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Concept
C preprocessor
Summary
The C preprocessor is the macro preprocessor for several computer programming languages, such as C, Objective-C, C++, and a variety of Fortran languages. The preprocessor provides inclusion of , macro expansions, conditional compilation, and line control.
In many C implementations, it is a separate program invoked by the compiler as the first part of translation.
The language of preprocessor directives is only weakly related to the grammar of C, and so is sometimes used to process other kinds of .
The preprocessor was introduced to C around 1973 at the urging of Alan Snyder and also in recognition of the usefulness of the file inclusion mechanisms available in BCPL and PL/I. Its original version offered only file inclusion and simple string replacement using #include and #define for parameterless macros, respectively. It was extended shortly after, firstly by Mike Lesk and then by John Reiser, to incorporate macros with arguments and conditional compilation.
The C preprocessor was part of a long macro-language tradition at Bell Labs, which was started by Douglas Eastwood and Douglas McIlroy in 1959.
Preprocessing is defined by the first four (of eight) phases of translation specified in the C Standard.
Trigraph replacement: The preprocessor replaces trigraph sequences with the characters they represent. This phase will be removed in C23 following the steps of C++17.
Line splicing: Physical source lines that are continued with escaped newline sequences are spliced to form logical lines.
Tokenization: The preprocessor breaks the result into preprocessing tokens and whitespace. It replaces comments with whitespace.
Macro expansion and directive handling: Preprocessing directive lines, including file inclusion and conditional compilation, are executed. The preprocessor simultaneously expands macros and, since the 1999 version of the C standard, handles _Pragma operators.
One of the most common uses of the preprocessor is to include another source file:
#include
Official source
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