进程通信之读写锁

进程通信之读写锁

读写锁

读写锁的分配规则 1. 没有线程持有读写锁进行写，任意数量的线程可以持有该读写锁用于读 2. 只有没有线程持有给定的读写锁用于读或者写的时候，才能分配读写锁用于写。 如果修改数据频繁，那么可以考虑用读写锁替代互斥锁。

获取与释放

如果对应的读写锁已由某个写入者持有，那么阻塞pthread_rwlock_rdlock获取读出锁如果对应的读写锁已由另一个写入者持有，那就阻塞pthread_rwlock_wrlock获取写入锁。pthread_rwlock_unlock用于释放读出锁或者写入锁。三者成功时返回0,出错时返回正的错误值pthread_rwlock_tryrdlock(pthread _rwlock_t *rwptr)尝试获取读出锁，如果不能马上获得，返回EBUSY错误。pthread_rwlock_trywrlock(pthread _rwlock_t *rwptr)尝试获取写入锁，如果不能马上获得，返回EBUSY错误。

初始化和摧毁

动态初始化​​​int pthread_rwlock_init(pthread_rwlock_t *rwptr, const pthread_rwlockattr_t *attr)​​静态初始化​​​PTHREAD_RWLOCK_INITIALIZER​​摧毁​​​int pthread_rwlock_destroy(pthread_rwlock_t *rwptr)​​

属性设置

int pthread_rwlockattr_init(pthread_rwlockattr_t * attr)int pthread_rwlockattr_destroy(pthread_rwlockattr_t *attr)int pthread_rwlockattr_getpshared(const pthread_rwlockattr_t * attr,int *valptr)int pthread_rwlockattr_setpshared(pthread_rwlockattr_t * attr,int

读写锁很适合读的次数远大于写的次数的情况。 例子： 一个写者，多个读者。用读写锁进行协调工作。 code:

#include #include #include #include struct data{ int number; char info[105];};struct data *pdata = NULL; pthread_rwlock_t lock;void *read(void *arg){ int id = *((int *)arg); while(1){ int ret; ret = pthread_rwlock_tryrdlock(&lock); if(ret != 0) continue; printf("reader %d is reading!\n",id); if(pdata == NULL) printf("data is null.\n"); else printf("data: number is %d, info are %s.\n",pdata->number,pdata->info); pthread_rwlock_unlock(&lock); } pthread_exit(0);}void *write(void *arg){ int id = *((int *)arg); while(1){ int ret; ret = pthread_rwlock_trywrlock(&lock); if(ret !=0) continue; printf("writer %d is writing!\n",id); if(pdata == NULL){ pdata = (struct data *)malloc(sizeof(struct data)); pdata->number = 1; strcpy(pdata->info,"I love linux."); printf("finish, wrote it.\n"); } else printf("the pdata is not null.\n"); pthread_rwlock_unlock(&lock); } pthread_exit(0);}int main(){ pthread_t reader[10]; pthread_t writer; int i; pthread_create(&writer,NULL,write,(void *)&i); for(i=0; i<10; i++){ pthread_create(&reader[i],NULL,read,(void *)&i); } //pthread_create(&writer,NULL,write,NULL); if(pdata != NULL) free(pdata); getchar(); return 0;}

运行：

writer 1 is writing!finish, wrote it.reader 2 is reading!data: number is 1, info are I love linux..writer 1 is writing!the pdata is not null.reader 2 is reading!data: number is 1, info are I love linux..reader 2 is reading!data: number is 1, info are I love linux..

线程取消函数： ​​int pthread_cancel(pthread_t tid)​​​ 一个线程可以被同一进程中的其他线程取消。 与之对应的自愿终止： ​​​void pthread_exit（void *retval）​​

#include #include #include "unix98.h"#include pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;pthread_t tid1, tid2;void *thread1(void *), *thread2(void *);int main(){ pthread_setconcurrency(2); pthread_create(&tid1,NULL,thread1,NULL); sleep(1); /* let tid2 get lock. */ pthread_create(&tid2,NULL,thread2,NULL); void *status; /* store returned value from waited tid */ pthread_join(tid2,&status); if(status != PTHREAD_CANCELED){ printf("thread2 status = %p, status); } pthread_join(tid1, &status); if(status != NULL){ printf("thread1 status = %p\n",status); } //printf("rw_refcount = %d, rw_nwaitreaders = %d, rw_nwaitwriter = %d\n", // rwlock.rw_refcount,rwlock.rw_nwaitreaders, rwlock.rw_nwaitwriters); pthread_rwlock_destroy(&rwlock); return 0;}void *thread1(void *arg){ pthread_rwlock_rdlock(&rwlock); printf("thread1 got a read lock.\n"); sleep(3); /* let thread2 block in pthread_rwlock_wrlock */ pthread_cancel(tid2); sleep(3); pthread_rwlock_unlock(&rwlock); return NULL;}void *thread2(void *arg){ printf("thread2 try to obtain a write lock.\n"); pthread_rwlock_wrlock(&rwlock); printf("thread2() get a write lock.\n"); sleep(1); pthread_rwlock_unlock(&rwlock); return

