关键部分代码如下：

#include "curl.h"

#pragma comment(lib, "libcurl.lib")

size_t CROS_DownloadDlg::getContentLengthFunc(void * ptr, size_t size, size_t nmemb, void * stream)
{
LONGLONG len = 0;
int r = sscanf((const char *)ptr, "Content-Length:%I64d\n",&len);
if(r)
*((LONGLONG *) stream) = len;
return size * nmemb;
}


//根据URL获取下载文件大小
LONGLONG CROS_DownloadDlg::getDownloadFileSize(const char*  url)
{
double lensize = 0.0;
for(int iTry = 0 ; iTry < 3 ; iTry ++)//由于curl_easy_perform可能会有偶发性的CURLE_WRITE_ERROR错误，所以添加重试机制
{
CURL *handle = curl_easy_init();
curl_easy_setopt(handle, CURLOPT_URL, url);
curl_easy_setopt(handle, CURLOPT_HEADER, 1);
curl_easy_setopt(handle, CURLOPT_FOLLOWLOCATION, 1);
curl_easy_setopt(handle, CURLOPT_NOBODY, 1);
CURLcode res = curl_easy_perform(handle), resGetInfo = CURLE_OK;
if (res == CURLE_OK) {
resGetInfo = curl_easy_getinfo(handle, CURLINFO_CONTENT_LENGTH_DOWNLOAD, &lensize);
if(CURLE_OK == resGetInfo){
curl_easy_cleanup(handle);
return (LONGLONG)lensize;
}
}
curl_easy_cleanup(handle);
Sleep(200);
}
return 0;
}


size_t CROS_DownloadDlg::downLoadPackage(void *ptr, size_t size, size_t nmemb, void *userdata)
{
if(!ptr || !userdata)
return 0;
return fwrite(ptr, size, nmemb, (FILE *)userdata);
}


int CROS_DownloadDlg::assetsManagerProgressFunc(void *ptr, double totalToDownload, double nowDownloaded, double totalToUpLoad, double nowUpLoaded)
{
if(m_bStop)
return -1;
if(totalToDownload > 0.0)
{
CROS_DownloadDlg* pDlg = (CROS_DownloadDlg*)ptr;
pDlg->m_lProcessSize = (LONGLONG)nowDownloaded;
pDlg->SendMessage(WM_DOWNLOADPROC_MSG, 1, 0);
}
return 0;
}


/*
0 设置总大小
1 设置当前下载
2 下载完成
3 用户停止下载，其他异常终止下载
4 网络问题引发的下载异常
5 设备断开连接
6 磁盘剩余空间不足
*/
UINT CROS_DownloadDlg::DownloadProc(LPVOID lpParam)

{

//////////////////////////////////////////////////////////////////////////使用CURL实现下载
string sURL = CStringConverter::Unicode2Ansi(strURL.GetString()), sSavePath = CStringConverter::Unicode2Ansi(strSavePath.GetString());


LONGLONG local_file_len = GetFileSize(strSavePath);
pDlg->m_lLocalFileCurSize = local_file_len;
curl_global_init(CURL_GLOBAL_DEFAULT);
LONGLONG remote_file_len = getDownloadFileSize(sURL.c_str());
if(!remote_file_len)
{
pDlg->SendMessage(WM_DOWNLOADPROC_MSG, 4, 0);//网络问题获取失败
return 0;
}
pDlg->m_lTotalSize = remote_file_len;
pDlg->SendMessage(WM_DOWNLOADPROC_MSG, 0, 0);
if(local_file_len == remote_file_len)
{
pDlg->SendMessage(WM_DOWNLOADPROC_MSG, 2, 0);
return 0;
}
//得到准确大小后应判断strSavePath空间是否足够先
if(remote_file_len >= (pDlg->m_ulDiskSpaceMax))
{
pDlg->SendMessage(WM_DOWNLOADPROC_MSG, 6, 0);
return 0;
}
FILE *fp = fopen(sSavePath.c_str(), "ab+");
if(!fp)
{
pDlg->SendMessage(WM_DOWNLOADPROC_MSG, 3, 0);
return 0;
}
LONGLONG filesize = 0;
m_pCurl = curl_easy_init();//curl初始化
CURLcode res;
curl_easy_setopt(m_pCurl, CURLOPT_URL, sURL.c_str());//设定curl的链接
curl_easy_setopt(m_pCurl, CURLOPT_CONNECTTIMEOUT,5);//设置连接超时，单位秒
curl_easy_setopt(m_pCurl, CURLOPT_HEADERFUNCTION,getContentLengthFunc);
curl_easy_setopt(m_pCurl, CURLOPT_HEADERDATA, &filesize);
curl_easy_setopt(m_pCurl, CURLOPT_RESUME_FROM_LARGE, local_file_len);//断点下载设置
curl_easy_setopt(m_pCurl, CURLOPT_WRITEFUNCTION, downLoadPackage);
curl_easy_setopt(m_pCurl, CURLOPT_WRITEDATA, fp);//设置将data写入到文件流fp中
curl_easy_setopt(m_pCurl, CURLOPT_FOLLOWLOCATION, 1L);//设置重定位URL，使用自动跳转，返回的头部中有Location(一般直接请求的url没找到)，则继续请求Location对应的数据
curl_easy_setopt(m_pCurl, CURLOPT_MAXREDIRS,5);//查找次数，防止查找太深
curl_easy_setopt(m_pCurl, CURLOPT_NOPROGRESS, false);//设为false 下面才能设置进度响应函数
curl_easy_setopt(m_pCurl, CURLOPT_PROGRESSFUNCTION, assetsManagerProgressFunc);//进度响应函数
curl_easy_setopt(m_pCurl, CURLOPT_PROGRESSDATA, pDlg);//数据传输的对象
curl_easy_setopt(m_pCurl, CURLOPT_VERBOSE,1L);
res = curl_easy_perform(m_pCurl);
curl_easy_cleanup(m_pCurl);
curl_global_cleanup();
m_pCurl = NULL;
if (res != CURLE_OK)
{
fclose(fp);
if(res == CURLE_ABORTED_BY_CALLBACK)//点击停止下载
pDlg->SendMessage(WM_DOWNLOADPROC_MSG, 3, 0);
else if(res == CURLE_RECV_ERROR)//断网     /* 56 - failure in receiving network data */
pDlg->SendMessage(WM_DOWNLOADPROC_MSG, 4, 0);
else
pDlg->SendMessage(WM_DOWNLOADPROC_MSG, 3, 0);
return 0;
}
fclose(fp);
pDlg->SendMessage(WM_DOWNLOADPROC_MSG, 2, 0);
return 0;

}

void CROS_DownloadDlg::OnClickBtnStop()
{
if(m_pCurl){
curl_easy_pause(m_pCurl, CURLPAUSE_RECV);
if(IDYES == ShowMessage(IDS_DEF_ROS_SURE_TO_STOP, CiPhoneUtil::GetApplicationName(), SM_ICON_QUESTION|SM_YES_AND_NO)){
m_btnStop.EnableWindow(FALSE);
m_bStop = true;
return;
}
curl_easy_pause(m_pCurl, CURLPAUSE_RECV_CONT);
}
}