Preprocessor

The preprocessor is executed at translation phase 4 , before the compilation. The result of preprocessing is a single file which is then passed to the actual compiler.

Contents 1 Directives 2 Capabilities 3 Footnotes 4 Example 5 References 6 See also

Directives

The preprocessing directives control the behavior of the preprocessor. Each directive occupies one line and has the following format:

• # character
• preprocessing instruction (one of define , undef , include , if , ifdef , ifndef , else , elif , elifdef , elifndef (since C23) , endif , line , embed (since C23) , error , warning (since C23) , pragma ) ^[1]
• arguments (depends on the instruction)
• line break.

The null directive ( # followed by a line break) is allowed and has no effect.

Capabilities

The preprocessor has the source file translation capabilities:

• conditionally compile of parts of source file (controlled by directive #if , #ifdef , #ifndef , #else , #elif , #elifdef , #elifndef (since C23) and #endif ).
• replace text macros while possibly concatenating or quoting identifiers (controlled by directives #define and #undef , and operators # and ## )
• include other files (controlled by directive #include and checked with __has_include (since C23) )
• cause an error or warning (since C23) (controlled by directive #error or #warning respectively (since C23) )

The following aspects of the preprocessor can be controlled:

• implementation defined behavior (controlled by directive #pragma and operator _Pragma (since C99) )
• file name and line information available to the preprocessor (controlled by directives #line )

Footnotes

1. ↑ These are the directives defined by the standard. The standard does not define behavior for other directives: they might be ignored, have some useful meaning, or make the program ill-formed. Even if otherwise ignored, they are removed from the source code when the preprocessor is done. A common non-standard extension is the directive #warning which emits a user-defined message during compilation. (until C23)

Example

This section is incomplete Reason: no example

References

See also

C documentation for Predefined Macro Symbols

C documentation for Macro Symbol Index

C++ documentation for Preprocessor