.TH DUP 3 
.SH NAME
dup \- dups of open files
.SH SYNOPSIS
.nf
.B bind #d /fd

.B /fd/0
.B /fd/0ctl
.B /fd/1
.B /fd/1ctl
\&...
.fi
.SH DESCRIPTION
The
.I dup
device serves a one-level directory containing files whose
names are decimal numbers.
Each such file also has an associated control file.
A file of name
.I n
corresponds to open file descriptor
.I n
in the current process.
.PP
An
.IR open (2)
of file
.I n
results in a file descriptor identical to
what would be returned from a system call
.IB dup ( n ,
.BR -1) .
Note that the result is no longer a file in the
.I dup
device.
.PP
The
.I stat
operation returns information about the device file, not the open file it points to.
A stat of
.BI #d/ n
will contain
.I n
for the name, 0 for the length, and 0400, 0200, or 0600
for the mode, depending on whether the dup target is open
for reading, writing, or both.
.PP
A file of name
.IB n ctl
may be read to discover the properties of the associated file descriptor, in format identical to that of the
.B fd
file in
.IR proc (3).
.SH SEE ALSO
.IR dup (2)
.SH SOURCE
.B /sys/src/9/port/devdup.c