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Android C++ 系列:Linux 线程(四)线程同步

作者:轻口味
  • 2021 年 12 月 07 日
  • 本文字数:4848 字

    阅读完需:约 16 分钟

Android C++系列:Linux线程(四)线程同步

多个线程同时访问共享数据时可能会冲突,这跟我们前面信号文章所说的可重入性是同样的问题。比如两个线程都要把某个全局变量增加 1,这个操作在某平台需要三条指令完成:


  • 从内存读变量值到寄存器;

  • 寄存器的值加 1;

  • 将寄存器的值写回内存


假设两个线程在多处理器平台上同时执行这三条指令,则可能导致下图所示的结果,最后变量只加了一次而非两次。



实例:


#include <stdio.h> #include <stdlib.h> #include <pthread.h>#define NLOOP 5000 int counter;void *doit(void *);/* incremented by threads */int main(int argc, char **argv) {  pthread_t tidA, tidB;   pthread_create(&tidA, NULL, &doit, NULL);  pthread_create(&tidB, NULL, &doit, NULL);  /* wait for both threads to terminate */   pthread_join(tidA, NULL);  pthread_join(tidB, NULL);  return 0; }void *doit(void *vptr) {  int i, val;  for (i = 0; i < NLOOP; i++) {     val = counter;    printf("%x: %d\n", (unsigned int)pthread_self(), val + 1);    counter = val + 1;   }  return NULL; }
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我们创建两个线程,各自把 counter 增加 5000 次,正常情况下最后 counter 应该等于 10000,但事实上每次运行该程序的结果都不一样,有时候数到 5000 多,有时候数到 6000 多。

1. 线程为什么要同步

  1. 共享资源,多个线程都可对共享资源操作;

  2. 线程操作共享资源的先后顺序不确定;

  3. 处理器对存储器的操作一般不是原子操作。

2. 互斥量

mutex 操作原语:


  • pthread_mutex_t

  • pthread_mutex_init

  • pthread_mutex_destroy

  • pthread_mutex_lock

  • pthread_mutex_trylock

  • pthread_mutex_unlock

2.1 临界区(Critical Section)

保证在某一时刻只有一个线程能访问数据的简便办法。在任意时刻只允许一个线程对共 享资源进行访问。如果有多个线程试图同时访问临界区,那么 在有一个线程进入后其他所有试图访问此临界区的线程将被挂起,并一直持续到进入临界区的线程离开。临界区在被释放后,其他线程可以继续抢占,并以此达到用原子方式操作共享资源的目的。

2.2 临界区的选定

临界区的选定因尽可能小,如果选定太大会影响程序的并行处理性能。

2.3 互斥量实例

#include <pthread.h>pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;int pthread_mutex_destroy(pthread_mutex_t *mutex);int pthread_mutex_init(pthread_mutex_t *restrict mutex, const pthread_mutexattr_t *restrict attr); int pthread_mutex_lock(pthread_mutex_t *mutex);int pthread_mutex_trylock(pthread_mutex_t *mutex);int pthread_mutex_unlock(pthread_mutex_t *mutex);
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实例:


#include <stdio.h> #include <stdlib.h> #include <pthread.h> #define NLOOP 5000int counter; /* incremented by threads */ pthread_mutex_t counter_mutex = PTHREAD_MUTEX_INITIALIZER;void *doit(void *);int main(int argc, char **argv) {  pthread_t tidA, tidB;  pthread_create(&tidA, NULL, doit, NULL);   pthread_create(&tidB, NULL, doit, NULL);  /* wait for both threads to terminate */   pthread_join(tidA, NULL);  pthread_join(tidB, NULL);  return 0; }void *doit(void *vptr) {  int i, val;  for (i = 0; i < NLOOP; i++) {     pthread_mutex_lock(&counter_mutex);    val = counter;    printf("%x: %d\n", (unsigned int)pthread_self(), val + 1);     counter = val + 1;    pthread_mutex_unlock(&counter_mutex);   }  return NULL; }
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这样运行结果就正常了,每次运行都能数到 10000。

3. 死锁

  1. 同一个线程在拥有 A 锁的情况下再次请求获得 A 锁;

  2. 线程一拥有 A 锁,请求获得 B 锁;线程二拥有 B 锁,请求获得 A 锁死锁导致的结果是什么?

