同步——互斥锁

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互斥锁是最基本的同步形式，用于保护临界区

#include <pthread.h>int pthread_mutex_lock(pthread_mutex_t *mptr);int pthread_mutex_trylock(pthread_mutex_t *mptr);int pthread_mutex_unlock(pthread_mutex_t *mptr);// return : 0/errno

// 互斥锁静态初始化pthread_mutex_t mMutex = PTHREAD_MUTEX_INITIALIZER// 动态初始化pthread_mutex_t *mptr = malloc(sizeof(pthread_mutex_t));pthread_mutex_init(mptr, 0);

// 通常情况下，我们将互斥锁和需要保护的对象封装一个结构体内struct {       pthread_mutex_t mutex;       int shObj;} shared = {       PTHREAD_MUTEX_INITIALIZER};

// 线程间共享互斥锁的生产者消费者#include"unpipc.h"#defineMAXNITEMS 1000000#defineMAXNTHREADS100intnitems;/* read-only by producer and consumer */struct {  pthread_mutex_tmutex;  intbuff[MAXNITEMS];  intnput;  intnval;} shared = { PTHREAD_MUTEX_INITIALIZER };void*produce(void *), *consume(void *);intmain(int argc, char **argv){inti, nthreads, count[MAXNTHREADS];pthread_ttid_produce[MAXNTHREADS], tid_consume;if (argc != 3)err_quit("usage: prodcons2 <#items> <#threads>");nitems = min(atoi(argv[1]), MAXNITEMS);nthreads = min(atoi(argv[2]), MAXNTHREADS);Set_concurrency(nthreads);/* 4start all the producer threads */for (i = 0; i < nthreads; i++) {count[i] = 0;Pthread_create(&tid_produce[i], NULL, produce, &count[i]);}/* 4wait for all the producer threads */for (i = 0; i < nthreads; i++) {Pthread_join(tid_produce[i], NULL);printf("count[%d] = %d\n", i, count[i]);}/* 4start, then wait for the consumer thread */Pthread_create(&tid_consume, NULL, consume, NULL);Pthread_join(tid_consume, NULL);exit(0);}/* end main */void *produce(void *arg){for ( ; ; ) {Pthread_mutex_lock(&shared.mutex);if (shared.nput >= nitems) {Pthread_mutex_unlock(&shared.mutex);return(NULL);/* array is full, we're done */}shared.buff[shared.nput] = shared.nval;shared.nput++;shared.nval++;Pthread_mutex_unlock(&shared.mutex);*((int *) arg) += 1;}}void *consume(void *arg){inti;for (i = 0; i < nitems; i++) {if (shared.buff[i] != i)printf("buff[%d] = %d\n", i, shared.buff[i]);}return(NULL);}

互斥锁可以用在进程间共享：互斥锁建在共享内存区，并设置PTHREAD_PROCESS_SHARED属性，这样该互斥锁为系统级别

pthread_mutex_t *mptr //point to the mutex in shmpthread_mutexattr_t mattr;mptr = /* mutex in shm */Pthread_mutexattr_init(&mattr);Pthread_mutexattr_setpshared(&mattr, PTHEAD_PROCESS_SHARED);Pthread_mutex_init(mptr, &mattr);

当在进程间共享一个互斥锁时，持有该互斥锁的进程在持有期间终止，没有办法让系统在进程终止时自动释放所持有的锁，读写锁和Posix信号量亦然；进程终止时内核总是自动清理的唯一同步锁类型是fcntl记录锁。线程在获取锁的情况下被另一个线程Pthread_cancel掉时，我们可以设置清理处理程序pthread_cleanup_pop/push应对该状况。