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
,endif
,line
,error
,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
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 (controlled by directive
#error
)
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
- ↑ 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.
References
- 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