C preprocessor

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<tool, programming> (cpp) The standard Unix macro-expansion utility run as the first phase of the C compiler, cc. Cpp interprets lines beginning with "#" such as

	#define BUFFER_SIZE 256

as a textual assignment giving the symbol BUFFER_SIZE a value "256". Symbols defined with cpp are traditionally given upper case names to distinguish them from C identifiers. This symbol can be used later in the input, as in

	char input_buffer[BUFFER_SIZE];

This use of cpp to name constants, rather than writing these magic numbers inline, makes a program easier to read and maintain, especially if there is more than one occurrence of BUFFER_SIZE all of which must all have the same value.

Cpp macros can have parameters:

	#define BIT(n) (1<<(n))

This can be used with any appropriate actual argument:

	msb = BIT(nbits-1);

Note the parentheses around the "n" in the definition of BIT. Without these, operator precedence might mean that the expression substituted in place of n might not be interpreted correctly (though the example above would be OK).

Cpp also supports conditional compilation with the use of

	#ifdef SYMBOL
	...
	#else
	...
	#endif
and
	#if EXPR
	...
	#else
	...
	#endif

constructs, where SYMBOL is a Cpp symbol which may or may not be defined and EXPR is an arithmetic expression involving only Cpp symbols, constants and C operators which Cpp can evaluate to a constant at compile time.

Decus cpp is a free implementation for VMS.

The most widely used C preprocessor today is the GNU CPP, distributed as part of GCC.

(2001-12-31)

Nearby terms: cpo « cpp « cppp « C preprocessor » C Programmer's Disease » C-Prolog » cproto

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