A preprocessor is a program that takes text and performs lexical conversions on it. The conversions may include macro substitution , conditional inclusion, and inclusion of other files.

The use of preprocessors has been getting less common as recent languages provide more abstract features rather than lexical-oriented ones. Indeed, the overuse of the preprocessor might yield quite chaotic code. In designing a new language based on C, Stroustrup introduced features such as inline and templates into C++ in an attempt to make the C preprocessor less relevant. Nevertheless, there is an abundance of installed C code which relies on the preprocessor.

New languages proposed recently have little or no preprocessor ability. Java has no preprocessor. D, designed as a replacement of C and C++, supports features such as imports, nested functions, versioning, debug statements, etc. that help make it practical to eliminate the preprocessor entirely.

Other preprocessors include m4 and Oracle Pro*C. The m4 preprocessor is general-purpose; Oracle Pro*C converts embedded PL/SQL into C.

Preprocessing can be quite cumbersome in incremental parsing or incremental lexical analysis because changes to preprocessing rules can affect the entire text to be preprocessed.

Most C compilers have a flag which allows generation of post-processed code so that static analysis can be performed on the output if desired.

See Also

• The C preprocessor