Linux下Socket写http协议下载文件代码以及说明

看代码：

#include <iostream>
#include <stdio.h>
#include <winsock.h>
#include <conio.h>


using namespace std;

#pragma comment("ws2_32.lib")

string host2ip(const string& strdomain);
void save2file(char *buffer,const string& filename);
int getlength(const string& sheader);
bool getstatus(const string& sheader);
int getleftlength(const string& sheader);
void sethttphead(const string& file,const string& server);


int port = 80;
int error=0;
string serveraddr;
int i = 0;
const int BUFFER_SIZE = 1024;
char buffer[BUFFER_SIZE] = {0};
char    msg[BUFFER_SIZE];
FILE *f;
const string file = "/BT/200801/3/117906.torrent";
const string url = "www.supbt.com";
const string filename = "117906.torrent";


int main(void)
{
        int      r;
        WSADATA  wsa;
        SOCKET   sock;
        struct   sockaddr_in sin;
        f = fopen(filename.c_str(),"ab");
        sethttphead(file,url);
        if(WSAStartup(MAKEWORD(2,2),&wsa))
        {
                printf("WSAStartup Error: %d/n",WSAGetLastError());
                getche();
                return -1;
        }

        if((sock=socket(AF_INET,SOCK_STREAM,IPPROTO_TCP))==INVALID_SOCKET)
        {
                printf("Socket Error: %d/n",WSAGetLastError());
                getche();
                return -1;
        }

        sin.sin_family=AF_INET;
        sin.sin_port=htons(port);
        serveraddr = host2ip(url);

        cout << serveraddr << endl;
        sin.sin_addr.s_addr=inet_addr(serveraddr.c_str());


        if(connect(sock,(struct sockaddr *)&sin,sizeof(sin))==SOCKET_ERROR)
        {
                printf("Connect Error: %d/n",WSAGetLastError());
                printf("请检测网络连接！/n");
                getche();
                return -1;
        }


        if((r=send(sock,msg,sizeof(msg),0))==SOCKET_ERROR)
        {
                printf("Send Error: %d/n",WSAGetLastError());
                getche();
                return -1;
        }

        memset(buffer,0,BUFFER_SIZE);
        if((r=recv(sock,buffer,BUFFER_SIZE,0))==SOCKET_ERROR)
        {
                printf("Recv Error: %d/n",WSAGetLastError());
                closesocket(sock);
                return -1;
        }

        string header = string(buffer);
        int ilength = getlength(header);
        int leftlength = ilength;
        int icount;
        if(getstatus(header)){
                //current is r
                char *q = buffer;
                char *p = strstr(buffer, "/r/n/r/n");

                p += 4;
                //注意这里得 r - (p-q) 非常关键得问题
                fwrite(p,r - (p - q),1,f);
                leftlength -= r - (p - q);
                int i=0;
                icount = r - (p - q);
                while(true)
                {
                        memset(buffer, 0, BUFFER_SIZE);
                        r = recv(sock, buffer, BUFFER_SIZE, 0);

                        if (r < 0) {
                                closesocket(sock);
                                return 2;
                        }
                        if (r == 0) {
                                break;
                        }
                        int tmp;
                        tmp = leftlength;    //209  100 100 92
                        leftlength -= r;
                        if(leftlength > 0){
                                fwrite(buffer,r,1,f);
                                icount += r;
                        }
                        else if(leftlength == 0){
                                //
                                break;
                        }
                        else{
                                icount += tmp;
                                fwrite(buffer,tmp,1,f);
                                break;
                        }
                        cout << i++ << " r is "<< r << endl;

                }
        }else{
                cout << "http protocal error" << endl;
        }
        cout << icount << endl;
        fclose(f);
        closesocket(sock);
        WSACleanup();
        getche();
        return 0;
}
/*
构造HTTP头
*/
void sethttphead(const string& file,const string& server)
{
        strcat(msg,"GET ");
        strcat(msg,file.c_str());

        strcat(msg," HTTP/1.1");
        strcat(msg,"/r/n");

        strcat(msg,"Accept:*/*");
        strcat(msg,"/r/n");

        //strcat(msg,"Accept-Language: zh-cn");
        //strcat(msg,"/r/n");

        //strcat(msg,"Accept-Encoding: gzip, deflate");
        //strcat(msg,"/r/n");

        strcat(msg,"User-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)");
        strcat(msg,"/r/n");

        strcat(msg,"Host:");
        strcat(msg,server.c_str());
        strcat(msg,"/r/n");

        strcat(msg,"Connection: Keep-Alive");
        strcat(msg,"/r/n");
        strcat(msg,"/r/n");
}
//---------------------------------------------------------------------------

/*
将domain 转换为IP地址
*/
string host2ip(const string& strdomain)
{
        SOCKADDR_IN   saddr;
        WORD   wVersionRequested;
        WSADATA   wsaData;
        hostent   *pHostIp;
        int   nErrorNum;
        wVersionRequested   =   MAKEWORD(2,   2);
        nErrorNum   =   WSAStartup(wVersionRequested,   &wsaData);
        if(nErrorNum   !=   0)
        {
                return   "";
        }
        saddr.sin_family   =   AF_INET;
        pHostIp   =   gethostbyname(strdomain.c_str());
        if(pHostIp   ==   NULL)
        {
                return   "0.0.0.0";
        }
        WSACleanup();
        return   (string)inet_ntoa(*(LPIN_ADDR)*(pHostIp->h_addr_list));
}
/*
该方法为测试用法，并为应用到项目中
*/
void save2file(char *buffer,const string& filename)
{
        FILE *f;
        f = fopen(filename.c_str(),"a+");
        fwrite(buffer,strlen(buffer),1,f);
        fclose(f);
}
/*
该方法获取待下载文件大小
*/
int getlength(const string& sheader){
        char * cont_len_pos;
        unsigned long cont_length;
        const char* header = sheader.c_str();

        cont_len_pos =(char *)strstr(header,"Content-Length:");
        cont_len_pos = (char *)strchr(cont_len_pos,' ');
        cont_len_pos++;
        sscanf(cont_len_pos, "%ld", &cont_length);

        return cont_length;
}
/*
该方法判断是否Get成功，不成功就取消下载文件
*/
bool getstatus(const string& sheader){
        string start = "HTTP/1.1 200 OK";
        if(sheader.find(start,0) != string::npos){
                return true;
        }
        else{
                return false;
        }
}
/*
该方法并没有用到，开始考虑多了
*/
int getleftlength(const string& sheader){
        int nreturnl = 0;
        int ilength = sheader.length();
        string _send = "/r/n/r/n";
        int istart = sheader.find_last_of(_send);
        if(istart != string::npos){
                nreturnl = ilength - istart -_send.length();
        }
        return nreturnl;
}