[JakobProgsch]

The first line is why I'm not 100% convinced of the severity of this mistake compared to the alternatives. The problem fundamentally is the use of magic values/numbers to represent the concept of "no value". You don't need explicit language support to have that concept and the bugs it causes. I guess having that as an intrinsic concept in the language makes it more likely that people use it badly. On the other hand debuggers etc. also intrinsically understand this and segfaults due to null pointers are usually very easy to localize once you see them. On the other hand if a "bad programmer" introduced their own magic non-value in a supposedly safe language, debugging that becomes way more confusing.

[andrepd]

No, that's not the "fundamental problem". The fundamental problem is a type system that lies. A "pointer to string" is not actually a pointer to a string, it's a pointer to a string or to nothing. If your api returns a pointer of the latter type, it should signal this by making the return type "maybe-pointer to string" (although it has the same memory representation as "pointer to string"). Then, if the user tries to dereference a maybe-pointer (that is, to use a maybe-pointer as a pointer), the type system can statically catch this and make it a simple type check failure compilation error. The user must first check if it's null through a function that casts a maybe-pointer to a pointer.

Nothing about this precludes the usage of sentinel values.