When is a segmentation fault guaranteed?

Question

I'm learning c++ and I've learned that segfaults can happen because of undefined behaviour. Are there ever cases though, where segfaults are guaranteed to happen? Or is it always undefined behaviour?

Answer 1

Segfaults occur in an operating system with memory protection when you attempt to read or write memory that you do not have permission to access — special values like nullptr , memory reserved for the OS, or memory of another process.

Because segfaults are raised by the OS, they are an inherently platform-specific concept. Something that causes a segfault on Windows will not necessarily cause a segfault on eg AmigaOS. The C++ standard, being platform-agnostic, doesn't even mention segfaults.

On modern OSes with memory protection, you can always be assured a segfault when you access memory that you don't own. Dereferencing nullptr will cause it for sure. Accessing a random address is probably not within your process's address space, so that will likely cause a segfault (hence why accessing uninitialized pointers or dangling pointers often causes a segfault). On AmigaOS or classic Mac OS, which lack memory protection, you won't get a segfault from accessing random pointers. Instead, you could overwrite the memory of the OS or another process (which would likely be disastrous).

Basically, a segfault is an OS concept, not a C++ concept, so what you can do to cause one in C++ depends on what you're running on.