./pthread_cancel thread1 got a read lock.thread2 try to obtain a write lock.thread2() get a write

fcntl函数和读写锁

文件锁：建议性锁，强制性锁 lock结构体：

Struct flock{short l_type;off_t l_start;short

flock可施加建议性锁，fcntl既可以施加建议性锁，也可施加强制性锁。 fcntl也可以对某一记录进行上锁，也就是所谓的记录锁。记录锁分为读取锁（共享锁），写入锁（排斥锁，互斥锁）。 fcntl建立记录锁： 下面是一个学习的例子，参考 ​​​ 使用fcntl增加和释放锁：

#include #include #include #include #include #include void lock_set(int fd,int type){ struct flock lock; lock.l_whence = SEEK_SET; /* 相对位移量的起点是文件的开头 */ lock.l_start = 0; /* 相对位移量 */ lock.l_len = 0; /* 加锁区域的长度 */ while(1){ lock.l_type=type; if(fcntl(fd,F_SETLK,&lock) == 0){ if(lock.l_type == F_RDLCK) printf("read lock by %d\n", getpid()); else if(lock.l_type == F_WRLCK) printf("write lock by %d\n", getpid()); else if(lock.l_type == F_UNLCK) printf("release lock by %d\n", getpid()); return ; } fcntl(fd,F_GETLK,&lock); if(lock.l_type != F_UNLCK){ if(lock.l_type == F_RDLCK) printf("read lock already set by %d\n",lock.l_pid); else if(lock.l_type == F_WRLCK) printf("write lock already set by %d\n",lock.l_pid); printf("please enter any key to continue: "); getchar(); } }}int main(){ int fd = open("path",O_RDWR | O_CREAT, 0755); /* 当前路径下创建path */ if(fd == -1){ perror("open: "); exit(1); } lock_set(fd,F_WRLCK); puts("wait for a secend...\n"); sleep(6); lock_set(fd,F_UNLCK); puts("wait for a secend...\n"); sleep(1); close(fd); exit(0);}

不修改上面的代码，试试写入锁 + 写入锁， 看看效果。 在不同的标签内运行此程序：

write + write

./writewrite lock by 2904wait for a secend...release lock by 2904wait for a secend..../writewrite lock already set by 2904please enter any key to continue: write lock already set by 2904please enter any key to continue: write lock by 2905wait for a secend...release lock by 2905wait for

在已有write锁的情况下不能加上write锁。 现在，稍作修改

lock_set(fd,F_WRLCK);--->lock_set(fd,F_RDLCK);

write + read

./write write lock by 3005wait for a secend...release lock by 3005wait for a secend..../read write lock already set by 3005please enter any key to continue: write lock already set by 3005please enter any key to continue: write lock already set by 3005please enter any key to continue: read lock by 3006wait for a secend...release lock by 3006wait for

在已有write锁的情况下不能加上read锁

read + write

./read read lock by 3019wait for a secend...release lock by 3019wait for a secend..../write read lock already set by 3019please enter any key to continue: read lock already set by 3019please enter any key to continue: read lock already set by 3019please enter any key to continue: read lock already set by 3019please enter any key to continue: write lock by 3020wait for a secend...release lock by 3020wait for

在已有read锁的情况下不能加上write锁

read + read

./readread lock by 3035wait for a secend...release lock by 3035wait for a secend..../read read lock by 3036wait for a secend...release lock by 3036wait for a

read锁和read锁不冲突。在已有read锁的情况下可以加上read锁。

版权声明：本文内容由网络用户投稿，版权归原作者所有，本站不拥有其著作权，亦不承担相应法律责任。如果您发现本站中有涉嫌抄袭或描述失实的内容，请联系我们jiasou666@gmail.com 处理，核实后本网站将在24小时内删除侵权内容。