Preprocessor
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:
- Replaces trigraphs with equivalents.
- Concatenates source lines.
- Replaces comments with whitespace.
- 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 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
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.
Referenced By
AspectJ | List of computing topics
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