/dev/full is an always-full device. I have yet to find a use for it. Any writes to /dev/full will fail. Reads from /dev/full will return \0. Any seeks on /dev/full will succeed.
The "full" man page:
FULL(4) Linux Programmer's Manual FULL(4)
full - always full device
File /dev/full has major device number 1 and minor device
Writes to the /dev/full device will fail with an ENOSPC
Reads from the /dev/full device will return \0 characters.
Seeks on /dev/full will always succeed.
If your system does not have /dev/full created already, it
can be created with the following commands:
mknod -m 666 /dev/full c 1 7
chown root:root /dev/full
mknod(1), null(4), zero(4)
Linux 1997-08-02 FULL(4)