[ncruces]

I actually encountered it a couple weeks ago.

Can you spot the infinite loop in this function?

  char* strcpy(char* restrict d, const char* restrict s) {
    stpcpy(d, s);
    return d;
  }

I'll help. A call to `stpcpy` that ignores the return value can be swapped with a call to the (more likely to be optimized) `strcpy`. Since that's infinite recursion, and there is no forward progress, it's undefined behavior and anything goes.

This isn't just theory, it actually broke things in practice for me.

[rssoconnor]

Naming an externally linked function with the prefix "str" is by itself UB since that prefix is reserved for <string.h>.

[ncruces]

Right. I am doing a standard library replacement, and forgot I needed to compile freestanding. Oops. My bad.

So `str` and `mem` are reserved. But then so are `to` and `is` (by <ctype.h>). Just forget about having a function named `is_valid_user`.

And so are `mtx_`, `cnd_`, `thrd_`, `atomic_`, `memory_`...

Which is why... everything in C is UB.