1

I have written this simple C code,

int main()
{
        int *p = NULL;
        if (p && *p);
        //if (*p && p);
        return 0;
}

When I run this code, unexpectedly I didn't any seg fault. But if I change order of p and *p in if ()(like in the comment) I do get seg fault.

Can someone explain why?

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1
  • The first is a standard ideom. You would have found out yourself if you had done a little research. Or if you just had read the description of the && operator. But apparently first asking is easier than learning. Re. the 2nd variant (*p %% p): why do you expect a segfault? You invoke undefined behaviour; think about the meaning of the word undefined. Commented Aug 7, 2016 at 13:26

1 Answer 1

Reset to default
4

The && operator has short-circuit evaluation. If the result can be determined by just looking at the value of the left operand, then the right operand is not evaluated. For the && operator, this means that if the left side evaluates to false, it doesn't evaluate the right side, and the result is false. In other words, 

if (p && *p) ...

is equivalent to

if (p) if (*p) ...

From the C11 standard, section 6.5.13/4:

Unlike the bitwise binary & operator, the && operator guarantees left-to-right evaluation; if the second operand is evaluated, there is a sequence point between the evaluations of the first and second operands. If the first operand compares equal to 0, the second operand is not evaluated.

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7 Comments

any reference to your answer?
@ParthaBera: Added.
this applies to c also?
@ParthaBera yes it does apply to C also
@ParthaBera: Sorry, I didn't notice the C tag. I've changed my answer to use the equivalent reference from the C11 standard.
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