As far as I know, using NULL explicitly makes programs more portable (a low 
priority for code specific to any OS kernel obviously).

I have always seen NULL defined as 0. However, NULL simply represents an 
invalid value for a pointer, and its actually value is PLATFORM SPECIFIC as 
far as ANSI compliant code goes. That is, a platform can define NULL to be 
any value which the platform would interprit to be an invalid pointer. NULL 
is usually zero because the first address in memory is usually never 
referenced or reserved.

Assuming it is zero is technically incorrect. It *could* be non-zero, in 
which case the resulting branch of the if() block would be different. Since 
this is code which I assume is for the NetBSD kernel, then it only depends 
on how NetBSD defines NULL.

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