C preprocessor
C preprocessor
(tool, programming)#define BUFFER_SIZE 256
as a textual assignment giving the symbol BUFFER_SIZE avalue "256". Symbols defined with cpp are traditionally givenupper case names to distinguish them from C identifiers. Thissymbol can be used later in the input, as in
char input_buffer[BUFFER_SIZE];
This use of cpp to name constants, rather than writing thesemagic numbers inline, makes a program easier to read andmaintain, especially if there is more than one occurrence ofBUFFER_SIZE all of which must all have the same value.
Cpp macros can have parameters:
#define BIT(n) (1<
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 theexpression substituted in place of n might not be interpretedcorrectly (though the example above would be OK).
Cpp also supports conditional compilation with the use of
#ifdef SYMBOL...#else...#endifand#if EXPR...#else...#endif
constructs, where SYMBOL is a Cpp symbol which may or may notbe defined and EXPR is an arithmetic expression involving onlyCpp symbols, constants and C operators which Cpp can evaluateto 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.