mnt – attach to 9P servers
The mount driver is used by the mount system call (but not bind;
see bind(2)) to connect the name space of a process to the service
provided by a 9P server over a communications channel. After the
mount, system calls involving files in that portion of the name
space will be converted by the mount driver into the
appropriate 9P messages to the server.
The mount system call issues session and attach(5) messages to the server to identify and validate the user of the connection. Each distinct user of a connection must mount it separately; the mount driver multiplexes the access of the various users and their processes to the service.
File–oriented system calls are converted by the kernel into messages in the 9P protocol. Within the kernel, 9P is implemented by procedure calls to the various kernel device drivers. The mount driver translates these procedure calls into remote procedure calls to be transmitted as messages over the communication channel to the server. Each message is implemented by a write of the corresponding protocol message to the server channel followed by a read on the server channel to get the reply. Errors in the reply message are turned into system call error returns.
A read(2) or write system call on a file served by the mount driver may be translated into more than one message, since there is a maximum data size for a 9P message. The system call will return when the specified number of bytes have been transferred or a short reply is returned.
The string #M is an illegal file name, so this device can only
be accessed directly by the kernel.
When mounting a service through the mount driver, that is, when
the channel being multiplexed is itself a file being served by
the mount driver, large messages may be broken in two.