streamsize sgetn(char_type *store,streamsize n) 返回缓冲区下n个字符并存储到store中，并将缓冲区位置后移n个字节
代码说明：本来是想不断的通过sgetn函数获取到streambuf的内容，由于没有完全理解sgetn获取流的方式，导致了问题的产生
int Teststreambuf()
{
boost::asio::streambuf request;
std::ostream request_stream(&request);
request_stream << "GET /cs/restfull/operationRestfullApi/testGet/ HTTP/1.1\r\n";
request_stream << "Accept: application/x-ms-application, image/jpeg, application/xaml+xml, image/gif, image/pjpeg, application/x-ms-xbap, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*\r\n";
request_stream << "Accept-Language: zh-CN\r\n";
request_stream << "User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E; InfoPath.3)\r\n";
request_stream << "Host: " << "192.168.0.88:8080" << "\r\n";
request_stream << "Connection: Keep-Alive\r\n\r\n";
//当第一次使用sgetn函数的时候，已经将所有的数据读完，指向流的结束位置
char buf[1024] = { 0 };
int len = request.size();
request.sgetn(buf, len);
//当第二次使用sgetn函数的时候，由于第一次函数调用指向了流的结束位置，
//所以第二次读取到的buf1是空的
char buf1[1024] = { 0 };
int len1 = request.size();
request.sgetn(buf1, len1);
return 0;
}
实际的测试环境：希望组装完成一次HTTP请求之后，循环发送构建的数据包，达到压力测试的结果，但是在接收端只是接收到一次的数据，初步怀疑是服务器的代码有问题，后面才发现是sgetn函数的使用方法有问题
#include <iostream>
#include <fstream>
#include <string>
#include <boost/asio.hpp>
using namespace std;
using namespace boost::asio;
int Teststreambuf()
{
io_service iosev;
ip::tcp::socket socket(iosev);
ip::tcp::endpoint ep(ip::address_v4::from_string("127.0.0.1"), 7002);
boost::system::error_code ec;
socket.connect(ep, ec);
if (ec) return -1;
boost::asio::streambuf request;
std::ostream request_stream(&request);
request_stream << "POST /cs/restfull/operationRestfullApi/testPost HTTP/1.1\r\n";
request_stream << "Accept: application/x-ms-application, image/jpeg, application/xaml+xml, image/gif, image/pjpeg, application/x-ms-xbap, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*\r\n";
request_stream << "Accept-Language: zh-CN\r\n";
request_stream << "User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E; InfoPath.3)\r\n";
request_stream << "Host: " << "192.168.0.88:8080" << "\r\n";
request_stream << "Connection: Keep-Alive\r\n\r\n";
while (1)
{
//第一次调用write之后，request的数据已经发送完毕，作为一个流，已经指向数据的末尾，所以
//当再次循环的时候发送的数据len==0,本意是循环发送上面的HTTP数据，对服务器进行压力测试
size_t len = boost::asio::write(socket, request);
std::cout << len << std::endl;
::Sleep(300);
}
return 0;
}
修改之后的代码
#include <iostream>
#include <fstream>
#include <string>
#include <boost/asio.hpp>
using namespace std;
using namespace boost::asio;
int Teststreambuf()
{
io_service iosev;
ip::tcp::socket socket(iosev);
ip::tcp::endpoint ep(ip::address_v4::from_string("127.0.0.1"), 7002);
boost::system::error_code ec;
socket.connect(ep, ec);
if (ec) return -1;
boost::asio::streambuf request;
std::ostream request_stream(&request);
request_stream << "POST /cs/restfull/operationRestfullApi/testPost HTTP/1.1\r\n";
request_stream << "Accept: application/x-ms-application, image/jpeg, application/xaml+xml, image/gif, image/pjpeg, application/x-ms-xbap, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*\r\n";
request_stream << "Accept-Language: zh-CN\r\n";
request_stream << "User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E; InfoPath.3)\r\n";
request_stream << "Host: " << "192.168.0.88:8080" << "\r\n";
request_stream << "Connection: Keep-Alive\r\n\r\n";
//跟上面的不一样的做法，是将request的数据一次性读取到buf数组中，然后循环发送buf的数据内容
char buf[1024] = { 0 };
request.sgetn(buf, request.size());
while (1)
{
size_t len = boost::asio::write(socket, boost::asio::buffer(buf));
std::cout << len << std::endl;
::Sleep(300);
}
return 0;
}