操作系統實驗九之寫者優先問題

一、代碼實現

讀者進程和寫者進程如下：
Main.cpp

#include <pthread.h>
#include <sys/types.h>
#include <semaphore.h>
#include <unistd.h>
#include "buffer.h"
#include "deal.h"
#define MAX 20

struct sto{
    sem_t mutex1;//保證read操作原子性
    sem_t mutex2;//保證write操作原子性
    sem_t single;//保證每次只有一個讀者進入臨界區。這樣，寫者即到即寫(只用等一個人)。
    sem_t r;//讀者是否可以讀；可以讀時r>0.
    sem_t wrt;//寫者是否可以讀取共享數據段
};
struct sto shared;
int readcount;
int writecount;

void *Read(void *arg)
{
    Instruction *ins = (Instruction*)arg;
    sleep(ins->start);//等待一段時間，開始申請資源

    sem_wait(&shared.single);//只有一個讀者準備進入臨界區，其他被阻塞
    sem_wait(&shared.r); //最後一個寫者完成操作，則把權利交還給讀者
    sem_wait(&shared.mutex1);//保證操作原子性
    readcount++;
    if(readcount == 1) sem_wait(&shared.wrt);//爲了保證在讀取期間，沒有寫者操作數據;互斥
    sem_post(&shared.mutex1);
    sem_post(&shared.r);//給寫者創造佔領先機的機會
    sem_post(&shared.single);

    //開始讀取共享數據段
    cout<<"Reader "<<ins->serial_num<<" is reading."<<endl;
    sleep(ins->last);
    cout<<"Reader "<<ins->serial_num<<" end reading."<<endl;

    //讀取完畢後,讀者數目減去1。
    //只有全部讀者讀取完畢後，喚醒寫者線程
    sem_wait(&shared.mutex1);
    readcount--;
    if(readcount == 0) sem_post(&shared.wrt);
    sem_post(&shared.mutex1);

    return(NULL);
}

void *Write(void *arg)
{
    Instruction *ins = (Instruction*)arg;
    sleep(ins->start);

    //來一個寫者，寫者數量即加1
    sem_wait(&shared.mutex2);
    writecount++;
    if(writecount == 1) sem_wait(&shared.r);
    sem_post(&shared.mutex2);

    //寫者讀取共享數據段
    sem_wait(&shared.wrt);
    cout<<"Writer "<<ins->serial_num<<" is writing."<<endl;
    sleep(ins->last);
    cout<<"Writer "<<ins->serial_num<<" end writing."<<endl;
    sem_post(&shared.wrt);

    //如果所有寫者寫完，把權利交還讀者
    sem_wait(&shared.mutex2);
    writecount--;
    if(writecount == 0) sem_post(&shared.r);
    sem_post(&shared.mutex2);

    return(NULL);
}

int main()
{
    FILE *fp;
    char file[MAX];
    int i;

    pthread_t tid_read;
    pthread_t tid_write;
    Instruction *ins = new Data();
    sem_init(&shared.mutex1, 0, 1);
    sem_init(&shared.mutex2, 0, 1);
    sem_init(&shared.single, 0, 1);
    sem_init(&shared.wrt, 0, 1);
    sem_init(&shared.r, 0, 1);
    readcount = 0;
    writecount = 0;

    fp = fopen("test.txt", "r");
    while(fgets(file, MAX, fp)){
        ins = Deal(file);
        if(ins->character == 'R'){
            pthread_create(&tid_read, NULL, Read, (void*)ins);
        }
        else{
            pthread_create(&tid_write, NULL, Write, (void*)ins);
        }
        /*cout<<ins->serial_num<<" "<<ins->character<<" ";
        cout<<ins->start<<" "<<ins->last<<" ";
        cout<<endl;*/

    }

    sem_destroy(&shared.mutex1);
    sem_destroy(&shared.mutex2);
    sem_destroy(&shared.wrt);
    sem_destroy(&shared.r);

    sleep(50);

    return 0;
}

爲了便於測試，完成以下兩段輔助代碼：
deal.h

#ifndef DEAL_H_
#define DEAL_H_

#include <stdio.h>
#insem_destroy(&shared.mutex2);
    sem_destroy(&shared.wrt);
    sem_destroy(&shared.r);

    sleep(50);

    return 0;
}
clude <stdlib.h>
#include <string.h>
#include <cstdlib>
#include <iostream>
using namespace std;

typedef struct Data{
    Data(){
        serial_num = 0;
        character = ' ';
        start = 0;
        last = 0;
    }

    int serial_num;
    char character;
    int start;
    int last;
} Instruction;

Instruction *Deal(char file[]);

#endif

deal.cpp

#include "deal.h"

Instruction *Deal(char file[]){
    Instruction *ins = new Data();
    int i;
    int j = 0;
    string tmp = "";

    int len = strlen(file);
    for(i = 0; i < len; i++){
        if(file[i] != ' ' && file[i] != '\n')
            tmp += file[i];

        else{
            if(tmp != ""){
                switch(j){
                    case 0: ins->serial_num = atoi(tmp.c_str());
                            tmp = "";
                            break;
                    case 1: ins->character = tmp.at(0);
                            tmp = "";
                            break;
                    case 2: ins->start = atoi(tmp.c_str());
                            tmp = "";
                        break;
                    case 3: ins->last = atoi(tmp.c_str());
                            tmp = "";
                        break;
                }
                j++;
            }
        }
    }
    return ins;
}

二、測試數據及結果

　　測試數據文件包括n行測試數據，分別描述創建的n個線程是讀者還是寫者，以及讀寫操作的開始時間和持續時間。每行測試數據包括四個字段，各個字段間用空格分隔。第一字段爲一個正整數，表示線程序號。第二字段表示相應線程角色，R表示讀者，W表示寫者。第三字段爲一個正數，表示讀寫操作的開始時間：線程創建後，延遲相應時間(單位爲秒)後發出對共享資源的讀寫申請。第四字段爲一個正數，表示讀寫操作的持續時間。當線程讀寫申請成功後，開始對共享資源的讀寫操作，該操作持續相應時間後結束，並釋放共享資源。例如：

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1 R 3 5
2 W 4 5
3 R 5 2
4 R 6 5
5 W 7 3

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　　實驗結果如圖：