大家好,又见面了,我是你们的朋友全栈君。 futex_t::wake 实际是一个计数器,防止在调用futex_wait函数前调用futex_wake而出现的死等现象, 函数futex只在满足(*addr1 == val)时等待。
futex_wait函数与futex_wake函数配合使用,前者等待后者唤醒。 futex_lock函数与futex_unlock函数配合使用,前者加锁后者解锁。
应该是对数据加锁,而不应该对代码加锁。但这只是测试, 实际编程中不应如此使用。
编译使用如下命令: g++ -g -W -Wall -Wextra -o mytest main.cpp -lpthread 执行: ./mytest 停止: Ctrl-C
参考文献: futex(2),futex(7),Linux内核文档,Linux内核源代码futex.c。
main.cpp:
// 2010年 07月 28日 星期三 13:01:43 CST // author: 李小丹(Li Shao Dan) 字 殊恒(shuheng) // K.I.S.S // S.P.O.T // linux-2.6.XX/Document/ // linux-2.6.xx/kernel/futex.c
#include <iostream> #include <cstdlib>
#include <unistd.h> #include <pthread.h> #include <errno.h> #include <linux/futex.h> #include <sys/syscall.h>
using namespace std;
#define futex(addr1, op, val, rel, addr2, val3) / syscall(SYS_futex, addr1, op, val, rel, addr2, val3)
struct futex_t { int wake; int lock; int wlock; };
inline static void futex_init(futex_t *); inline static int futex_wake(futex_t *); inline static int futex_wait(futex_t *); inline static int futex_lock(futex_t *); inline static int futex_unlock(futex_t *);
static void *work_thread(void *); static void *work_send(void *);
static int count = 0;
int main() { struct futex_t ftx; pthread_t tid[6];
futex_init(&ftx);
pthread_create(&tid[3], 0, work_send, (void *)&ftx); pthread_create(&tid[1], 0, work_thread, (void *)&ftx); pthread_create(&tid[2], 0, work_thread, (void *)&ftx); pthread_create(&tid[5], 0, work_send, (void *)&ftx); pthread_create(&tid[0], 0, work_thread, (void *)&ftx); pthread_create(&tid[4], 0, work_send, (void *)&ftx);
for(int i = 0; i < 6; ++i) pthread_join(tid[i], 0);
return 0; }
void *work_thread(void *p) { struct futex_t *ftx = (struct futex_t *)p; //sleep(2);
for(;;) { futex_wait(ftx);
futex_lock(ftx); cout << pthread_self() << endl; cout << “Hello,world!/n”; cout << count++ << endl; cout << “******************************/n”; futex_unlock(ftx); } return (void *)0; }
void *work_send(void *p) { futex_t *ftx = (struct futex_t *)p; for(int i = 0; i < 3000; ++i) futex_wake(ftx); return (void *)0; }
inline static void futex_init(futex_t *ftx) { ftx->lock = 0; ftx->wake = 0; ftx->wlock = 0; }
inline static int futex_wake(futex_t *ftx) { __sync_fetch_and_add(&ftx->wake, 1); //__sync_lock_test_and_set(&ftx->wake, 1); return futex(&ftx->wake, FUTEX_WAKE, 1, 0, 0, 0); }
inline static int futex_wait(futex_t *ftx) { futex(&ftx->wlock, FUTEX_LOCK_PI, 0, 0, 0, 0); int ret = futex(&ftx->wake, FUTEX_WAIT, 0, 0, 0, 0); __sync_fetch_and_sub(&ftx->wake, 1); futex(&ftx->wlock, FUTEX_UNLOCK_PI, 0, 0, 0, 0); return (ret && errno == EWOULDBLOCK ? 1 : ret); }
inline static int futex_lock(futex_t *ftx) { return futex(&ftx->lock, FUTEX_LOCK_PI, 0, 0, 0, 0); }
inline static int futex_unlock(futex_t *ftx) { return futex(&ftx->lock, FUTEX_UNLOCK_PI, 0, 0, 0, 0); }
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