meaning of pre processor

1. C preprocessor 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 BITn 1< This can be used with any appropriate actual argument: msb = BITnbits-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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