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Meaning of C PREPROCESSOR

 
Computing Dictionary
 
 Definition: 

(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.

 

 

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