Question

When testing for NULL, I see a lot of code that uses !var. Is there a reason to use this kind of test as opposed to the more explicit var == NULL. Likewise would if (var) be a correct test for an item being non-null?

Answer 1

The difference between:

!var

and

var == NULL

is in the second case the compiler have to issue a diagnostic if var is not of a pointer type and NULL is defined with a cast (like (void *) 0).

Also (as pointed by @bitmask in the comments) to use the NULL macro, you need to include a standard header that defines the NULL macro. In C the NULL macro is defined in several headers for convenience (like stddef.h, stdio.h, stdlib.h, string.h, etc.).

Otherwise the two expressions are equivalent and it is just a matter of taste. Use the one you feel more confortable at.

And for your second question if (var) is the same as if (var != NULL) with the difference noted above.