Unable to obtain POST request response using wininet

I need to make a POST request to an API to get some XML data (http://freecite.library.brown.edu/welcome/api_instructions). This works fine with curl:

curl -H "Accept: application/xml" --data "citation=Udvarhelyi, I.S., Gatsonis, C.A., Epstein, A.M., Pashos, C.L., Newhouse, J.P. and McNeil, B.J. Acute Myocardial Infarction in the Medicare population: process of care and clinical outcomes. Journal of the American Medical Association, 1992; 18:2530-2536. " http://freecite.library.brown.edu:80/citations/create

So I am trying to do a similar thingy using Win32 SDK. This is my code:

void LoadData()
{
    wil::unique_hinternet hInternet(InternetOpen(L"Dummy", INTERNET_OPEN_TYPE_PRECONFIG, NULL, NULL, 0));
    wil::unique_hinternet hConnect(InternetConnect(hInternet.get(), L"http://freecite.library.brown.edu", 80, NULL, NULL, INTERNET_SERVICE_HTTP, 0, 0));
    wil::unique_hinternet hRequest(HttpOpenRequest(hConnect.get(), L"POST", L"/citations/create", NULL, NULL, NULL, NULL, NULL));
    wstring data = L"citation=Udvarhelyi, I.S., Gatsonis, C.A., Epstein, A.M., Pashos, C.L., Newhouse, J.P. and McNeil, B.J. Acute Myocardial Infarction in the Medicare population: process of care and clinical outcomes. Journal of the American Medical Association, 1992; 18:2530-2536.";
    PCWSTR szHeaders = L"Accept: application/xml";

    HttpSendRequest(hRequest.get(), szHeaders, 0, (LPVOID)data.c_str(), static_cast<int>(data.length()));

    DWORD availableBytes = 0;
    InternetQueryDataAvailable(hRequest.get(), &availableBytes, 0, 0);
    PBYTE outputBuffer = (PBYTE)HeapAlloc(GetProcessHeap(), 0, availableBytes);
    PBYTE nextBytes = outputBuffer;
    DWORD bytesUsed = 0; // number of used bytes.
    while (availableBytes)
    {
        DWORD downloadedBytes;
        InternetReadFile(hRequest.get(), nextBytes, availableBytes, &downloadedBytes);
        bytesUsed = bytesUsed + downloadedBytes;

        InternetQueryDataAvailable(hRequest.get(), &availableBytes, 0, 0);
        if (availableBytes > 0)
        {
            // lazy buffer growth here. Only alloc for what we need. could be optimized if we end up with huge payloads (>10MB).
            // otherwise, this is good enough.
            outputBuffer = (PBYTE)HeapReAlloc(GetProcessHeap(), 0, outputBuffer, bytesUsed + availableBytes);
            nextBytes = outputBuffer + bytesUsed; // careful, pointer might have moved! Update cursor.
        }
    }

    // Convert outputed XML to wide char
    int size_needed = MultiByteToWideChar(CP_UTF8, 0, (PCCH)outputBuffer, bytesUsed, NULL, 0);
    std::wstring wstrTo(size_needed, 0);
    MultiByteToWideChar(CP_UTF8, 0, (PCCH)outputBuffer, bytesUsed, &wstrTo[0], size_needed);

    wstring res = wstrTo;
}

The problem is, before entering the for loop, even after the call to InternetQueryDataAvailable, availableBytes comes out to be 0. As a result, I finally end up getting a blank string as response, whereas I was expecting a XML response.

Can anyone point me what am I doing wrongly, and how to fix it?

InternetConnect expects server name or IP address, so don't include "http://" in the address. Change to:

InternetConnect(handle, L"freecite.library.brown.edu"...);

Use UTF-8 for data. Other parameters for WinAPI functions are correctly using UTF-16, they automatically make the necessary conversions.

Change the header:

std::wstring szHeaders = L"Content-Type: application/x-www-form-urlencoded\r\n";

accept should be sent through HttpOpenRequest

const wchar_t *accept[] = { L"text/xml", NULL };
HINTERNET hrequest = HttpOpenRequest(hconnect, L"POST", L"/citations/create",
    NULL, NULL, accept, 0, 0);

Note, if you don't specify accept (use NULL in its place) then the result can be in plain html.

The example below should return XML.

Note, for simplicity I put optional as ANSI string, but it should be UTF8, then you convert it to UTF16 woptional and send it. result will be UTF8 string, it should be converted to UTF16 for Windows's display.

#include <iostream>
#include <string>
#include <Windows.h>
#include <WinINet.h>

#pragma comment(lib, "wininet.lib")//include WinINet library

int main()
{
    std::string result;
    std::wstring server = L"freecite.library.brown.edu";
    std::wstring objectname = L"/citations/create"; //file in this case!
    std::wstring header = L"Content-Type: application/x-www-form-urlencoded\r\n";
    std::string optional = "citation=Udvarhelyi, I.S., Gatsonis, C.A., Epstein";

    HINTERNET hsession = InternetOpen(L"appname", 
        INTERNET_OPEN_TYPE_PRECONFIG, NULL, NULL, 0);

    HINTERNET hconnect = InternetConnect(hsession, server.c_str(),
        INTERNET_DEFAULT_HTTP_PORT, NULL, NULL, INTERNET_SERVICE_HTTP, 0, 0);

    const wchar_t* accept[] = { L"text/xml", NULL };
    HINTERNET hrequest = HttpOpenRequest(hconnect, L"POST", objectname.c_str(),
        NULL, NULL, accept, 0, 0);

    if(HttpSendRequest(hrequest, header.c_str(), header.size(),
        &optional[0], optional.size()))
    {
        DWORD blocksize = 4096;
        DWORD received = 0;
        std::string block(blocksize, 0);
        while (InternetReadFile(hrequest, &block[0], blocksize, &received) 
            && received)
        {
            block.resize(received);
            result += block;
        }
        std::cout << result << std::endl;
    }

    if (hrequest)  InternetCloseHandle(hrequest);
    if (hconnect)  InternetCloseHandle(hconnect);
    if (hsession)  InternetCloseHandle(hsession);
    return 0;
}