互斥锁的robust属性的介绍和使用
一个具体的场景：在多线程中，当一个线程获得锁之后异常退出后，应该怎么处理？
方案一 使用锁的robust特性
简单地讲，就是当拥有这个锁的线程挂了后，下一个尝试去获得锁的线程会得到EOWNWERDEAD的返回值，新的拥有者应该再去调用pthread_mutex_consistent_np（）来保持锁状态的一致性，并解锁。
直接上代码看示例：
1 /*================================================================ 2 * Copyright (C) 2019 Ltd. All rights reserved. 3 * 4 * File Name ：robust_mutex.c 5 * Author ：Hamilton 6 * Date ：2019-07-30 7 * Descriptor： 8 * 9 ================================================================*/ 10 11 #include <stdlib.h> 12 #include <stdio.h> 13 #include <unistd.h> 14 #include <pthread.h> 15 #include <errno.h> 16 17 #define handle_error_en(en, msg) \ 18 do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0) 19 20 static pthread_mutex_t mtx; 21 22 static void *original_owner_thread(void *ptr) 23 { 24 printf("\n[original owner] Setting lock...\n"); 25 pthread_mutex_lock(&mtx); 26 printf("[original owner] Locked. Now exiting without unlocking.\n"); 27 pthread_exit(NULL); 28 } 29 static void *bad_thread(void *ptr) 30 { 31 printf("\n[bad owner] Setting lock...\n"); 32 pthread_mutex_lock(&mtx); 33 printf("[bad owner] Locked. Now exiting without unlocking.\n"); 34 pthread_exit(NULL); 35 } 36 static void *second_thread(void *ptr) 37 { 38 int i = 5; 39 40 while (i--) 41 { 42 int s = pthread_mutex_lock(&mtx); 43 44 if (s == EOWNERDEAD) 45 { 46 printf("\n[second thread] pthread_mutex_lock() returned EOWNERDEAD\n"); 47 48 printf("[second thread] Now make the mutex consistent\n"); 49 s = pthread_mutex_consistent(&mtx); 50 if (s != 0) 51 handle_error_en(s, "pthread_mutex_consistent"); 52 53 printf("[second thread] Mutex is now consistent; unlocking\n"); 54 s = pthread_mutex_unlock(&mtx); 55 if (s != 0) 56 handle_error_en(s, "pthread_mutex_unlock"); 57 58 } 59 else if (s < 0) 60 { 61 printf("\n[second thread] pthread_mutex_lock() unexpectedly failed\n"); 62 handle_error_en(s, "pthread_mutex_lock"); 63 } 64 else 65 { 66 printf("\n[second thread] pthread_mutex_lock success.\n"); 67 printf("do somthing.... \n"); 68 s = pthread_mutex_unlock(&mtx); 69 if (s != 0) 70 handle_error_en(s, "pthread_mutex_unlock"); 71 } 72 sleep(1); 73 } 74 75 pthread_exit(NULL); 76 } 77 78 int main(int argc, char *argv[]) 79 { 80 pthread_t thr; 81 pthread_mutexattr_t attr; 82 int s; 83 84 pthread_mutexattr_init(&attr); 85 /* initialize the attributes object */ 86 pthread_mutexattr_setrobust(&attr, PTHREAD_MUTEX_ROBUST); 87 /* set robustness */ 88 89 pthread_mutex_init(&mtx, &attr); /* initialize the mutex */ 90 91 pthread_create(&thr, NULL, original_owner_thread, NULL); 92 93 sleep(1); 94 pthread_create(&thr, NULL, second_thread, NULL); 95 sleep(1); 96 pthread_create(&thr, NULL, bad_thread, NULL); 97 98 /* "original_owner_thread" should have exited by now */ 99 100 int i = 5; 101 while(i--) 102 { 103 s = pthread_mutex_lock(&mtx); 104 105 if (s == EOWNERDEAD) 106 { 107 printf("\n[main thread] pthread_mutex_lock() returned EOWNERDEAD\n"); 108 109 printf("[main thread] Now make the mutex consistent\n"); 110 s = pthread_mutex_consistent(&mtx); 111 if (s != 0) 112 handle_error_en(s, "pthread_mutex_consistent"); 113 114 printf("[main thread] Mutex is now consistent; unlocking\n"); 115 s = pthread_mutex_unlock(&mtx); 116 if (s != 0) 117 handle_error_en(s, "pthread_mutex_unlock"); 118 119 } 120 else if (s < 0) 121 { 122 printf("\n[main thread] pthread_mutex_lock() unexpectedly failed\n"); 123 handle_error_en(s, "pthread_mutex_lock"); 124 } 125 else 126 { 127 printf("\n[main thread] pthread_mutex_lock success.\n"); 128 printf("do somthing.... \n"); 129 s = pthread_mutex_unlock(&mtx); 130 if (s != 0) 131 handle_error_en(s, "pthread_mutex_unlock"); 132 } 133 134 sleep(1); 135 } 136 exit(EXIT_SUCCESS); 137 }
