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

易揮發的，避免優化
===>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;
}

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

1,加了O3優化纔會變成while(true)


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

適合地點：在一些硬件設備上，設備驅動上（來回切換的代價比較高)