DUP(3) Library Functions Manual DUP(3)
dup - dups of open files
bind #d /fd
The dup device serves a one-level directory containing files whose
names are decimal numbers. A file of name n corresponds to open file
descriptor n in the current process.
An open(2) of file n results in a file descriptor identical to what
would be returned from a system call dup(n, -1). Note that the result
is no longer a file in the dup device.
The stat operation returns information about the device file, not the
open file it points to. A stat of #d/n will contain 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.