Is NULL always false?

The 'C' language dates from an era where (void*)0 could actually be a valid pointer. It is not that long ago, the 8080 and Z80 microprocessors had an interrupt vector at address 0. Faced with such architecture choices, it couldn't do anything but let a header file declare the value of NULL. There were some compilers out there, now long forgotten, where NULL was not equal to (void*)0 (0xffff was the next alternative), thus giving your if() statement undefined behavior.

C++ mercifully put an end to this, a null pointer is assignable from and testable against 0.

Yes. NULL evaluates to false, since C considers any non-zero value true and any zero value false. NULL is essentially the zero address and is treated as such in comparisons, and I believe would be promoted to an int for the boolean check. I would expect that your code is readable to anyone familiar with C although I would probably make the check explicit.

In C and C++ programming, two null pointers are guaranteed to compare equal; ANSI C guarantees that any null pointer will be equal to 0 in a comparison with an integer type; furthermore the macro NULL is defined as a null pointer constant, that is value 0 (either as an integer type or converted to a pointer to void), so a null pointer will compare equal to NULL.

Ref: http://en.wikipedia.org/wiki/Null_pointer#Null_pointer