an open file
A file descriptor obtained
from the open function.
Nil if file-desc is
valid; otherwise results in an error message.
After a close, the file descriptor
is unchanged but is no longer valid. Data added to an open file
is not actually written until the file is closed.
The following code counts
the number of lines in the file somefile.txt and
sets the variable ct equal to that
(setq fil "SOMEFILE.TXT")
(setq x (open fil "r") ct 0)
(while (read-line x)
(setq ct (1+ ct))