Reserved words and an example

C Programming Tutorial

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C programs are constructed from a set of reserved words which provide control and from libraries which perform special functions. The basic instructions are built up using a reserved set of words, such as main, for, if,while, default, double, extern, for, and int, to name just a few. These words may not be used in just any old way: C demands that they are used only for giving commands or making statements. You cannot use default, for example, as the name of a variable. An attempt to do so will result in a compilation error.

See All the Reserved Words, for a complete list of the reserverd words.

Words used in included libaries are also, effectively, reserved. If you use a word which has already been adopted in a library, there will be a conflict between your choice and the library.

Libraries provide frequently used functionality and, in practice, at least one library must be included in every program: the so-called C library, of standard functions. For example, the stdio library, which is part of the C library, provides standard facilities for input to and output from a program.

In fact, most of the facilities which C offers are provided as libraries that are included in programs as plug-in expansion units. While the features provided by libraries are not strictly a part of the C language itself, they are essential and you will never find a version of C without them. After a library has been included in a program, its functions are defined and you cannot use their names.