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

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