Before you can read or write the contents of a file, you must establish
a connection or communications channel to the file. This process is
called opening the file. You can open a file for reading, writing,
or both.
The connection to an open file is represented either as a stream or as a
file descriptor. You pass this as an argument to the functions that do
the actual read or write operations, to tell them which file to operate
on. Certain functions expect streams, and others are designed to
operate on file descriptors.
When you have finished reading to or writing from the file, you can
terminate the connection by closing the file. Once you have
closed a stream or file descriptor, you cannot do any more input or
output operations on it.