4. 读写锁

读共享,写独占


  • pthread_rwlock_t

  • pthread_rwlock_init

  • pthread_rwlock_destroy

  • pthread_rwlock_rdlock

  • pthread_rwlock_wrlock

  • pthread_rwlock_tryrdlock

  • pthread_rwlock_trywrlock

  • pthread_rwlock_unlock


实例:


#include <stdio.h>#include <pthread.h>int counter;pthread_rwlock_t rwlock; //3个线程不定时写同一全局资源,5个线程不定时读同一全局资源 void *th_write(void *arg){  int t;  while (1) {     pthread_rwlock_wrlock(&rwlock);     t = counter;    usleep(100);    printf("write %x : counter=%d ++counter=%d\n", (int)pthread_self(), t, ++counter);    pthread_rwlock_unlock(&rwlock);     usleep(100);  } }void *th_read(void *arg) {  while (1) {    pthread_rwlock_rdlock(&rwlock);    printf("read %x : %d\n", (int)pthread_self(), counter);           pthread_rwlock_unlock(&rwlock);    usleep(100);  } }int main(void) {  int i;  pthread_t tid[8];   pthread_rwlock_init(&rwlock, NULL);   for (i = 0; i < 3; i++)    pthread_create(&tid[i], NULL, th_write, NULL);   for (i = 0; i < 5; i++)    pthread_create(&tid[i+3], NULL, th_read, NULL);  pthread_rwlock_destroy(&rwlock);  for (i = 0; i < 8; i++)    pthread_join(tid[i], NULL);   return 0;}
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5. 条件变量

条件变量给多个线程提供了一个汇合的场所,条件变量控制原语:


  • pthread_cond_t

  • pthread_cond_init

  • pthread_cond_destroy

  • pthread_cond_wait

  • pthread_cond_timedwait

  • pthread_cond_signal

  • pthread_cond_broadcast


生产者消费者模型:


#include <stdlib.h> #include <pthread.h> #include <stdio.h>struct msg {  struct msg *next;   int num;};struct msg *head;pthread_cond_t has_product = PTHREAD_COND_INITIALIZER; pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;void *consumer(void *p) {  struct msg *mp;  for (;;) {   pthread_mutex_lock(&lock);   while (head == NULL)    pthread_cond_wait(&has_product, &lock);   mp = head;  head = mp->next;   pthread_mutex_unlock(&lock);   printf("Consume %d\n", mp->num); free(mp);  sleep(rand() % 5);  } }void *producer(void *p) {  struct msg *mp;   for (;;) {    mp = malloc(sizeof(struct msg));     mp->num = rand() % 1000 + 1;     printf("Produce %d\n", mp->num);     pthread_mutex_lock(&lock);     mp->next = head;    head = mp;     pthread_mutex_unlock(&lock);     pthread_cond_signal(&has_product);     sleep(rand() % 5);  } }int main(int argc, char *argv[]) {  pthread_t pid, cid;  srand(time(NULL));  pthread_create(&pid, NULL, producer, NULL);   pthread_create(&cid, NULL, consumer, NULL);   pthread_join(pid, NULL);  pthread_join(cid, NULL);  return 0;}
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6. 信号量

信号量控制原语


  • sem_t

  • sem_init

  • sem_wait

  • sem_trywait

  • sem_timedwait

  • sem_post

  • sem_destroy


生产者消费者实例:


#include <stdlib.h> #include <pthread.h> #include <stdio.h> #include <semaphore.h>#define NUM 5int queue[NUM];sem_t blank_number, product_number;void *producer(void *arg) {  int p = 0;   while (1) {    sem_wait(&blank_number);    queue[p] = rand() % 1000 + 1;     printf("Produce %d\n", queue[p]);     sem_post(&product_number);    p = (p+1)%NUM;    sleep(rand()%5);  } }void *consumer(void *arg) {  int c = 0;   while (1) {    sem_wait(&product_number);     printf("Consume %d\n", queue[c]); queue[c] = 0; sem_post(&blank_number);    c = (c+1)%NUM;    sleep(rand()%5);   }}int main(int argc, char *argv[]) {  pthread_t pid, cid;  sem_init(&blank_number, 0, NUM);  sem_init(&product_number, 0, 0);   pthread_create(&pid, NULL, producer, NULL);   pthread_create(&cid, NULL, consumer, NULL);  pthread_join(pid, NULL);   pthread_join(cid, NULL);   sem_destroy(&blank_number);   sem_destroy(&product_number);   return 0;}
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7. 进程间锁

