Preprocessor
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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
#defineand#undef, and operators#and##) - include other files (controlled by directive
#include) - cause an error or warning (since C23) (controlled by directive
#erroror#warningrespectively (since C23))
The following aspects of the preprocessor can be controlled:
- implementation defined behavior (controlled by directive
#pragmaand operator_Pragma(since C99)) - file name and line information available to the preprocessor (controlled by directives
#line)
Footnotes
- ↑ 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
#warningwhich 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