The second item is warning against a particular misuse of pointers that was apparently widespread at the time. On VAX Unix, it happened that the memory at address zero of a process was readable and contained zeroes. So a lot of code would misuse a NULL pointer as being equivalent to a pointer-to-an-empty-string, for instance: worked fine on the VAX!. So it's kind of a specific flavour of commandment 10. These days the common machines don't have a readable address zero, and all the old non-portable code has faded into history.

right. compat for this still exists on solaris at least, as lib0@0: see the notes section at the bottom of [0].

[0] - https://docs.oracle.com/cd/E19683-01/816-0210/6m6nb7md6/inde...

That could enable optimization. The compiler could remove many NULL-pointer checks.

On many platforms, all that is required is a change to the compiler. It could emit a ".quad 0" or equivalent, in a common data section at absolute address zero. Some platforms would require linker changes or even kernel changes.

16-bit x86 also had the interrupt handler table at linear address 0, I seem to recall some compilers for DOS that would make NULL point at that.

Yes. At least writing to interrupt vector table usually crashed your computer right at the point. I remember a certain DSP architecture that system flags near zero address. It was an interesting experience to debug a program that failed because a write to struct member through null pointer switched the machine to a different instruction set.

Every MMU-less micro lets you read address 0.

A lot of micros have MPUs that'll catch address 0 accesses without a full MMU. AFAIK all ARMs newer than ARM7 (ie. ARMv5) have one for instance.

Yeah, they will, if you activated it. And if you made access to 0 address be caught explicitly ( you might have the MMU active but not have it configured in such a way)

Yes, I know and I agree, I just said I'd rather put a more broad message there

Can you program in C effectively without pointers?

Nope, therefore you cannot do it effectively if you do not understand them :)

Depends on what you mean by "effectively." If you wanna, say, do an exhaustive search over all 4-dimensional integer vectors in a rectangular region, and print off the ones that satisfy some relationship, you don't need pointers (nevermind what printf and string literals are doing under the hood). It's a wonderfully beefy desktop calculator, in that way. But for most applications, pointers are pretty much necessary.

Arrays in C (once passed to functions) are pointers, even when they use array syntax:

https://www.digitalmars.com/articles/C-biggest-mistake.html

You don't need arrays for the code I was describing: it's a nested for-loop on 4 ints.

But, yeah, that's a really common mistake.

You can program C effectively without pointer arithmetic. Storing copies of pointers for later is usually a bad idea.