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Comment by richo | Hacker News Reader

that's what mapping an -rwx page at 0x0 does, and as a result it segfaults, which is an access violation.

There is some subtlety about dereferencing a null pointer. Many languages (C, C++, Rust) state that *NULL is undefined behaviour, that is, the compiler can assume that it never happens and optimises based on this. This can lead to a "misoptimised" program that doesn't actually segfault when the source suggests it should.

http://blog.llvm.org/2011/05/what-every-c-programmer-should-...

No, you are wrong.

First, mapping that page doesn't cause all null pointer dereferences to segfault.

And second, the language doesn't require a segfault. In fact, it explicitly permits the implementation to do whatever it likes.

That is the difference between safe and unsafe. It is in the language definition.

"the language"... which one? AIUI, Go doesn't state that null dereferences are undefined behaviour, but rather that they are guaranteed to panic.

"The language" ?

C doesn't define behaviour of a null deref, but most compilers map a -rwx page there to ensure that attempts to deref fault.

In what circumstance do they not?

The compilers don't map pages there, the operating system does. The problem is the compiler will optimise assuming that a null deref never happens, so you can have source that looks like it should crash due to a null deref, but the compiler has "misoptimised" it to have very different behaviour.

http://blog.llvm.org/2011/05/what-every-c-programmer-should-...

...I'm talking about throwing an exception or something similar, not getting a segfault. Like in Java.

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