Test program to see what compilers enforce C and C++ having different function pointer types (and different calling conventions).

gistfile1.cpp

// What systems does this break on?
// The C++ standard says that function pointers to C and C++ functions are
// different types even if they have the same signature.

#include <assert.h>

extern "C" int run(int (*f)(int), int x) { return f(x); }

int times2(int x) { return x * 2; }

int main(int argc, char *argv[]) {
  int a = times2(argc);
  // This is undefined behavior according to C because I am passing an
  // "extern C++" function pointer to an "extern C" function.
  int b = run(&times2, argc);
  assert(a == b);
  return a;
}

That should work as expected.
There is no undefined behavior in this code.

In this code the function run() is compiled by the C++ compiler.
So even though its ABI is C it can still call C++ functions (even through a function pointer). In fact the code planted is going to call a C++ function as the parameter f is not declared to have extern "C" interface. Thus it is expecting f to point at a C++ function and will plant the appropriate code.