The basic control of a computer rests with its operating system. This
is a layer of software which drives the hardware and provides users with a
comfortable environment in which to work. An operating system has two
main components which are of interest to users: a user interface (often a
command language) and a filing system. The operating system is the route
to all input and output, whether it be to a screen or to files on a disk. A
programming language has to get at this input and output easily so that
programs can send out and receive messages from the user and it has to
be in contact with the operating system in order to do this. In C the link
between these two is very efficient.
Operating systems vary widely but most have a command language or
shell which can be used to type in commands. Recently the tendency has
been to try to eliminate typing completely by providing graphical user in-
terfaces (GUIs) for every purpose. GUIs are good for carrying out simple
procedures like editing, but they are not well suited to giving complicated
instructions to a computer. For that one needs a command language. In the
network version of this book we shall concentrate on Unix shell commands
since they are the most important to programmers. On microcomputers
command languages are usually very similar in concept, though more prim-
itive, with only slightly different words for essentially the same commands.
(This is a slightly superficial view).
When most compiler languages were developed, they were intended to
be run on large mainframe computers which operated on a multi-user, time-
sharing principle and were incapable of interactive communication with the
user. Many compiler languages still have this inadequacy when carried over
to modern computers, but C is an exception, because of its unique design.
Input and output are not actually defined as a fixed, unchanging part of
the C language. Instead there is a standard file which has to be included
in programs and defines the input/output commands that are supported by
the language for a particular computer and operating system. This file is
called a standard C library. (See the next chapter for more information.)
The library is standard in the sense that C has developed a set of functions
which all computers and operating systems must implement, but which are
specially adapted to your system.

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