c中的volatile关键字

volatile说明变量是容易变化的，与编译过程中的优化有关。比如在循环中的指示变量并不在循环体中重新赋值，编译器可能会把这个指示变量拷贝到寄存器中以加快速度。但是实际上这个指示变量在循环过程中可能被其它进程所改变。 以下举sinal机制中的一个例子以说明。

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>

/* This flag controls termination of the main loop. */
volatile sig_atomic_t keep_going = 1;

/* The signal handler just clears the flag and re-enables itself. */
void
catch_alarm (int sig)
{
  keep_going = 0;
  signal (sig, catch_alarm);
}

void
do_stuff (void)
{
  puts ("Doing stuff while waiting for alarm....");
}

int
main (void)
{
  /* Establish a handler for SIGALRM signals. */
  signal (SIGALRM, catch_alarm);

  /* Set an alarm to go off in a little while. */
  alarm (2);

  /* Check the flag once in a while to see when to quit. */
  while (keep_going)
    do_stu#include <signal.h>
#include <stdio.h>
#include <stdlib.h>

/* This flag controls termination of the main loop. */
volatile sig_atomic_t keep_going = 1;

/* The signal handler just clears the flag and re-enables itself. */
void
catch_alarm (int sig)
{
  keep_going = 0;
  signal (sig, catch_alarm);
}

void
do_stuff (void)
{
  puts ("Doing stuff while waiting for alarm....");
}

int
main (void)
{
  /* Establish a handler for SIGALRM signals. */
  signal (SIGALRM, catch_alarm);

  /* Set an alarm to go off in a little while. */
  alarm (2);

  /* Check the flag once in a while to see when to quit. */
  while (keep_going)
    do_stuff ();

  return EXIT_SUCCESS;
}
ff ();

  return EXIT_SUCCESS;
}