dup - dups of open files

 bind #d /fd 
 /fd/0  /fd/0ctl  /fd/1  /fd/1ctl ...

The 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 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.

A file of name n ctl may be read to discover the properties of the associated file descriptor, in format identical to that of the fd file in proc (3).

dup (2) /sys/src/9/port/devdup.c