Why macros need parentheses in C

Substitution ignores precedence, so every parameter and the whole body get brackets.

#define BAD_SQUARE(x) x * x
#define SQUARE(x) ((x) * (x))
#define MAX(a, b) ((a) > (b) ? (a) : (b))

int compare_expansions(void) {
    int wrong = BAD_SQUARE(1 + 2);
    int right = SQUARE(1 + 2);
    return right - wrong;
}

int double_evaluation(void) {
    int i = 1;
    int bigger = MAX(i++, 0);
    return bigger + i;
}

How it works

1. Without brackets SQUARE(1 + 2) expands to 1 + 2 * 1 + 2.
2. Wrapping the parameters and the result fixes it.
3. A parameter used twice also evaluates its argument twice.

Keywords and builtins used here

• compare_expansions
• double_evaluation
• int
• return
• void

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Step 2 of 4 in Macros, step 2 of 12 in Preprocessor & headers.

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