Preprocessor

The preprocessor is executed at translation phase 4

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