以正常、安全和有效的方式复制文件

提问者：小点点

以正常、安全和有效的方式复制文件

我搜索一个好的方法来复制一个文件（二进制或文本）。我写了几个样本，每个人都工作。但我想听听经验丰富的程序员的意见。

我错过了好的例子，并搜索了一种与C++一起工作的方法。

ANSI-C方式

#include <iostream>
#include <cstdio>    // fopen, fclose, fread, fwrite, BUFSIZ
#include <ctime>
using namespace std;

int main() {
    clock_t start, end;
    start = clock();

    // BUFSIZE default is 8192 bytes
    // BUFSIZE of 1 means one chareter at time
    // good values should fit to blocksize, like 1024 or 4096
    // higher values reduce number of system calls
    // size_t BUFFER_SIZE = 4096;

    char buf[BUFSIZ];
    size_t size;

    FILE* source = fopen("from.ogv", "rb");
    FILE* dest = fopen("to.ogv", "wb");

    // clean and more secure
    // feof(FILE* stream) returns non-zero if the end of file indicator for stream is set

    while (size = fread(buf, 1, BUFSIZ, source)) {
        fwrite(buf, 1, size, dest);
    }

    fclose(source);
    fclose(dest);

    end = clock();

    cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
    cout << "CPU-TIME START " << start << "\n";
    cout << "CPU-TIME END " << end << "\n";
    cout << "CPU-TIME END - START " << end - start << "\n";
    cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";

    return 0;
}

POSIX方法(K&R在“C编程语言”中使用该方法，更低级别）

#include <iostream>
#include <fcntl.h>   // open
#include <unistd.h>  // read, write, close
#include <cstdio>    // BUFSIZ
#include <ctime>
using namespace std;

int main() {
    clock_t start, end;
    start = clock();

    // BUFSIZE defaults to 8192
    // BUFSIZE of 1 means one chareter at time
    // good values should fit to blocksize, like 1024 or 4096
    // higher values reduce number of system calls
    // size_t BUFFER_SIZE = 4096;

    char buf[BUFSIZ];
    size_t size;

    int source = open("from.ogv", O_RDONLY, 0);
    int dest = open("to.ogv", O_WRONLY | O_CREAT /*| O_TRUNC/**/, 0644);

    while ((size = read(source, buf, BUFSIZ)) > 0) {
        write(dest, buf, size);
    }

    close(source);
    close(dest);

    end = clock();

    cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
    cout << "CPU-TIME START " << start << "\n";
    cout << "CPU-TIME END " << end << "\n";
    cout << "CPU-TIME END - START " << end - start << "\n";
    cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";

    return 0;
}

Kiss-C++-StreamBuffer-Way

#include <iostream>
#include <fstream>
#include <ctime>
using namespace std;

int main() {
    clock_t start, end;
    start = clock();

    ifstream source("from.ogv", ios::binary);
    ofstream dest("to.ogv", ios::binary);

    dest << source.rdbuf();

    source.close();
    dest.close();

    end = clock();

    cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
    cout << "CPU-TIME START " << start << "\n";
    cout << "CPU-TIME END " << end << "\n";
    cout << "CPU-TIME END - START " <<  end - start << "\n";
    cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";

    return 0;
}

复制-算法-C++-方法

#include <iostream>
#include <fstream>
#include <ctime>
#include <algorithm>
#include <iterator>
using namespace std;

int main() {
    clock_t start, end;
    start = clock();

    ifstream source("from.ogv", ios::binary);
    ofstream dest("to.ogv", ios::binary);

    istreambuf_iterator<char> begin_source(source);
    istreambuf_iterator<char> end_source;
    ostreambuf_iterator<char> begin_dest(dest); 
    copy(begin_source, end_source, begin_dest);

    source.close();
    dest.close();

    end = clock();

    cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
    cout << "CPU-TIME START " << start << "\n";
    cout << "CPU-TIME END " << end << "\n";
    cout << "CPU-TIME END - START " <<  end - start << "\n";
    cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";

    return 0;
}

Own-Buffer-C++-Way

#include <iostream>
#include <fstream>
#include <ctime>
using namespace std;

int main() {
    clock_t start, end;
    start = clock();

    ifstream source("from.ogv", ios::binary);
    ofstream dest("to.ogv", ios::binary);

    // file size
    source.seekg(0, ios::end);
    ifstream::pos_type size = source.tellg();
    source.seekg(0);
    // allocate memory for buffer
    char* buffer = new char[size];

    // copy file    
    source.read(buffer, size);
    dest.write(buffer, size);

    // clean up
    delete[] buffer;
    source.close();
    dest.close();

    end = clock();

    cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
    cout << "CPU-TIME START " << start << "\n";
    cout << "CPU-TIME END " << end << "\n";
    cout << "CPU-TIME END - START " <<  end - start << "\n";
    cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";

    return 0;
}

LINUX-WAY//需要kernel>=2.6.33

#include <iostream>
#include <sys/sendfile.h>  // sendfile
#include <fcntl.h>         // open
#include <unistd.h>        // close
#include <sys/stat.h>      // fstat
#include <sys/types.h>     // fstat
#include <ctime>
using namespace std;

int main() {
    clock_t start, end;
    start = clock();

    int source = open("from.ogv", O_RDONLY, 0);
    int dest = open("to.ogv", O_WRONLY | O_CREAT /*| O_TRUNC/**/, 0644);

