为什么在c++中使用http post失败

我有下面的代码，但nginx给了我500。我不知道为什么，请帮帮我。

和标题在这里，

我找不到任何错误。有人遇到过吗？

POST http://sss.com/ HTTP/1.1
Host: sss.com
Connection: keep-alive
Content-Length: 241047
Cache-Control: max-age=0
Origin: http://sss.com
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/15.0.849.0 Safari/535.1
Content-Type: multipart/form-data; boundary=----WebKitFormBoundarybZRmyZBq9p09AotO
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Referer: http://sss.com/
Accept-Encoding: gzip,deflate,sdch
Accept-Language: zh-CN,zh;q=0.8
Cookie: PHPSESSID=krje6jlnls57ejc0p4jqhkd2k7
------WebKitFormBoundarybZRmyZBq9p09AotO
Content-Disposition: form-data; name="type"

4
------WebKitFormBoundarybZRmyZBq9p09AotO
Content-Disposition: form-data; name="upfile"; filename="/home/heida/test/1152_648.png"
Content-Type: application/octet-stream

------WebKitFormBoundarybZRmyZBq9p09AotO--

nginx给我的结果如下：

HTTP/1.1 500 Internal Server Error
Server: nginx
Date: Tue, 29 Mar 2016 12:28:23 GMT
Content-Type: text/html
Content-Length: 537
Connection: close
ETag: "54b4def1-219"

代码如下：

#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <iostream>
#include <string.h>
#include <netdb.h>
#include <cerrno>
using namespace std;
std::string get_file_contents(const char *filename)
{
  std::FILE *fp = std::fopen(filename, "rb");
  if (fp)
  {
    std::string contents;
    std::fseek(fp, 0, SEEK_END);
    contents.resize(std::ftell(fp));
    std::rewind(fp);
    std::fread(&contents[0], 1, contents.size(), fp);
    std::fclose(fp);
    return(contents);
  }
  throw(errno);
}
int request(char* hostname, char* api,string & content)
{
    //socket
    struct hostent* host_addr = gethostbyname(hostname);
    if (host_addr == NULL)
    {
        cout<<"Unable to locate host"<<endl;
        return -103;
    }

    sockaddr_in sin;
    sin.sin_family = AF_INET;
    sin.sin_port = htons((unsigned short)80);
    sin.sin_addr=  *((struct in_addr *)host_addr->h_addr);
    bzero(&(sin.sin_zero),8);
    int sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock == -1)
    {
        return -100;
    }

    if (connect(sock, (struct sockaddr *)&sin, sizeof(struct sockaddr) ) == -1)
    {
        cout<<"connect failed"<<endl;
        return -101;
    }

    char send_str[2048] = {0};

    strcat(send_str, "POST ");
    strcat(send_str, api);
    strcat(send_str, " HTTP/1.1rn");
    strcat(send_str, "Host: ");
    strcat(send_str, hostname);
    strcat(send_str, "rn");
    strcat(send_str, "Connection: keep-alivern");

    char content_header[100];
    sprintf(content_header,"Content-Length: %drn", content.size());

    strcat(send_str, content_header);
    strcat(send_str, "Cache-Control: max-age=0rn");
    strcat(send_str, "Origin: http://yyfs.yy.comrn");
    strcat(send_str, "Upgrade-Insecure-Requests: 1rn");
    strcat(send_str, "User-Agent: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/15.0.849.0 Safari/535.1rn");
    strcat(send_str, "Content-Type: multipart/form-data; boundary=----WebKitFormBoundarybZRmyZBq9p09AotOrn");
    strcat(send_str, "Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8rn");
    strcat(send_str, "Referer: http://yyfs.yy.com/rn");
    strcat(send_str, "Accept-Encoding: gzip,deflate,sdchrn");
    strcat(send_str, "Accept-Language: zh-CN,zh;q=0.8rn");
    strcat(send_str,"Cookie: PHPSESSID=krje6jlnls57ejc0p4jqhkd2k7rn");    

    strcat(send_str, "rn");
    strcat(send_str,"------WebKitFormBoundarybZRmyZBq9p09AotOrn");
    strcat(send_str,"Content-Disposition: form-data; name="type"rnrn");
    strcat(send_str, "rn");
    strcat(send_str, "4rn");
    strcat(send_str,"------WebKitFormBoundarybZRmyZBq9p09AotOrn");
    strcat(send_str,"Content-Disposition: form-data; name="upfile"; filename="/home/heida/test/1152_648.png"rn");
    strcat(send_str,"Content-Type: application/octet-streamrnrn");
    cout<<send_str<<endl;
    if (send(sock, send_str, strlen(send_str),0) == -1)
    {
        cout<<"send failed"<<endl;
        return -101;
    }
    if (send(sock, content.c_str(), content.size(),0) == -1)
    {
       cout<<"send failed"<<endl;
       return -1;
    }
    char ends[190] = {0};
    strcat(ends, "rn------WebKitFormBoundarybZRmyZBq9p09AotO--rn");
    cout<<ends<<endl;
    if (send(sock, ends, strlen(ends),0) == -1)
    {
      cout<<"send failed"<<endl;
      return -1;
    }

    char recv_str[4096] = {0};
    if (recv(sock, recv_str, sizeof(recv_str), 0) == -1)
    {
        cout<<"recv failed"<<endl;
        return -101;
    }

    cout<<recv_str<<endl;
    return 0;
}
int main()
{
    std::string files =  get_file_contents("1152_648.png");
    cout<<files.size()<<endl;
    request("sss.com","http://ssy.com/",files);

    return 0;
}