The master driver and slave driver have the following characteristics:
Each master driver has one-to-one relationship with a slave device based on major/minor device numbers.
Only one open is allowed on a master device. Multiple opens are allowed on the slave device according to standard file mode and ownership permissions.
Each slave driver minor device has a node in the file system.
An open on a master device automatically locks out an open on the corresponding slave driver.
A slave cannot be opened unless the corresponding master is open and has unlocked the slave.
To provide a TTY interface to the user, the ldterm and ptem modules are pushed on the slave side.
A close on the master sends a hang up to the slave and renders both streams unusable, after all data have been consumed by the process on the slave side.