    // struct required, rationale: function stat() exists also
    struct stat stat_source;
    fstat(source, &stat_source);

    sendfile(dest, source, 0, stat_source.st_size);

    close(source);
    close(dest);

    end = clock();

    cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
    cout << "CPU-TIME START " << start << "\n";
    cout << "CPU-TIME END " << end << "\n";
    cout << "CPU-TIME END - START " <<  end - start << "\n";
    cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";

    return 0;
}

环境

GNU/Linux(Archlinux)
内核3.3
glibc-2.15,libstdc++4.7（GCC-LIBS）,GCC 4.7,Coreutils 8.16
使用RUNLEVEL 3（多用户、网络、终端、无GUI)
Intel SSD-PostVille 80 GB，最多填充50%
复制270 MB的OGG-VIDEO-FILE

重现的步骤

 1. $ rm from.ogg
 2. $ reboot                           # kernel and filesystem buffers are in regular
 3. $ (time ./program) &>> report.txt  # executes program, redirects output of program and append to file
 4. $ sha256sum *.ogv                  # checksum
 5. $ rm to.ogg                        # remove copy, but no sync, kernel and fileystem buffers are used
 6. $ (time ./program) &>> report.txt  # executes program, redirects output of program and append to file

结果（使用的CPU时间）

Program  Description                 UNBUFFERED|BUFFERED
ANSI C   (fread/frwite)                 490,000|260,000  
POSIX    (K&R, read/write)              450,000|230,000  
FSTREAM  (KISS, Streambuffer)           500,000|270,000 
FSTREAM  (Algorithm, copy)              500,000|270,000
FSTREAM  (OWN-BUFFER)                   500,000|340,000  
SENDFILE (native LINUX, sendfile)       410,000|200,000

文件大小不变。
sha256sum打印相同的结果。
视频文件仍可播放。

问题

您喜欢什么方法？

您知道更好的解决方案吗？

您看到我的代码中有错误吗？

你知道避免解决方案的原因吗？

FSTREAM（KISS,Streambuffer)
我真的很喜欢这个，因为它真的很短很简单。据我所知，操作符<<对于rdbuf（）是重载的，并且不转换任何内容。正确？

谢谢

更新1
我以这种方式更改了所有示例中的源，即文件描述符的打开和关闭包含在clock（）的度量中。它们在源代码中没有其他重大变化。结果没变！我还用时间仔细检查了一下我的结果。

更新2
更改了ANSI C示例：while-loop的条件不再调用feof（），而是将fread（）移到条件中。看起来，代码现在运行快了10,000个时钟。

度量发生了变化：以前的结果总是被缓冲的，因为我对每个程序重复了几次旧的命令行rm to.ogv&&sync&&time./program。现在我为每个程序重新启动系统。未缓冲的结果是新的，并不令人惊讶。未缓冲的结果并没有真正改变。

如果我不删除旧版本，程序的反应就会不同。用POSIX和SENDFILE覆盖缓冲的现有文件更快，所有其他程序都更慢。可能选项truncate或create对这种行为有影响。但是用相同的副本覆盖现有的文件并不是一个真实的用例。

使用cp执行复制需要0.44秒（无缓冲）和0.30秒（有缓冲）。所以cp比POSIX示例慢一点。对我来说很好。

也许我还会添加来自Boost::FileSystem的mmap()和copy_file()的示例和结果。

更新3
我也把它放在了一个博客页面上，并对它进行了一点扩展。包括splice（）,它是Linux内核的一个低级函数。也许更多带有Java的样本会接踵而至。http://www.ttyhoney.com/blog/?page_id=69

共2个答案

匿名用户

以正常的方式复制文件：

#include <fstream>

int main()
{
    std::ifstream  src("from.ogv", std::ios::binary);
    std::ofstream  dst("to.ogv",   std::ios::binary);

    dst << src.rdbuf();
}

这是如此简单和直观的阅读它是值得的额外成本。如果我们经常这样做，那么最好还是依靠操作系统对文件系统的调用。我确信boost在其filesystem类中有一个copy file方法。

有一个C方法用于与文件系统交互：

#include <copyfile.h>

int
copyfile(const char *from, const char *to, copyfile_state_t state, copyfile_flags_t flags);

匿名用户

在C++17中，复制文件的标准方法是包含头，并使用：

bool copy_file( const std::filesystem::path& from,
                const std::filesystem::path& to);

bool copy_file( const std::filesystem::path& from,
                const std::filesystem::path& to,
                std::filesystem::copy_options options);

第一个表单与第二个表单等效，其中copy_options::none用作选项（另请参见copy_file)。

filesystem库最初是作为boost.filesystem开发的，最后从C++17开始合并到ISO C++。