GCC: suggest parentheses around comparison parentheses not able to solve myself

Question

I am trying to check whether this equal to zero as

//Address already malloc

void MyCheck (void *Address){

    if ( (long)Address & (~(sizeof(long)-1)) != 0 ){
           printf("Invalid");
    }

}

When try to compile it give me:

Error:  suggest parentheses around comparison in operand 
         of '&' [-Werror=parentheses]

score 9 · Accepted Answer · answered Feb 20 '14 at 21:28

9

!= has higher precedence than & , so your code is really doing the same as :

if ((long)Address &  ( (~(sizeof(long)-1)) != 0 ) )
                     ^                          ^

gcc suggests this might be an error, which it probably is. You'd want:

if ( ( (long)Address &  (~(sizeof(long)-1)) ) != 0 )
     ^                                      ^

answered Feb 20 '14 at 21:28

nos

@haccks Um. that chart has `!=` *above* `&` , unless you're suggesting the chart is in low-to-high order, which it isn't. – WhozCraig Feb 20 '14 at 21:36
@haccks C operators in order of precedence (highest to lowest). – Vineet1982 Feb 20 '14 at 21:38
@WhozCraig; My bad. I looked over here `= += -= *= /= %= &= ^= |= <<= >>=` and thought `|=` to be `!=` :) – haccks Feb 20 '14 at 21:38
In case anyone cares : [Near-Duplicate Question Here](http://stackoverflow.com/questions/4685072/c-operator-precedence-bitwise-lower-than) – WhozCraig Feb 20 '14 at 21:39

score 2 · Answer 2 · answered Feb 20 '14 at 21:26

2

Place a braces around (long)Address & (~(sizeof(long)-1)) in if statement.

if ( ( (long)Address & (~(sizeof(long)-1)) ) != 0 ){

answered Feb 20 '14 at 21:26

haccks

score 2 · Answer 3 · answered Feb 20 '14 at 21:30

2

!= is of a higher level of precedence than &, so what you're actually saying is:

if ((long)Address & ((~(sizeof(long)-1)) != 0)){
       printf("Invalid");
}

Instead, put parentheses around the first part:

if (((long)Address & (~(sizeof(long)-1))) != 0){
       printf("Invalid");
}

However, having a lot of background in TI-Basic and C, I would personally leave out the != 0 part:

if ((long)Address & (~(sizeof(long)-1))){
       printf("Invalid");
}

answered Feb 20 '14 at 21:30

bb94

Your answer is also correct but for validation i need !=0 – Vineet1982 Feb 20 '14 at 21:34
@Vineet1982 Just in case someday 0 != 0, you want to be absolutely sure ? The last snippet in this is defined to do exactly what the second does. If by validate you mean appeasing some code reviewer that doesn't know C from A, B, or ice-cream, ugh, but ok. – WhozCraig Feb 20 '14 at 21:42

score 1 · Answer 4 · answered Feb 20 '14 at 21:49

1

Although other answers have addressed the compiler error, you could sidestep the problem with:

if ((uintptr_t) Address % sizeof(long)) …

(To get uintptr_t, include <stdint.h>. And, if you are using C 2011, you might want to replace sizeof(long) with _Alignof(long).)

answered Feb 20 '14 at 21:49

Eric Postpischil

That's probably what he meant to do, but with the `~`, is that really what it does? – Marc Glisse Feb 20 '14 at 22:39
@MarcGlisse: Good point, but who can tell with all the parentheses and absence of documentation? – Eric Postpischil Feb 20 '14 at 23:12

4 Answers4