Why doesn’t my program seg fault when I dereference a NULL pointer inside of malloc?

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You are not really dereferencing anything. The argument of sizeof is not evaluated, unless it is a VLA. It is explicitly allowed by the language to put whatever “garbage” you want as the argument of sizeof. The language guarantees that it will not evaluate anything, just perform compile-time analysis of the type of the expression. For example, expression sizeof i++ is guaranteed not to change the value of i.

The only exception from that rule is Variable Length Arrays. The result of sizeof for VLAs is a run-time value, which means that the argument is evaluated and must be valid.

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