Working of the C Preprocessor

Question

How does the following piece of code work, in other words what is the algorithm of the C preprocessor? Does this work on all compilers?

#include 

        

Answer 1

I think you are trying to get the information how the source code is processed by compiler. To know exactly you have to go through Translation Phases. The general steps that are followed by every compiler (tried to give every detail - gathered from different blogs and websites) are below:

1. First Step by Compiler - Physical source file characters are mapped to the source character set (introducing new-line characters for end-of-line indicators) if necessary. Trigraph sequences are replaced by corresponding single-character internal representations.

2. Second Step by Compiler - Each instance of a new-line character and an immediately preceding backslash character is deleted, splicing physical source lines to form logical source lines. A source file that is not empty shall end in a new-line character, which shall not be immediately preceded by a backslash character.

3. Third Step by Compiler - The source file is decomposed into preprocessing tokens and sequences of white-space characters (including comments). A source file shall not end in a partial preprocessing token or comment. Each comment is replaced by one space character. New-line characters are retained. Whether each nonempty sequence of other white-space characters is retained or replaced by one space character is implementation-defined.

4. Fourth Step by Compiler - Preprocessing directives are executed and macro invocations are expanded. A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively.

5. Fivth Step by Compler - Each escape sequence in character constants and string literals is converted to a member of the execution character set.

6. Sixth Step by Compiler - Adjacent character string literal tokens are concatenated and adjacent wide string literal tokens are concatenated.

7. Seventh Step by Compiler - White-space characters separating tokens are no longer significant. Preprocessing tokens are converted into tokens. The resulting tokens are syntactically and semantically analyzed and translated.

8. Last Step - All external object and function references are resolved. Library components are linked to satisfy external references to functions and objects not defined in the current translation. All such translator output is collected into a program image which contains information needed for execution in its execution environment.