我已经读过,升压iostreams据称支持64位访问大文件的半便携式方式.他们的常见问题解答提到
64 bit offset functions,但没有关于如何使用它们的例子.有没有人用这个库来处理大文件？打开两个文件,寻找他们的middles,并将一个文件复制到另一个文件的一个简单例子将非常有用.
谢谢.
解决方法
简短的回答
只是包括
#include <boost/iostreams/seek.hpp>
boost::iostreams::seek(device,offset,whence);
哪里
> device是一个文件,流,streambuf或任何可转换为可搜索的对象;
> offset是类型stream_offset的64位偏移量;
> whence是BOOST_IOS :: beg,BOOST_IOS :: cur或BOOST_IOS :: end.
seek的返回值是std :: streampos类型,可以使用position_to_offset函数将其转换为stream_offset.
例
这是一个冗长乏味且重复的例子,它展示了如何打开两个文件,寻找超过4GB的余量,以及在它们之间复制数据.
警告：此代码将创建非常大的文件(几GB).在支持稀疏文件的OS /文件系统上尝试此示例. Linux还可以;我没有在其他系统上测试它,例如Windows.
/*
* WARNING: This creates very large files (several GB)
* unless your OS/file system supports sparse files.
*/
#include <boost/iostreams/device/file.hpp>
#include <boost/iostreams/positioning.hpp>
#include <cstring>
#include <iostream>
using boost::iostreams::file_sink;
using boost::iostreams::file_source;
using boost::iostreams::position_to_offset;
using boost::iostreams::seek;
using boost::iostreams::stream_offset;
static const stream_offset GB = 1000*1000*1000;
void setup()
{
file_sink out("file1",BOOST_IOS::binary);
const char *greetings[] = {"Hello","Boost","World"};
for (int i = 0; i < 3; i++) {
out.write(greetings[i],5);
seek(out,7*GB,BOOST_IOS::cur);
}
}
void copy_file1_to_file2()
{
file_source in("file1",BOOST_IOS::binary);
file_sink out("file2",BOOST_IOS::binary);
stream_offset off;
off = position_to_offset(seek(in,-5,BOOST_IOS::end));
std::cout << "in: seek " << off << std::endl;
for (int i = 0; i < 3; i++) {
char buf[6];
std::memset(buf,'\0',sizeof buf);
std::streamsize nr = in.read(buf,5);
std::streamsize nw = out.write(buf,5);
std::cout << "read: \"" << buf << "\"(" << nr << "),"
<< "written: (" << nw << ")" << std::endl;
off = position_to_offset(seek(in,-(7*GB + 10),BOOST_IOS::cur));
std::cout << "in: seek " << off << std::endl;
off = position_to_offset(seek(out,BOOST_IOS::cur));
std::cout << "out: seek " << off << std::endl;
}
}
int main()
{
setup();
copy_file1_to_file2();
}
