我正在寻找一种好的方法来复制文件（二进制或文本）。我已经写了几个样例，每个都可以工作。但是我想听听有经验的程序员的意见。
我缺少好的示例，并且正在寻找一种适用于C ++的方法。
ANSI-C-WAY
#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-WAY（K&R 在《C程序设计语言》中使用此方式，更低级）
#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++实现方法
#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;
}
自有缓冲区C++实现方法
#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 // 需要内核版本 >= 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）
- Kernel 3.3
- GLIBC-2.15，LIBSTDC++4.7（GCC-LIBS），GCC 4.7，Coreutils8.16
- 使用RUNLEVEL 3（多用户，网络，终端，无GUI）
- INTEL SSD-Postville 80 GB，填充到50%
- 复制270 MB OGG-VIDEO-FILE
重现步骤
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 (CPU 时间使用)
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
文件大小不会改变。
sha256sum打印相同的结果。
视频文件仍然可播放。
问题
- 您更喜欢哪种方法？
- 您是否了解更好的解决方案？
- 您在我的代码中看到任何错误吗？
您知道避免某种解决方案的原因吗？
FSTREAM(KISS，Streambuffer)
我真的很喜欢这个方法，因为它非常简短和简单。据我所知，运算符<<被重载为rdbuf()，不会转换任何内容。正确吗？
谢谢
更新1
我已经按照这种方式更改了所有示例中的源代码，即将文件描述符的打开和关闭包含在clock()的测量中。其它方面没有显著的源代码改动。结果没有改变！我还使用time再次检查我的结果。
更新2
ANSI C示例已更改：while循环的条件不再调用feof()，而是将fread()移到条件中。看起来，该代码现在快10,000个钟。
测量已更改：以前的结果总是缓冲的，因为我为每个程序重复了旧的命令行rm to.ogv && sync && time ./program。现在我重新启动系统运行每个程序。未缓冲的结果是新的，并且没有什么意外。未缓冲的结果并没有真正改变。
如果我不删除旧副本，则程序会有所不同。使用POSIX和SENDFILE覆盖现有文件缓冲速度更快，所有其他程序都较慢。也许选项truncate或create影响此行为。但是，使用相同副本覆盖现有文件并不是真实的用例。
使用cp进行复制需要0.44秒未缓冲和0.30秒缓冲。因此，cp比POSIX示例略慢。对我来说看起来很好。
也许我还会添加使用mmap()和copy_file()从boost::filesystem的样例和结果。
更新3
我将其放在博客页面上并进行了扩展。包括splice()，它是Linux内核的低级函数。可能会有更多使用Java的示例。
http://www.ttyhoney.com/blog/?page_id=69
fstream绝对是进行文件操作的良好选择。 - chris