互斥鎖，讀寫鎖

一、互斥量（互斥鎖）

將輸出作爲共享資源，加鎖，進行父線程輸出大寫HELLO WORLD，子線程輸出小寫hello world,利用隨機數，使得睡眠時間，模擬線程佔用cpu時間，

調用pthread_mutex_init初始化的互斥鎖，在釋放內存前需要調用pthread_mutex_destroy 

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#include<stdio.h>
#include<unistd.h>
#include<pthread.h>
#include<stdlib.h>
#include<string.h>

pthread_mutex_t mutex; //鎖的定義

void *tfn(void *arg)
{
    srand(time(NULL));

    while(1){

        pthread_mutex_lock(&mutex);
        printf("hello");
        sleep(rand() % 3); /*模擬長時間操作共享資源，導致cpu易主，產生與時間>有關的錯誤*/
        printf("world\n");
        pthread_mutex_unlock(&mutex);
        sleep(rand() % 3); 
    }   

    return NULL;
}

int main(void)
{

    pthread_t tid;
    srand(time(NULL));
    pthread_mutex_init(&mutex,NULL); //mutex == 1

    pthread_create(&tid,NULL,tfn,NULL);
    while(1){

        pthread_mutex_lock(&mutex);
        printf("HELLO");
        sleep(rand()%3);
        printf("WORLD\n");
        pthread_mutex_unlock(&mutex); 
//將 unlock 挪至第二個 sleep 後，發現交替現象很難出現。
//線程在操作完共享資源後本應該立即解鎖，但修改後，線程抱着鎖睡眠。睡醒解鎖後又立即加鎖，這將導致其他線程很難搶到鎖 
        sleep(rand() % 3);
    }

    pthread_mutex_destroy(&mutex);

} 

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 二、讀寫鎖，同時多個進程多共享數據進行讀寫操作，，讀共享，寫獨佔，讀寫並行阻塞是，寫的優先級高

---

#include<stdio.h>                                                            
#include<unistd.h>
#include<pthread.h>

int counter;    //全局資源

pthread_rwlock_t rwlock;

void *th_write(void *arg) //寫線程
{
    int t;
    int i = (int)arg;

    while(1){
        t = counter;
        usleep(1000);

        pthread_rwlock_wrlock(&rwlock);
        printf("========write %d: %lu: counter=%d ++counter=%d\n",i,pthread_self(),t,++counter);
        pthread_rwlock_unlock(&rwlock);

        usleep(5000);
    }   

    return NULL;
}

void *th_read(void *arg)
{
    int i = (int)arg;

    while(1){
        pthread_rwlock_rdlock(&rwlock);
        printf("---------read %d: %lu: %d\n",i,pthread_self(),counter);
        pthread_rwlock_unlock(&rwlock);

        usleep(900);
    }   
    return NULL;
}
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,(void *)i);

    for(i = 0; i < 5; i++)
        pthread_create(&tid[i+3],NULL,th_read,(void *)i);
    
    //三個寫線程，5個讀線程
    for(i = 0; i < 8; i++)
        pthread_join(tid[i],NULL);
    
    pthread_rwlock_destroy(&rwlock);   //釋放讀寫鎖

    return 0;
}