c++ - ifstream read 读取整个流的随机字符

Question

我正在尝试实现 list 5.1 中的功能 here但是当从文件中读取并复制到缓冲区时，我只得到整个数组的相同字符 (Í)，其中 string.txt 是从先前的链接内容复制和粘贴的。 这是我的代码:

#include <iostream>
#include <fstream>
#include <string>
#include <cinttypes>
#include <cstdio>
#include <cstring>

const int block_size = 0x4000;   //16KB

int search(char* buffer, int searchLength, char* stringToSearch, int stringToSearchLength) {
    char * potentialMatch;
    while (searchLength) {
        potentialMatch = reinterpret_cast<char *>(memchr(buffer, *stringToSearch, searchLength));
        if (potentialMatch == NULL)
            break;

        if (stringToSearchLength == 1) {
            return 1;
        } else {
            if (!memcmp(potentialMatch + 1, stringToSearch + 1, stringToSearchLength - 1))
                return 1;
        }

        searchLength -= potentialMatch - buffer + 1;
        buffer = potentialMatch + 1;
    }

    return 0;
}


int main(int argc, char* argv[]) {
    char *toSearch = "Interpreting Where";
    int done = 0;
    int found = 0;  
    char *buffer;
    int64_t fileSizeLeft = 0;

    std::ifstream myFile("string.txt");
    if (!myFile.fail()) {
        buffer = new char[block_size];   
        myFile.seekg(0, std::ios::end);   //Get file's size
        fileSizeLeft = myFile.tellg();    
    } else {
        std::cout << "Cannot open file" << std::endl;
        return 1;
    }

    int toSearchLength = strlen(toSearch);
    int stringLeft = toSearchLength - 1;
    int first_time = 1;


    while (!done && fileSizeLeft > toSearchLength) {
        if (first_time) {
            myFile.read(buffer, block_size);
            found = search(buffer, block_size, toSearch, toSearchLength);
        } else {
            memcpy(buffer, buffer + stringLeft, stringLeft);
            myFile.read(buffer+stringLeft, fileSizeLeft-stringLeft);        
            found = search(buffer, block_size, toSearch, toSearchLength);
        }

        fileSizeLeft = fileSizeLeft - block_size;
        first_time = 0;
    }


    if (found) {
        std::cout << "String found" << std::endl;
    } else {
        std::cout << "String not found" << std::endl;
    }

    myFile.close();
    delete[] buffer;

    return 0;
}

希望你能帮我看看我做错了什么，谢谢!

Answer 1

您正在使用 seekg 将 myFile 的位置设置为 ios_base::end :

myFile.seekg(0, ios::end);

然后尝试从中读取:

myFile.read(buffer, block_size);

显然不会读取任何数据，因为 myFile 已经在 ios_base::end。您将读取 buffer

中已有的任何未初始化数据

你可能打算做的是在阅读之前通过这样做将你的 myFile 位置设置回开头:

myFile.seekg(0, ios::beg);