示例中总共包含四个线程，original_owner_thread() 和 bad_thread() 两个线程获得锁后立马退出不释放，其它两个线程main thread 及 second_thread() 轮流抢占锁，并对锁的异常进行恢复处理，看下打印结果：
是不是很简单，通过设置robust特性，并在每次获取锁时判断锁的异常状态，便能很好的处理锁异常退出的情况。
关于锁的robust特性及consistent设定，可参考以下更多资料：
https://docs.oracle.com/cd/E19455-01/806-5257/6je9h032m/index.html
http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_consistent.html
本文示例改编自：http://manpages.ubuntu.com/manpages/bionic/man3/pthread_mutexattr_setrobust.3.html
后记：再看看进程间有没有类似的机制，但是google并没有找到相关介绍，便想看看pthreas_mutex_lock这套机制在process下工作是否正常，先看示例代码：
1 /*================================================================ 2 * Copyright (C) 2019 Ltd. All rights reserved. 3 * 4 * File Name ：robust_mutex.c 5 * Author ：Hamilton 6 * Date ：2019-07-30 7 * Descriptor： 8 * 9 ================================================================*/ 10 11 #include <stdlib.h> 12 #include <stdio.h> 13 #include <unistd.h> 14 #include <pthread.h> 15 #include <errno.h> 16 #include <sys/mman.h> 17 #include <fcntl.h> 18 #include <sys/types.h> 19 #include <sys/stat.h> 20 21 #define SHM_NAME "fasdfasfasfas" 22 23 24 #define handle_error_en(en, msg) \ 25 do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0) 26 27 static pthread_mutex_t *mtx; 28 static int fd_shm; 29 30 void shm_mutex_init(pthread_mutex_t **mutex) 31 { 32 pthread_mutexattr_t attr; 33 34 pthread_mutexattr_init(&attr); 35 /* initialize the attributes object */ 36 pthread_mutexattr_setrobust(&attr, PTHREAD_MUTEX_ROBUST); 37 /* set robustness */ 38 39 // Get shared memory 40 if ((fd_shm = shm_open (SHM_NAME, O_RDWR | O_CREAT, 0660)) == -1) 41 perror ("shm_open"); 42 43 if (ftruncate (fd_shm, sizeof (pthread_mutex_t)) == -1) 44 perror ("ftruncate"); 45 46 if ((*mutex = mmap (NULL, sizeof (pthread_mutex_t), PROT_READ | PROT_WRITE, MAP_SHARED, 47 fd_shm, 0)) == MAP_FAILED) 48 perror ("mmap"); 49 50 pthread_mutex_init(*mutex, &attr); /* initialize the mutex */ 51 } 52 int main(int argc, char *argv[]) 53 { 54 int s; 55 56 shm_mutex_init(&mtx); 57 58 if ((s = fork()) < 0) 59 { 60 perror("fork."); 61 } 62 else if (s == 0) // child 63 { 64 sleep(1); 65 printf("\n[bad owner] Setting lock...\n"); 66 pthread_mutex_lock(mtx); 67 printf("[bad owner] Locked. Now exiting without unlocking.\n"); 68 } 69 else 70 { 71 int i = 5; 72 while(i--) 73 { 74 s = pthread_mutex_lock(mtx); 75 76 if (s == EOWNERDEAD) 77 { 78 printf("\n[main thread] pthread_mutex_lock() returned EOWNERDEAD\n"); 79 80 printf("[main thread] Now make the mutex consistent\n"); 81 s = pthread_mutex_consistent(mtx); 82 if (s != 0) 83 handle_error_en(s, "pthread_mutex_consistent"); 84 85 printf("[main thread] Mutex is now consistent; unlocking\n"); 86 s = pthread_mutex_unlock(mtx); 87 if (s != 0) 88 handle_error_en(s, "pthread_mutex_unlock"); 89 90 } 91 else if (s < 0) 92 { 93 printf("\n[main thread] pthread_mutex_lock() unexpectedly failed\n"); 94 handle_error_en(s, "pthread_mutex_lock"); 95 } 96 else 97 { 98 printf("\n[main thread] pthread_mutex_lock success.\n"); 99 printf("do somthing.... \n"); 100 s = pthread_mutex_unlock(mtx); 101 if (s != 0) 102 handle_error_en(s, "pthread_mutex_unlock"); 103 } 104 105 sleep(1); 106 } 107 } 108 exit(EXIT_SUCCESS); 109 }
编译执行下看看：
进程间通信的锁得放在共享内存中，编译运行OK，也能正常工作。
