C preprocessor

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)