Как применить постоянную опцию подключения libcurl

Я использую libcurl для загрузки файла в виде блоков с FTP-сервера.
Я написал это, используя простой интерфейс.
псевдокод выглядит так:

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
#include <unistd.h>
#include <curl/curl.h>

static size_t
WriteCallback(void *buffer, size_t size, size_t nmemb, void *user_param)
{
size_t realsize = size * nmemb;
std::ostream *ftp_file_p = static_cast<std::ofstream*>(user_param);
if (realsize && ftp_file_p)
{
ftp_file_p->write(static_cast<char *>(buffer), realsize);
}
return realsize;
}

std::string
get_file_name_from_url(const std::string& url)
{
size_t pos = url.rfind('/');
return url.substr(pos + 1);

return "";
}

void easy_setup_constants(CURL *curl, std::ostream& results_file)
{
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
curl_easy_setopt(curl, CURLOPT_HEADER, 1L);

curl_easy_setopt(curl, CURLOPT_NOBODY, 1L);
curl_easy_setopt(curl, CURLOPT_FTP_FILEMETHOD, CURLFTPMETHOD_SINGLECWD);
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteCallback);
//curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, write_response);
curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, WriteCallback);
curl_easy_setopt(curl, CURLOPT_HEADERDATA, static_cast<void*>(&results_file));
}

CURLcode
download_file(CURL *curl, const std::string& url)
{
const size_t BLOCK_SIZE = 0x10000;
if (!curl)
{
return CURLE_OK;
}

const std::string file_name = get_file_name_from_url(url);
std::ofstream ftp_file(file_name.c_str(), std::ofstream::binary);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, static_cast<void*>(&ftp_file));
curl_easy_setopt(curl, CURLOPT_URL, url.c_str());double filesize = 0.0;
CURLcode res = curl_easy_getinfo(curl, CURLINFO_CONTENT_LENGTH_DOWNLOAD, &filesize);
filesize = 205901924;
size_t file_full_size = static_cast<size_t>(filesize);
size_t remaining = file_full_size;

curl_easy_setopt(curl, CURLOPT_NOBODY, 0L);

for (size_t actual_offset = 0; CURLE_OK == res && actual_offset < file_full_size; actual_offset += BLOCK_SIZE, remaining -= BLOCK_SIZE)
{
size_t length = std::min(BLOCK_SIZE, remaining);
size_t range_end = actual_offset + length - 1;
std::string range_buf = std::to_string(static_cast<long long>(actual_offset)) + "-" + std::to_string(static_cast<long long>(range_end));
std::clog << "range_buf: " << range_buf << std::endl;
curl_easy_setopt(curl, CURLOPT_RANGE, range_buf.c_str());
res = curl_easy_perform(curl);
}if (CURLE_OK != res)
{
std::cerr << __FUNCTION__ << "[" << __LINE__ << "]" << ":: curl failed " << res << " '" << curl_easy_strerror(res) << "'" << std::endl;
}

return res;
}

int main(void)
{
std::ofstream log_file("persistent.log");
std::ofstream results_file("persistent.results");
std::cout.rdbuf(log_file.rdbuf());
std::cerr.rdbuf(log_file.rdbuf());
std::clog.rdbuf(log_file.rdbuf());
std::vector<std::string> urls = {"ftp://user_name:[email protected]/some/path/Very_Large.File"};
curl_global_init(CURL_GLOBAL_ALL);

CURL *curl = curl_easy_init();
if(curl) {
easy_setup_constants(curl, results_file);

for (auto it = urls.begin(); it != urls.end(); ++it)
{
/* Perform the request, res will get the return code */
CURLcode res = download_file(curl, *it);

/* Check for errors */
if(res != CURLE_OK)
std::cerr << __FUNCTION__ << "[" << __LINE__ << "]" << "::curl_easy_perform() failed: " << curl_easy_strerror(res) << std::endl;
}

/* always cleanup */
curl_easy_cleanup(curl);
}

curl_global_cleanup();
return 0;
}

Командная строка компиляции:

g++ -g -Wall -std=c++0x -lcurl persistent.cpp -o persistent -lstdc++

Однако, используя tcpdump, я замечаю, что каждый раз, когда я читаю блок, происходит вход-чтение-выход из системы. По какой-то причине соединение не остается постоянным. Даже если libcurl обернут в класс и curl_easy_cleanup выполняется в деструкторе, я вообще не использую curl_easy_reset, а в конструкторе я использую curl_easy_init

Я включил режим многословия и увидел, что для каждого переданного фрагмента я получаю следующее:

* About to connect() to 127.0.0.1 port 21 (#0)
*   Trying 127.0.0.1... * connected
* Connected to 127.0.0.1 (127.0.0.1) port 21 (#0)
< 220 (vsFTPd 2.2.2)
> user_name
< 331 Please specify the password.
> user_pass
< 230 Login successful.
> PWD
< 257 "/data"* Entry path is '/data'
> CWD some/path
< 250 Directory successfully changed.
> EPSV
* Connect data stream passively
< 229 Entering Extended Passive Mode (|||63137|).
*   Trying 127.0.0.1... * connected
* Connecting to 127.0.0.1 (127.0.0.1) port 63137
> TYPE I
< 200 Switching to Binary mode.
> SIZE Very_Large.File
< 213 205901924
> RETR Very_Large.File
< 150 Opening BINARY mode data connection for Very_Large.File (205901924 bytes).
* Maxdownload = 65536
* Getting file with size: 65536
* Remembering we are in dir "some/path/"< 426 Failure writing network stream.
* Connection #0 to host 127.0.0.1 left intact
* Re-using existing connection! (#0) with host 127.0.0.1
* Connected to 127.0.0.1 (127.0.0.1) port 21 (#0)
* Request has same path as previous transfer
> EPSV
* Connect data stream passively
< 229 Entering Extended Passive Mode (|||36365|).
*   Trying 127.0.0.1... * connected
* Connecting to 127.0.0.1 (127.0.0.1) port 36365
> SIZE Very_Large.File
< 213 205901924
* Instructs server to resume from offset 65536
> REST 65536
< 350 Restart position accepted (65536).
> RETR Very_Large.File
< 150 Opening BINARY mode data connection for Very_Large.File (205901924 bytes).
* Maxdownload = 65536
* Getting file with size: 65536
* Remembering we are in dir "some/path/"< 426 Failure writing network stream.
* Connection #0 to host 127.0.0.1 left intact
* Re-using existing connection! (#0) with host 127.0.0.1
* Connected to 127.0.0.1 (127.0.0.1) port 21 (#0)
* Request has same path as previous transfer
> EPSV
* Connect data stream passively
< 229 Entering Extended Passive Mode (|||49089|).
*   Trying 127.0.0.1... * connected
* Connecting to 127.0.0.1 (127.0.0.1) port 49089
> SIZE Very_Large.File
< 213 205901924
* Instructs server to resume from offset 131072
> REST 131072
< 350 Restart position accepted (131072).
> RETR Very_Large.File
< 150 Opening BINARY mode data connection for Very_Large.File (205901924 bytes).
* Maxdownload = 65536
* Getting file with size: 65536
* Remembering we are in dir "some/path/"< 426 Failure writing network stream.
< 226 Transfer complete.
* Connection #0 to host 127.0.0.1 left intact
> QUIT
< 221 Goodbye.
* Closing connection #0