Preprocessor: Meaning (information, definition, explanation, facts)

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

1. Replaces trigraphs with equivalents.
2. Concatenates source lines.
3. Replaces comments with whitespace.
4. Reacts to lines starting with an octothorp (#), performing macro substitution, file inclusion, conditional inclusion, and other transformations.

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 proprecessor 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.

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.

C Example

A typical example in C is:

#include <stdio.h>

int main (void)
{
    printf("Hello, world!\n");
    return 0;
}

The preprocessor replaces the line #include <stdio.h> with the system header file of that name, which facilitates use of the printf function.