The preprocessor runs before the compilation begins. The result of preprocessing is 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 things included in it:

• # character
• preprocessing instruction (one of define, undef, include, if, ifdef, ifndef, else, elif, endif, line, error, pragma)
• arguments (depends on the instruction)

The preprocessor has the source file translation capabilities:

• conditionally include of parts of source file (controlled by directives #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 a warning (controlled by directive #warning)
• cause an error (controlled by directive #error)

The following aspects of the preprocessor can be controlled:

• implementation defined behavior (controlled by directives #pragma)
• file name and line information available to the preprocessor (controlled by directives #line)