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Comment by okl | Hacker News Reader

You can even do

  #define free(p) do { free(p); p = NULL; } while(0)

And when you want to call the original free:

  (free)(p);

The preprocessor will not substitute this occurence of free.

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C n00b here - why the `do {} while(0)`? Couldn't you just do something like `#define free(p) { free(p); p = NULL; }`?

Semi-experienced C user here, I believe the anonymous block is perfectly adequate here. No idea why they are wrapping it in a single instance do loop, unless they’re unaware of block scoping or I’m unaware of some UB here.

do {} while(0) is a common idiom for macros in C, because it consumes the trailing semicolon, which a bare {} block doesn't do.

    if(x) MACRO();
    else something();

expands to

    if(x) { ... }; // Error!
    else something();

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