NULL v. zero [LWN.net]

NULL v. zero

Posted Jul 15, 2004 17:03 UTC (Thu) by kamil (subscriber, #3802)
In reply to: NULL v. zero by ikm
Parent article: NULL v. zero

0 is not good enough if the compiler can't guess from the context that you want a pointer. This typically happens in cases such as this one:

int execl(const char *path, const char *arg, ...);

Here, one should pass null pointer as the last argument. Passing 0 is usually OK on 32-bit machines, but not on 64-bit ones. You must explicitly cast 0 to a pointer type to conform to the standard. But NULL will be just as erroneous here, given that the standard allows it to be defined as a plain int 0. So you would in fact have to write (void*)NULL to conform both to the C standard and to Linus :-).

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NULL v. zero

Posted Jul 15, 2004 19:25 UTC (Thu) by knobunc (subscriber, #4678) [Link]

Sorry, I don't see why you would need to cast NULL to conform to the spec as summarized above since it is says "implementation-defined null pointer constant" so it must provide something that is a 64-bit pointer.

-ben

NULL v. zero

Posted Jul 15, 2004 19:34 UTC (Thu) by Ross (subscriber, #4065) [Link]

I think the other person mean (const char *)NULL, but maybe not.

NULL v. zero

Posted Jul 15, 2004 19:32 UTC (Thu) by Ross (subscriber, #4065) [Link]

Yeah, but unfortunately you can't pass plain NULL either (though it works
on almost any system). You have to cast it to the correct pointer type.
(Of course using 0 is even worse.)

NULL v. zero

Posted Jul 15, 2004 22:45 UTC (Thu) by ikm (subscriber, #493) [Link]

I guess you can always safely pass NULL, as it would be declared as a plain 0 only in case it really physically is a plain 0. In any other case it would be ((void *)0). Any sane library would always declare NULL as ((void *)0), because it is the compiler that always knows the actual physical value for sure :)

Of course, all other values for NULL would be incorrect, as it was explained in my initial posting. One can not define NULL as 0xFFFFFFFF, for example, as it would not qualify as a null pointer constant.

Thanks for providing the example, that clarified things a bit!