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I think there are compiler flags available that will do this (at least the bounds checking).

When talking about C it's important to understand why the standard is the way it is. C was designed to be a portable language. Not in the sense that code is easily portable, but that the language itself is easily portable by writing a new compiler for it on whatever OS you're using. As such, the C standard is written to make C an easy language to write a compiler for, which is why it has so much undefined behavior.

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I don't think bounds checking can be done at compile time.

Otherwise Java, Rust, Go et al. wouldn't need to incur the runtime overhead of the extra bookkeeping int per array.

I'm not an expert, but you can do this in a language with dependent types such as Idris. But given the historical progress of the industry, it will probably be a long, long time before such an idea percolates down into an industrial language.

-fbounds-checking, I'm sure there is a runtime cost, but it's provided by the compiler.

Not applicable to C -- "Currently only supported by the Java and Fortran front ends."

https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Code-Gen-Option...

There have been bounds-checked implementations of C, but the runtime cost is very large. The problem is that to do it, you need to check not only explicit array references, but pointer dereferences as well -- and in order to make that work, each pointer needs to carry around with it its associated bounds. That, in turn, means that your pointer representation has to be something more (or other) than just a plain machine address.