Preprocessor

A preprocessor is a program that takes text and does lexical conversions to it. The type of lexical conversions may include substitution of macros, conditional inclusion, and inclusion of other files.

The C programming language has a preprocessor that performs the following transformations:

Replaces trigraphs with equivalents.
Concatenates source lines.
Replaces comments with white space.
Reacts to lines starting with an octothorp (#), performing macro substitution, file inclusion, conditional inclusion, and other transformations.

Overuse of the C preprocessor is considered bad style, especially in C++. Stroustrup introduced features such as templates into C++ in an attempt to make the C preprocessor irrelevant; however, his file inclusion alternative was never seriously considered as it was a poor imitation of the C preprocessor's file inclusion mechanism.

Other famous 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 a cumbersome in incremental parsing or incremental lexial analysis because changes on definition of rules of preprocessing can affect the entire text to be preprocessed.

Example

Typical example seen in hello program in C is

include
int
main ()
{
 printf ("hello mundo\\n");
}

In this case, #include is treated by preprocessor to include a file called stdio.h lexically.

Encyclopedia Home Page

Preprocessor

Example