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Comment by dataflow | Hacker News Reader

> That the annotation applies to variables and not types is surely an oversight or mistake right?

I don't think so. It doesn't make sense on the type. Otherwise, what should happen here?

  char s[1];
  char (__nonstring ns)[1];  // (I guess this would be the syntax?)
  s[0] = '1';
  ns[0] = '\0';
  char* p1 = s;  // Should this be legal?
  char* p2 = ns;  // Should this be legal?
  char* __nonstring p3 = s;  // Should this be legal?
  char* __nonstring p4 = ns;  // Should this be legal?

  foo(s, ns, p1, p2, p3, p4);  // Which ones can foo() assume to be NUL-terminated?
                               // Which ones can foo() assume to NOT be NUL-terminated??

By putting it in the type you're not just affecting the initialization, you're establishing an invariant throughout the lifetime of the object... which you cannot enforce in any desirable way here. That would be equivalent to laying a minefield throughout your code.

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Perhaps unsigned could help here with understanding.

unsigned means, don't use of an integer MSB as sign bit. __nonstring means, the byte array might not be terminated with a NUL byte.

So what happens if you use integers instead of byte arrays? I mean cast away unsigned or add unsigned. Of course these two areas are different, but one could try to design such features that they behave in similar ways where it makes sense.

I am unsure but it seems, if you cast to a different type you lose the conditions of the previous type. And "should this be legal", you can cast away a lot of things and it's legal. That's C.

But whatever because it's not implemented. This all is hypothetical. I understand GCC that they took the easier way. Type strictness is not C's forte.

> Perhaps unsigned could help here with understanding.

No, they're very different situations.

> unsigned means, don't use of an integer MSB as sign bit.

First: unsigned is a keyword. This fact is not insignificant.

But anyway, even assuming they were both keywords or both attributes: "don't use an MSB as a sign bit" makes sense, because the MSB otherwise is used as a sign bit.

> __nonstring means, the byte array might not be terminated with a NUL byte.

The byte array already doesn't have to contain a NUL character to begin with. It just so happens that you usually initialize it somewhere with an initializer that does, but it's already perfectly legal to strip that NUL away later, or to initialize it in a manner that doesn't include a NUL character (say, char a[1] = {'a'}). It doesn't really make sense to change the type to say "we now have a new type with the cool invariant that is... identical to the original type's."

> I understand GCC that they took the easier way. Type strictness is not C's forte.

People would want whatever they do to make sense in C++ too, FWIW. So if they introduce a type incompatibility, they would want it to avoid breaking the world in other languages that enforce them, even if C doesn't.

Do you mean s & ns to be swapped? ns starts with a NUL terminator and s does not.

No actually, that was the point. I was asking, what do you think should happen if you store a NUL when you're claiming you're not. Or if you don't store a NUL, when you claim it's there.

Well, as a human compiler, I said "Hey, you've non-NUL terminated a NUL terminated string". If that was what you intended you should use the type annotation for that, so I think that case worked as intended.

EDIT: > what do you think should happen if you store a NUL when you're claiming you're not

I don't believe nonstring implies it doesn't end with a NUL, just that it isn't required to.

But char[] already isn't required to be NUL-terminated to begin with. char a[1] = {'a'} is perfectly fine, as is a[0] = '1'. If all you want to do is to document the fact that a type can do exactly what it already can... changing the type to something new doesn't make sense.

Note that "works as intended" isn't sole the criterion for "does it make sense" or "should we do this." You can kill a fly with a cannon too, and it achieves the intended outcome, but that doesn't mean you should.

Is ns NUL terminated, or is it an array of chars that happens to end with NUL?

If ns is __nonstring, it could be the latter. Without it, it should be the former and warn if it's not. That's not ambiguous.

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