Thread: macro substitution rule

  1. #1
    Registered User
    Join Date
    Oct 2010

    macro substitution rule

    i want to know the rule behind the macro substitution

    #define p P
    int main()
    int P = 10;
    printf("\n P= %d",p);
    o/p: P=10

    my question is why dont macro substitution dont replace p in printf with P.

  2. #2
    Registered User
    Join Date
    Jun 2005
    The preprocessor understands enough of the C language to recognise that printf is a different identifier from p.

    Roughly speaking, the preprocessor only substitutes p for P where the p is in a context where it looks like a complete token.
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy or unhelpful in reply to you, or tell you you need to demonstrate more effort before you can expect help, it is likely you deserve it. Suck it up, Buttercup, and read this, this, and this before posting again.

  3. #3
    Registered User
    Join Date
    Oct 2010

    Smile tokenization before macro expansion

    There are 8 phases of Standard C translation
    1) Trigraph Replacement
    2) Line Splicing
    3) Tokenization
    4) Macro Expansion and Directive handling
    5) Character set mapping
    6) String concatenation
    7) Translation
    8) Linkage

    Since tokenization occur prior to macro expansion it does not expand p of printf(). The preprocessor does not parse the source text but it does break it up in tokens for the purpose of locationg macro calls.

    The Preprocessor (C/C++)

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