foo.cpp

#include <stdio.h>
#include <thread>
#include <sys/mman.h>
#include <assert.h>

struct Foo {
  int a;
  void *ptr;
  Foo() : a(4) {
    ptr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
    assert(ptr != MAP_FAILED);
  }
  ~Foo() {
    munmap(ptr, 4096);
  }
};

thread_local Foo foo;


static void f1(int a) {
  foo.a += a;
}

int main() {
  for (int i = 0; i < 10000; i++) {
    std::thread t1(f1, 1);
    std::thread t2(f1, 2);

    t1.join();
    t2.join();
  }
}