Just use array properly and you get run-time bounds checking: https://godbolt.or...

lelanthran · on Oct 7, 2023

TIL ...

I never knew that you got runtime bounds-checking with VLAs.

Do you have a link that explains this snippet in more detail? Why/how does it work?

It doesn't appear to work with non-VLAs though.

uecker · on Oct 7, 2023

It works because the type (char (buf)[n]) knows the dynamic size 'n'. So the compiler can simply add the bounds check to an array access (buf)[i] if instructed to do so.

If you replace the sizes with constants it also works: https://godbolt.org/z/sKPW6zT87

The safety story is not complete though: If you pass the wrong size to 'foo' this is not detected (this is easy to add to compilers and I submitted a patch to GCC which would do this): https://godbolt.org/z/T8844e1z8

(ASAN still catches the problem in this case, but ASAN does not work consistently and has a high run-time overhead.)