volatile busy wating

consequently C++, the keyword was intended to:[citation needed]

    * allow access to memory mapped devices
    * allow uses of variables between setjmp and longjmp
    * allow uses of variables in signal handler

易挥发的，避免优化
===>busy waiting (or spinning)
is a technique in which a process repeatedly checks to see if a condition is true.such as waiting for keyboard input or waiting for a lock to become available.
It's considered an anti-pattern.
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>

//volatile int i = 0;
int i = 0;

static void *
f1 (void *p)
{
  while (i == 0)
    {
      ;
    }
  printf ("i's value has changed to %d\n", i);
  return NULL;
}

static void *
f2 (void *p)
{
  sleep (2);
  i = 99;
  printf ("t2 has changed value\n");
  return NULL;
}

int
main ()
{
  int rc;
  pthread_t t1, t2;
  rc = pthread_create (&t1, NULL, f1, NULL);
  if (rc != 0)
    {
      fprintf (stderr, "pthread f1 failed\n");
      return EXIT_FAILURE;
    }

  rc = pthread_create (&t2, NULL, f2, NULL);
  if (rc != 0)
    {
      fprintf (stderr, "pthread f2 failed\n");
      return EXIT_FAILURE;
    }

  pthread_join (t1, NULL);
  pthread_join (t2, NULL);
  puts ("all pthreads finished.");
  return 0;
}

gcc -o busy_wati busy_wati.c -lpthread -O3[b][/b]

1,加了O3优化才会变成while(true)


替换busy_waiting的方法：
signals,
lock acquisitios,timers,I/O avail....

适合地点：在一些硬件设备上，设备驱动上（来回切换的代价比较高)