Preprocessing
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Preprocessing is the act of processing data before it is parsed. There are numerous situations where it makes sense to do the parsing in several stages. One is where humans are the parsers, and another is in the context of computer programming.
As the name suggests, preprocessing is performed by a preprocessor. The preprocessor modifies the data according to preprocessing directives that are usually placed in the input data itself. For instance, in the C programming language, where preprocessing directives are marked with a '#' at the beginning of the line, the preprocessor can implement macros, be used for including external files at different points in the file or to select blocks of code are to be sent to the compiler. The criteria can be several things, such as processor type (e.g. to resolve integer representation problems), availability of function calls (so you can provide a work-around if one is missing) and user preferences.
When preprocessors support macros, calls to macro functions within the code will expand to the whole implementation of the macro before it is sent to the compiler. This can be quite useful where speed is more important than the size of the binary code, and when you need expressions that expand to more than just a function; for instance a case-block.
Preprocessing is very useful to solve portability issues: depending on the target platform (that is specified to the preprocessor by some command-line argument) the application will contain specific code. For instance, when compiled for Linux, the application would read its configuation options from a file named .conf, whereas when compiled for Microsoft Windows, it would read the configuration from the registry.