写入二进制文件时，`std::ofstream::write` 有时会写入比应有的更多字节

Question

我正在尝试将 3D 几何体写入二进制 STL 文件。以下是主程序的工作原理：

ParseSTL stl();
//generate the 3D model
std::string outfname = "test.stl";
std::ofstream outf(outfname, std::ios_base::out & std::ios_base::binary);
stl.writeBinarySTL(outf);

STL 作者是：

void writeBinarySTL(std::ofstream &outf)
{
    char attr[2] = { 0 };
    char header[80] = { 0 };
    int count = 0;
    outf.write(header, 80);
    //count += 80;
    //std::cout << "after header: file position: " << outf.tellp() << "  bytes written: " << count << "\n";
    //if ((int)outf.tellp() != count)
    //    std::cout << "something is wrong!\n";
    outf.write(reinterpret_cast<char *>(&facecount), sizeof(int));
    //count += sizeof(int);
    //std::cout << "after number of faces: file position: " << outf.tellp() << "  bytes written: " << count << "\n";
    //if ((int)outf.tellp() != count)
    //    std::cout << "something is wrong!\n";
    for (int i = 0; i < facecount; i++)
    {
        struct face{
            float n[3];
            float v[3][3];
        } f;
        for (int j = 0; j < 3; ++j)
            f.n[j] = (float) normals[i][j];
        for (int j = 0; j < 3; ++j)
            for (int k = 0; k < 3; ++k)
                f.v[j][k] = (float)vertices[faces[i][j]][k];
        outf.write(reinterpret_cast<char *>(&f), sizeof(f));
        //count += sizeof(f);
        //std::cout << "after struct: file position: " << outf.tellp() << "  bytes written: " << count << "\n";
        //if ((int)outf.tellp() != count)
        //{
        //    std::cout << "something is wrong!\n";
        //    break;
        //}
        outf.write(attr, 2);
        //count += 2;
        //std::cout << "after attributes: file position: " << outf.tellp() << "  bytes written: " << count << "\n";
        //if ((int)outf.tellp() != count)
        //{
        //    std::cout << "something is wrong!\n";
        //    break;
        //}
    }
    //std::cout << "Bytes written: " << count << "\n";
}

注释部分仅供调试，facecount是class的成员，ParseSTL存储三角形数，normals是3D向量向量存储所有面的法线，vertices 是存储所有顶点坐标的 3D 向量向量，faces 是存储属于三角形的顶点索引的 3 个整数向量。现在，如果我运行程序，这里是我得到的输出：

Vertex count = 4243
Face count = 3168
Edge count = 0
after header: file position: 80  bytes written: 80
after number of faces: file position: 84  bytes written: 84
after struct: file position: 132  bytes written: 132
after attributes: file position: 134  bytes written: 134
...
after struct: file position: 532  bytes written: 532
after attributes: file position: 534  bytes written: 534
after struct: file position: 583  bytes written: 582
something is wrong!

因此，outf.write(reinterpret_cast<char *>(&f), sizeof(f)); 行似乎比应有的多写了 1 个字节。这种漂移偶尔会发生，最后，我得到的不是 158,484 字节的文件，而是 158,896 字节的文件。如果我在写完struct后加上一个outf.seekp(count)，最终的文件大小是正确的，但是有些float没有写正确（所以漂移并不总是在struct的末尾）。

我想知道我做错了什么导致将这些额外的字节写入文件。哦，我正在使用 Microsoft Visual Stusio Express 2013。

Answer 1

这一行

std::ofstream outf(outfname, std::ios_base::out & std::ios_base::binary);

需要

std::ofstream outf(outfname, std::ios_base::out | std::ios_base::binary);
                                              ^^^^^ The difference

可以通过省略 std::ios_base::out 标志来简化（感谢@Blastfurnace 的提示）。它还减少了出现此类错误的可能性。

std::ofstream outf(outfname, std::ios_base::binary);

写入二进制文件时，`std::ofstream::write` 有时会写入比应有的更多字节

When writing into a binary file, `std::ofstream::write` sometimes writes more bytes than it should

c++

file-io

binaryfiles