Preprocessor directives

The preprocessor is a program that is invoked by the compiler to process code before compilation. Commands for that program, known as directives, are lines of the source file beginning with the character #, which distinguishes them from lines of source program text. The effect of each preprocessor directive is a change to the text of the source code, and the result is a new source code file, which does not contain the directives. The preprocessed source code, an intermediate file, must be a valid C or C++ program, because it becomes the input to the compiler.

Preprocessor directives consist of the following:

• 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, 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.