The C preprocessor is a tool which filters the source code before it is compailed. The preprocessor allows constants to be named using the £define notation. The preprocessor provides several other facilities which will be described in this chapter. The preprocessing work in a program execution is shown in figure

It is important to remember that the preprocessor is not working to the same rules as the rest of C program. It works on a line-by-line basis. The preprocessor does not know about the scope rules of C. Preprocessor directives like #define take effect as soon as they are seen and remain in effect until the end of the file that contains them. This is one of the reasons why it is a good idea to make sparing use of these directives.

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