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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../../cpp/../c/preprocessor.html below:

Preprocessor - cppreference.com

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.

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.

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)

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)

• C23 standard (ISO/IEC 9899:2024):

• 6.10 Preprocessing directives (p: TBD)

• 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

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