fa fildes is not an active, valid file descriptor. .\" ========== .It Bq Er EINTR Execution is interrupted by a signal. .\" ========== .It Bq Er EMFILE Too many file descriptors are active. .El .Pp The .Fn dup2 system call will fail if: .Bl -tag -width Er .\" ========== .It Bq Er EBADF .Fa fildes2 is negative or greater than the maximum allowable number (see getdtablesize(2)). .El .Sh SEE ALSO .Xr accept 2 , .Xr close 2 , .Xr fcntl 2 , .Xr getdtablesize 2 , .Xr open 2 , .Xr pipe 2 , .Xr socket 2 , .Xr socketpair 2 .Sh STANDARDS .Fn Dup and .Fn dup2 are expected to conform to .St -p1003.1-88 .