[bigcheesegs]

In C++ this violates the data race rules both with and without volatile, but because it's sig_atomic_t it has a special carve out _only_ if it's volatile. See https://eel.is/c++draft/basic#intro.races-22

C however states :

> When the processing of the abstract machine is interrupted by receipt of a signal, the values of objects that are neither lock-free atomic objects nor of type volatile sig_atomic_t are unspecified, [...] The representation of any object modified by the handler that is neither a lock-free atomic object nor of type volatile sig_atomic_t becomes indeterminate when the handler exits.

This wording is not present in C++, as it instead defines how signal handlers fit into the memory model.

This means that (with adjustments for C atomics):

  int val = 0;
  std::atomic<bool> flag{false};
  
  void handler(int sig) {
    if (!set) {
      val = 1;
      flag = true;
    }
  }

  int main(void) {
    signal(SIGINT, handler);
    while (!flag) { /* Spin waiting for flag */ }
    return val;
  }

Is valid in C++, but not in C.

[kazinator]

std::anything<whatever> is obviously invalid in C.