c_cpp “Linux的环境高级编程”作业1

Posted 2021-05-14

#include <iostream>
#include <fcntl.h>
#include <unistd.h>
#include <cstring>

using namespace std;

int main() {
    const char* filePath = "/home/developerdong/task1.txt";
    const char* fileContent = "abcdefghijklmnopqrstuvwxyz";
    unsigned long contentLength = strlen(fileContent);
    int file = open(filePath, O_RDWR|O_CREAT|O_APPEND, 0777);
    if (file != -1){
        //首先写入测试数据
        long writedNum = write(file, fileContent, contentLength);
        if(writedNum != -1){
            //记录修改前和修改后的偏移量
            long oldPosition = lseek(file, 0, SEEK_CUR);
            long newPosition = lseek(file, 0, SEEK_SET);
            //如果修改前后偏移量相同，则说明以O_APPEND选项打开文件时不能修改偏移量
            if(oldPosition == newPosition){
                cout<<"不能修改偏移量"<<endl;
            }
            else{
                cout<<"可以修改偏移量"<<endl;
                char readContent[contentLength];
                long readNum = read(file, readContent, contentLength);
                //如果修改后读操作从文件尾部开始读，则读出数据的长度应该为0，否则说明从修改后偏移量开始读
                if(readNum > 0){
                    cout<<"修改后读操作从修改后的偏移量开始读"<<endl;
                }
                else{
                    cout<<"修改后读操作从文件尾部读"<<endl;
                }
            }
        }
        else{
            cout<<"写入测试数据失败"<<endl;
        }
        close(file);
    }
    else{
        cout<<"打开文件失败"<<endl;
    }
    return 0;
}