The C Preprocessor

The C preprocessor is not part of the C compiler. The C preprocessor runs before the compiler. During preprocessing, all occurrences of a macro name are replaced by the macro body that is associated with the macro name. Note that a macro statement ends with a newline character, not a semicolon.

The C preprocessor also enables you to include additional source files to the program or compile sections of C code conditionally.

The #define directive tells the preprocessor to replace every occurrence of a macro name defined by the directive with a macro body that is associated with the macro name. You can specify one or more arguments to a macro name defined by the #define directive.

The #undef directive is used to remove the definition of a macro name that has been previously defined.

The #ifdef directive controls whether a given group of statements is to be included as part of the program. The #ifndef directive is a mirror directive to the #ifdef directive; it enables you to define code that is to be included when a particular macro name is not defined.

The #if, #elif, and #else directives enable you to filter out portions of code to compile. #endif is used to mark the end of an #ifdef, #ifndef, or #if block because the statements under the control of these preprocessor directives are not enclosed in braces.