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.

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)
• 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)

References

• C17 standard (ISO/IEC 9899:2018):
• 6.10 Preprocessing directives (p: 117-129)
• C11 standard (ISO/IEC 9899:2011):
• 6.10 Preprocessing directives (p: 160-178)
• C99 standard (ISO/IEC 9899:1999):
• 6.10 Preprocessing directives (p: 145-162)
• C89/C90 standard (ISO/IEC 9899:1990):
• 3.8 PREPROCESSING DIRECTIVES

See also

C++ documentation for Preprocessor