通过设置CURLOPT_RANGE可以下载服务器端文件的特定数据块，这样如果服务器数据很大的情况下，如果只是需其中一小块数据就可以采用这种方法

当然这个也可以用来多线程加速下载同一个文件

#include "stdafx.h"
#include "curl/curl.h"
#include /*注意包含这个头文件后必须把share.h重命名一下，可能是stl里面也有这个头文件，比如curl_share.h,然后把包含到的地方替换一下*/

#include "curl/easy.h"

using namespace std;

struct RecordNode
{
public:
int idx;
int data[10000];
void SetData(int i)
{
idx = i;
for ( int j = 0; j < 10000; j ++ )
{
data[j] = i + j;
}
}
};

struct BufferNode
{
char ptrBuff[40004];
int cur;
int max;
};

BufferNode node;

static size_t downLoadPackage(void *ptr, size_t size, size_t nmemb, void *userdata)
{
//FILE *fp = (FILE*)userdata;
//size_t written = fwrite(ptr, size, nmemb, fp);
//return written;
BufferNode* pBuf = /*&node;*/(BufferNode*)userdata;
size_t cnt = size * nmemb;
if ( pBuf->cur + nmemb <= pBuf->max )
{
memcpy(&(pBuf->ptrBuff[pBuf->cur]), ptr, cnt);
pBuf->cur += cnt;
}
else
{
//printf("over flow...\n");
memcpy(&(pBuf->ptrBuff[pBuf->cur]), ptr, pBuf->max - pBuf->cur);
pBuf->cur += pBuf->max - pBuf->cur;

}

return nmemb;
}

int assetsManagerProgressFunc(void *ptr, double totalToDownload, double nowDownloaded, double totalToUpLoad, double nowUpLoaded)
{
static int percent = 0;
int tmp = 0;
if ( totalToDownload > 0 )
{
tmp = (int)(nowDownloaded / totalToDownload * 100);
}

printf("下载进度%0d%%\r", tmp);
return 0;
}

bool downLoad(void *_curl, std::string _packageUrl, int idx )
{
// Create a file to save package.
//BufferNode node;
node.cur = 0;
node.max = sizeof(RecordNode);
//node.ptrBuff = new char[node.max];
char szRange[128] ={};
size_t nodeSize = sizeof(RecordNode);
sprintf( szRange, "%d-%d", idx * nodeSize, (idx + 1)*nodeSize );

// Download pacakge
CURLcode res;
curl_easy_setopt(_curl, CURLOPT_URL, _packageUrl.c_str());
curl_easy_setopt(_curl, CURLOPT_WRITEFUNCTION, downLoadPackage);
curl_easy_setopt(_curl, CURLOPT_WRITEDATA, &node);
curl_easy_setopt(_curl, CURLOPT_NOPROGRESS, false);
curl_easy_setopt(_curl, CURLOPT_PROGRESSFUNCTION, assetsManagerProgressFunc);
//curl_easy_setopt(_curl, CURLOPT_PROGRESSDATA, this);
curl_easy_setopt(_curl, CURLOPT_NOSIGNAL, 1L);
curl_easy_setopt(_curl, CURLOPT_LOW_SPEED_LIMIT, 1L);
curl_easy_setopt(_curl, CURLOPT_LOW_SPEED_TIME, 5L);

curl_easy_setopt(_curl, CURLOPT_TIMEOUT, 20); //设置超时
curl_easy_setopt(_curl, CURLOPT_NOSIGNAL, 1); //屏蔽其它信号
curl_easy_setopt(_curl, CURLOPT_HEADER, 0); //下载数据包括HTTP头部
curl_easy_setopt(_curl, CURLOPT_RANGE, /*"0-500"*/szRange); //用于断点续传, 设置下载的分片

res = curl_easy_perform(_curl);
curl_easy_cleanup(_curl);
if (res != 0)
{
printf("error: failed to download data\n");
return false;
}

if(node.cur == node.max)
{
RecordNode record;
memcpy(&record, node.ptrBuff, sizeof(RecordNode));
printf(".....idx:%d,data3:%d.......\n", record.idx, record.data[2]);
}
else
{
printf("error: data not correct\n");
}

//delete[] node.ptrBuff;

return true;
}

void GenerateData()
{
FILE *fp = fopen("records.dat", "wb");
if (!fp)
{
return;
}

for ( int i = 0; i < 10000; i ++ )
{
RecordNode node;
node.SetData(i);
size_t written = fwrite(&node, sizeof(RecordNode), 1, fp);
}

fclose(fp);