7.1 进程间 pthread_mutex

#include <pthread.h>int pthread_mutexattr_init(pthread_mutexattr_t *attr);int pthread_mutexattr_setpshared(pthread_mutexattr_t *attr, int pshared); int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);
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pshared:


  • 线程锁:PTHREAD_PROCESS_PRIVATE ;

  • 进程锁:PTHREAD_PROCESS_SHARED;

  • 默认情况是线程锁


实例:


#include <stdio.h> #include <pthread.h> #include <unistd.h> #include <sys/stat.h> #include <sys/types.h> #include <fcntl.h> #include <sys/mman.h> #include <string.h>struct mt {   int num;  pthread_mutex_t mutex;  pthread_mutexattr_t mutexattr; };int main(void) {  int fd, i;  struct mt *mm;  pid_t pid;  fd = open("mt_test", O_CREAT | O_RDWR, 0777);  /* 不需要write,文件里初始值为0 */  ftruncate(fd, sizeof(*mm));  mm = mmap(NULL, sizeof(*mm), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);   close(fd);  memset(mm, 0, sizeof(*mm));  /* 初始化互斥对象属性 */   pthread_mutexattr_init(&mm->mutexattr);  /* 设置互斥对象为PTHREAD_PROCESS_SHARED共享,即可以在多个进程的线程访问,PTHREAD_PROCESS_PRIVATE 为同一进程的线程共享 */  pthread_mutexattr_setpshared(&mm->mutexattr,PTHREAD_PROCESS_SHARED);  pthread_mutex_init(&mm->mutex, &mm->mutexattr);  pid = fork();   if (pid == 0){    /* 加10次。相当于加10 */     for (i=0;i<10;i++){      pthread_mutex_lock(&mm->mutex);       (mm->num)++;       printf("num++:%d\n",mm->num);       pthread_mutex_unlock(&mm->mutex);       sleep(1);    }   }else if (pid > 0) {    /* 父进程完成x+2,加10次,相当于加20 */     for (i=0; i<10; i++){      pthread_mutex_lock(&mm->mutex);       mm->num += 2;       printf("num+=2:%d\n",mm->num);       pthread_mutex_unlock(&mm->mutex);       sleep(1);    }    wait(NULL);   }  pthread_mutex_destroy(&mm->mutex);   pthread_mutexattr_destroy(&mm->mutexattr);   /* 父子均需要释放 */   munmap(mm,sizeof(*mm));  unlink("mt_test");  return 0;}
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7.2 文件锁

使用 fcntl 提供文件锁


struct flock {   ...  short l_type; /*Type of lock: F_RDLCK, F_WRLCK, F_UNLCK */  short l_whece; /* How to interpret l_start: SEEK_SET,SEEK_CUR,SEEK_END */  off_t l_start; /* Starting offset for lock*/  off_t l_len; /*Number of bytes to lock*/  pid_t l_pid; /* PID of process blocking our lock (F_GETLK only) */
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实例:


#include <stdio.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h> #include <stdlib.h> void sys_err(char *str) {  perror(str);  exit(1); }int main(int argc, char *argv[]) {  int fd;  struct flock f_lock;   if (argc < 2) {    printf("./a.out filename\n");    exit(1);   }  if ((fd = open(argv[1], O_RDWR)) < 0)   sys_err("open");  //f_lock.l_type = F_WRLCK;   f_lock.l_type = F_RDLCK;   f_lock.l_whence = SEEK_SET;   f_lock.l_start = 0;  f_lock.l_len = 0; //0表示整个文件加锁  fcntl(fd, F_SETLKW, &f_lock);   printf("get flock\n");   sleep(10);  f_lock.l_type = F_UNLCK;   fcntl(fd, F_SETLKW, &f_lock);   printf("un flock\n");  close(fd);  return 0; }
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8. 总结

本文介绍了线程同步机制:为什么要同步、互斥量、死锁、读写锁、条件变量、信号量、进程间锁等概念与机制以及相关示例。

发布于: 15 小时前阅读数: 6
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Android C++系列:Linux线程(四)线程同步