基本概念：
何谓自旋锁？它是为实现保护共享资源而提出一种锁机制。其实，自旋锁与互斥锁比较类似，它们都是为了解决对某项资源的互斥使用。无论是互斥锁，还是自旋锁，在任何时刻，最多只能有一个保持者，也就说，在任何时刻最多只能有一个执行单元获得锁。但是两者在调度机制上略有不同。对于互斥锁，如果资源已经被占用，资源申请者只能进入睡眠状态。但是自旋锁不会引起调用者睡眠，如果自旋锁已经被别的执行单元保持，调用者就一直循环在那里看是否该自旋锁的保持者已经释放了锁，"自旋"一词就是因此而得名。
一、初始化和销毁锁
PTHREAD_SPIN_DESTROY(P)POSIX Programmer's Manual PTHREAD_SPIN_DESTROY(P)
NAME
pthread_spin_destroy, pthread_spin_init - destroy or initialize a spin
lock object (ADVANCED REALTIME THREADS)
SYNOPSIS
#include <pthread.h>
int pthread_spin_destroy(pthread_spinlock_t *lock);
int pthread_spin_init(pthread_spinlock_t *lock, int pshared);
两个函数的返回值：若成功，返回0；否则，返回错误编号
关于pthread_spin_init函数的参数pshard，单进程可以设置成PTHREAD_PROCESS_SHARED
二、加锁与解锁
PTHREAD_SPIN_LOCK(P) POSIX Programmer's Manual PTHREAD_SPIN_LOCK(P)
NAME
pthread_spin_lock, pthread_spin_trylock - lock a spin lock object
(ADVANCED REALTIME THREADS)
SYNOPSIS
#include <pthread.h>
int pthread_spin_lock(pthread_spinlock_t *lock);
int pthread_spin_trylock(pthread_spinlock_t *lock);
int pthread_spin_unlock(pthread_spinlock_t *lock);
两个函数的返回值：若成功，返回0；否则，返回错误编号
例子1，互斥锁的耗时， gcc pthread_mutex.c -pthread -o mutex：
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/timeb.h>
static int num = 0;
static int count = 10000000;
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
void Perror(const char *s)
{
perror(s);
exit(EXIT_FAILURE);
}
long long getSystemTime() {
struct timeb t;
ftime(&t);
return 1000 * t.time + t.millitm;
}
void* fun2(void *arg)
{
pthread_t thread_id = pthread_self();
printf("the thread2 id is %ld\n", (long)thread_id);
int i = 1;
for (; i<=count; ++i) {
pthread_mutex_lock(&mutex);
num += 1;
pthread_mutex_unlock(&mutex);
}
}
int main()
{
int err;
pthread_t thread1;
pthread_t thread2;
thread1 = pthread_self();
printf("the thread1 id is %ld\n", (long)thread1);
long long start = getSystemTime();
// Create thread
err = pthread_create(&thread2, NULL, fun2, NULL);
if (err != 0) {
Perror("can't create thread2\n");
}
int i = 1;
for (; i<=count; ++i) {
pthread_mutex_lock(&mutex);
num += 1;
pthread_mutex_unlock(&mutex);
}
pthread_join(thread2, NULL);
long long end = getSystemTime();
printf("The num is %d, pay %lld ms\n", num, (end-start));
return 0;
}
例子2，自旋锁的耗时，gcc pthread_spin.c -pthread -o spin：
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/timeb.h>
static int num = 0;
static int count = 10000000;
static pthread_spinlock_t spin;
void Perror(const char *s)
{
perror(s);
exit(EXIT_FAILURE);
}
long long getSystemTime() {
struct timeb t;
ftime(&t);
return 1000 * t.time + t.millitm;
}
void* fun2(void *arg)
{
pthread_t thread_id = pthread_self();
printf("the thread2 id is %ld\n", (long)thread_id);
int i = 1;
for (; i<=count; ++i) {
pthread_spin_lock(&spin);
num += 1;
pthread_spin_unlock(&spin);
}
}
int main()
{
int err;
pthread_t thread1;
pthread_t thread2;
pthread_spin_init(&spin, PTHREAD_PROCESS_PRIVATE);
thread1 = pthread_self();
printf("the thread1 id is %ld\n", (long)thread1);
long long start = getSystemTime();
// Create thread
err = pthread_create(&thread2, NULL, fun2, NULL);
if (err != 0) {
Perror("can't create thread2\n");
}
int i = 1;
for (; i<=count; ++i) {
pthread_spin_lock(&spin);
num += 1;
pthread_spin_unlock(&spin);
}
pthread_join(thread2, NULL);
long long end = getSystemTime();
printf("The num is %d, pay %lld ms\n", num, (end-start));
pthread_spin_destroy(&spin);
return 0;
}
运行结果，可以看到某些条件下，自旋锁是比较快的：
参考：《unix环境高级编程》·第三版