More undefined behavior in C

Shifting too far, reading uninitialized memory, dividing by zero — and the guards.

#include <limits.h>

int shift_safely(unsigned int value, unsigned int by) {
    if (by >= sizeof(unsigned int) * CHAR_BIT) {
        return 0;
    }
    return (int) (value << by);
}

int divide_safely(int a, int b, int *out) {
    if (b == 0 || (a == INT_MIN && b == -1)) {
        return 0;
    }
    *out = a / b;
    return 1;
}

int initialized(void) {
    int value = 0;
    return value;
}

How it works

1. Shifting by the width of the type is undefined, not zero.
2. INT_MIN / -1 overflows, so division needs two checks.
3. An uninitialized read may return anything at all.

Keywords and builtins used here

• divide_safely
• if
• initialized
• int
• return
• shift_safely
• sizeof
• unsigned
• void

The run, in numbers

Lines

21

Characters to type

345

Tokens

111

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 63 seconds.

Type this snippet

Step 3 of 3 in Undefined behavior, step 10 of 13 in Unions, bitfields & undefined behavior.

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