Undefined behavior: strict aliasing in C

Reading an object through an unrelated pointer type breaks the aliasing rule.

#include <string.h>

/* Undefined: an int object read as a float. */
float punned(int bits) {
    return *(float *) &bits;
}

/* Defined: copy the bytes into a real float. */
float reinterpreted(int bits) {
    float value;
    memcpy(&value, &bits, sizeof value);
    return value;
}

unsigned char first_byte(const void *object) {
    return *(const unsigned char *) object;
}

How it works

1. Only a char pointer may alias any object.
2. Casting an int pointer to a float pointer is undefined.
3. memcpy between the two is the defined way to reinterpret.

Keywords and builtins used here

• char
• const
• first_byte
• float
• int
• punned
• reinterpreted
• return
• sizeof
• unsigned
• void

The run, in numbers

Lines

17

Characters to type

358

Tokens

69

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 39 seconds.

Type this snippet

Step 2 of 3 in Undefined behavior, step 9 of 13 in Unions, bitfields & undefined behavior.

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