The C preprocessor is a macro processor that is used automatically by the C compiler to transform your program before actual compilation. It is called a macro processor because it allows you to define macros, which are brief abbreviations for longer constructs.
The C preprocessor provides four separate facilities that you can use as you see fit:
C preprocessors vary in some details. This manual discusses the GNU C preprocessor, the C Compatible Compiler Preprocessor. The GNU C preprocessor provides a superset of the features of ANSI Standard C.
ANSI Standard C requires the rejection of many harmless constructs commonly used by today's C programs. Such incompatibility would be inconvenient for users, so the GNU C preprocessor is configured to accept these constructs by default. Strictly speaking, to get ANSI Standard C, you must use the options `-trigraphs', `-undef' and `-pedantic', but in practice the consequences of having strict ANSI Standard C make it undesirable to do this. See section Invoking the C Preprocessor.
The C preprocessor is designed for C-like languages; you may run into problems if you apply it to other kinds of languages, because it assumes that it is dealing with C. For example, the C preprocessor sometimes outputs extra white space to avoid inadvertent C token concatenation, and this may cause problems with other languages.
Most C preprocessor features are inactive unless you give specific directives to request their use. (Preprocessing directives are lines starting with `#'; see section Preprocessing Directives). But there are three transformations that the preprocessor always makes on all the input it receives, even in the absence of directives.
The first two transformations are done before nearly all other parsing and before preprocessing directives are recognized. Thus, for example, you can split a line cosmetically with Backslash-Newline anywhere (except when trigraphs are in use; see below).
/* */ # /* */ defi\ ne FO\ O 10\ 20
is equivalent into `#define FOO 1020'. You can split even an escape
sequence with Backslash-Newline. For example, you can split
between the `\' and the `b' to get
This behavior is unclean: in all other contexts, a Backslash can be inserted in a string constant as an ordinary character by writing a double Backslash, and this creates an exception. But the ANSI C standard requires it. (Strict ANSI C does not allow Newlines in string constants, so they do not consider this a problem.)
But there are a few exceptions to all three transformations.