fwrite & fread 的使用

每一次切换文件操作模式必须调用fclose关闭文件。

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如果直接切换操作模式，文件将损坏（出现乱码）或操作失败。

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在调用了fclose时，作为参数的文件指针将被回收，必须再次定义，因此最好将功能封装。

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存数组时，fwrite参数size_t size可使用sizeof(buffer[0])，size_t count可使用sizeof(buffer)/sizeof(buffer[0])。

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fread返回了一个整数，是其成功读取的数据数目，最大值是其参数size_t count。

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使用循环顺序读取时while(!feof(stream))，fread在一次读取不完整后触发文件尾条件。

一个例子：

#include<iostream>
#include<fstream>

int main()
{
    using std::cin;
    using std::cout;
    using std::endl;
    cout << "Hello, I am a C++ test program." << endl; 
    cin.get();
    //++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    
    FILE* _f0;
    _f0 = fopen("f0.txt", "wb");
    if (_f0 != NULL) 
    {
        cout << "I created a file named \"f0\"." << endl;
        cin.get();
        int buf[8];
        cout << "size of buf[0] = " << sizeof(buf[0]) << endl
            << "size of buf = " << sizeof(buf) << endl;

        for (int i = 0; i < 8; i++)
        {
            buf[i] = '0' + i;
        }
        fwrite(buf, sizeof(buf[0]), sizeof(buf)/sizeof(buf[0]), _f0);
        cout << "Then put some numbers into this file." << endl;
        cin.get();
        cout << "Read out these numbers:" << endl;
        fclose(_f0);
        FILE* _f1 = fopen("f0.txt", "rb");
        cout << "f0 = " << _f1 << endl;
        int i = 0;
        int R = 0;
        int n = 0;
        while (!feof(_f1))
        {
            n = fread(&R, sizeof(R), 1, _f1);
            cout << "n = " << n << " buf[" << i << "] = " << R << endl;
            i++;
        }
        fclose(_f1);
        cout << "At last, add a number to the file." << endl;
        FILE* _f2 = fopen("f0.txt", "ab");
        R = '8';
        fwrite(&R, sizeof(R), 1, _f2);
        fclose(_f2);
    }
    else 
    {
        cout << "File creating failed." << endl;
    }
    
    //++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    cout << endl << "Press enter to end.";
    cin.get();
    return 0;
}