Preprocessing is an initial phase to process text before compilation. Preprocessor directives are lines of the source file where the first non-whitespace character is #, which distinguishes them from other lines of text. The effect of each preprocessor directive is a change to the text and the result is a transformation of the text that does not contain the directives nor comments. The compiler can optionally output the preprocessed text to a file that has a .i suffix. Preprocessing is always the initial phase of compilation, even when the text has already been preprocessed.
- Macro definition directives, which replace tokens in the current file with specified replacement tokens
- File inclusion directives, which imbed files within the current file
- Conditional compilation directives, which conditionally compile sections of the current file
- Message generation directives, which control the generation of diagnostic messages
Assertion directives (IBM extension), which specify
attributes of the system the program is to run on- The null directive (#), which performs no action
- Pragma directives, which apply compiler-specific rules to specified sections of code
Preprocessor directives begin with the # token followed by a preprocessor keyword. The # token must appear as the first character that is not white space on a line. The # is not part of the directive name and can be separated from the name with white spaces.
A preprocessor directive ends at the new-line character unless the last character of the line is the \ (backslash) character. If the \ character appears as the last character in the preprocessor line, the preprocessor interprets the \ and the new-line character as a continuation marker. The preprocessor deletes the \ (and the following new-line character) and splices the physical source lines into continuous logical lines. White space is allowed between backslash and the end of line character or the physical end of record. However, this white space is usually not visible during editing.
Except for some #pragma directives, preprocessor directives can appear anywhere in a program.