linux多线程之自旋锁

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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