if (!ptr) VS if (ptr == NULL) in C

Question

Sometimes I see programmers use:

void *ptr = <something>;
if (ptr == NULL)
    <do something>;

Instead of:

void *ptr = <something>;
if (!ptr)
    <do something>;

Is there anything that can go wrong with if (!ptr), or is it just a coding style preference?

There seem to be some info [here](https://stackoverflow.com/a/6187656/133203), but I'm not sure if this should be classed as a duplicate — Federico klez Culloca, Sep 01 '17 at 13:49
related https://stackoverflow.com/questions/9894013/is-null-always-zero-in-c — cleblanc, Sep 01 '17 at 13:58

score 5 · Accepted Answer · edited Jun 20 '20 at 09:12

5

In C, it is just a coding style preference.

Some people prefer if (NULL == ptr) with the argument that if the programmer made a typo (and mistyped the == as a single =) the compiler will complain. However, many compilers would emit a warning for if (ptr=NULL) (at least recent GCC do, when invoked as gcc -Wall -Wextra as you should).

In C++ (where you would use nullptr instead of NULL) there could be a difference, because one can redefine operator ! (e.g. on smart pointers). However, on raw plain pointers (like void*, or sometype* or SomeClass*), you cannot redefine operators like ! or !=.

BTW, some weird processors might have NULL pointers which are not an all zero-bits machine word (but the compiler should deal with this issue). I can't name any such processor in wide use today (however think of 1980s segmented 16 bits x86 as a counter example).

edited Jun 20 '20 at 09:12

Community

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answered Sep 01 '17 at 13:50

Basile Starynkevitch

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One could redefine `==`, too, so it's unclear which is better. Though if you are daft enough to define `a==0` != `!a`, you deserve all that comes to you... – Toby Speight Sep 01 '17 at 14:14
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IIRC, segmented x86 has null pointers that aren't necessarily all-bits-zero (due to use of segment registers), and the compiler must make sure that they compare equal to `0` . There's probably some x86 around today (notably in devices such as cameras and other embedded controllers). – Toby Speight Sep 01 '17 at 14:18
I thought of that, but I won't call 16 bits segmented x86 in wide use today. – Basile Starynkevitch Sep 01 '17 at 14:32
I thought that Canon DSLRs are 16-bit x86? Perhaps I misremember. – Toby Speight Sep 01 '17 at 14:36
Is `NULL` always a zero? because if it's not then `if (!ptr)` won't be equivalent to `if (ptr == NULL)`, when `ptr` is `NULL`. – Bite Bytes Sep 01 '17 at 16:07
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The compiler would compile both equivalently. I don't know what you mean by `NULL` always zero. – Basile Starynkevitch Sep 01 '17 at 16:10
I mean all it's bits are 0 – Bite Bytes Sep 01 '17 at 16:13
@Toby Speight: but that is a camera, not a programmable computer – PintoDoido Feb 13 '19 at 11:39
@PintoDoido: today, a camera *is* a programmable, embedded, computer system. – Basile Starynkevitch Feb 13 '19 at 12:07
@Toby Speight what programming tools do you use for that? – PintoDoido Feb 13 '19 at 13:11

if (!ptr) VS if (ptr == NULL) in C

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