Operating systems


C Programming Tutorial

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Operating sytems and environments

Where is a C program born? How is it created?

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 interfaces (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 primitive, 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.