adhoc_mutex.c

#include <stdatomic.h>

int count()
{
  static atomic_flag mutex = ATOMIC_FLAG_INIT;
  static unsigned volatile counter = 0;
  unsigned volatile retval;
  
  while (atomic_flag_test_and_set_explicit(&mutex, memory_order_acquire));
  retval = ++counter;
  atomic_flag_clear_explicit(&mutex, memory_order_release);
  
  return retval;
}

adhoc_mutex.cpp

#include <atomic>

int count()
{
  static std::atomic_flag mutex; // automatically initialized
  static unsigned volatile counter = 0;
  unsigned volatile retval;
  
  while (mutex.test_and_set(std::memory_order_acquire));
  retval = ++counter;
  mutex.clear(std::memory_order_release);
  
  return retval;
}

std_mutex.cpp

#include <mutex>

int count()
{
  static std::mutex mtx;
  static unsigned counter = 0;
  std::lock_guard<mutex> _(mtx); // automatically released when it goes out of scope
  return ++counter; // automatically saves the return value then unlocks mutex then returns this value. 
}