WinHttpQueryDataAvailable function

The WinHttpQueryDataAvailable function returns the amount of data, in bytes, available to be read with WinHttpReadData.

Syntax

BOOL WINAPI WinHttpQueryDataAvailable(
  _In_  HINTERNET hRequest,
  _Out_ LPDWORD   lpdwNumberOfBytesAvailable
);

Parameters

hRequest [in]

A valid HINTERNET handle returned by WinHttpOpenRequest. WinHttpReceiveResponse must have been called for this handle and have completed before WinHttpQueryDataAvailable is called.

lpdwNumberOfBytesAvailable [out]

A pointer to an unsigned long integer variable that receives the number of available bytes. When WinHTTP is used in asynchronous mode, always set this parameter to NULL and retrieve data in the callback function; not doing so can cause a memory fault.

Return value

Returns TRUE if the function succeeds, or FALSE otherwise. To get extended error data, call GetLastError. Among the error codes returned are the following.

Remarks

Even when WinHTTP is used in asynchronous mode (that is, when WINHTTP_FLAG_ASYNC has been set in WinHttpOpen), this function can operate either synchronously or asynchronously. If it returns FALSE, it failed and you can call GetLastError to get extended error information. If it returns TRUE, use the WINHTTP_CALLBACK_STATUS_DATA_AVAILABLE completion to determine whether this function was successful and the value of the parameters. The WINHTTP_CALLBACK_STATUS_REQUEST_ERROR completion indicates that the operation completed asynchronously, but failed.

This function returns the number of bytes of data that are available to read immediately by a subsequent call to WinHttpReadData. If no data is available and the end of the file has not been reached, one of two things happens. If the session is synchronous, the request waits until data becomes available. If the session is asynchronous, the function returns TRUE, and when data becomes available, calls the callback function with WINHTTP_STATUS_CALLBACK_DATA_AVAILABLE and indicates the number of bytes immediately available to read by calling WinHttpReadData.

The amount of data that remains is not recalculated until all available data indicated by the call to WinHttpQueryDataAvailable is read.

Use the return value of WinHttpReadData to determine when a response has been completely read.

For HINTERNET handles created by the WinHttpOpenRequest function and sent by WinHttpSendRequest, a call to WinHttpReceiveResponse must be made on the handle before WinHttpQueryDataAvailable can be used.

If a status callback function has been installed with WinHttpSetStatusCallback, then those of the following notifications that have been set in the dwNotificationFlags parameter of WinHttpSetStatusCallback indicate progress in checking for available data:

• WINHTTP_CALLBACK_STATUS_RECEIVING_RESPONSE
• WINHTTP_CALLBACK_STATUS_RESPONSE_RECEIVED
• WINHTTP_CALLBACK_STATUS_DATA_AVAILABLE

Examples

The following example shows how to use secure transaction semantics to download a resource from an HTTPS server. The sample code initializes the WinHTTP API, selects a target HTTPS server, and then opens and sends a request for this secure resource. WinHttpQueryDataAvailable is used with the request handle to determine how much data is available for download, then WinHttpReadData is used to read that data. This process repeats until the entire document has been retrieved and displayed.

    DWORD dwSize = 0;
    DWORD dwDownloaded = 0;
    LPSTR pszOutBuffer;
    BOOL  bResults = FALSE;
    HINTERNET  hSession = NULL, 
               hConnect = NULL,
               hRequest = NULL;

    // Use WinHttpOpen to obtain a session handle.
    hSession = WinHttpOpen( L"WinHTTP Example/1.0",  
                            WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
                            WINHTTP_NO_PROXY_NAME, 
                            WINHTTP_NO_PROXY_BYPASS, 0);

    // Specify an HTTP server.
    if (hSession)
        hConnect = WinHttpConnect( hSession, L"www.microsoft.com",
                                   INTERNET_DEFAULT_HTTPS_PORT, 0);

    // Create an HTTP request handle.
    if (hConnect)
        hRequest = WinHttpOpenRequest( hConnect, L"GET", NULL,
                                       NULL, WINHTTP_NO_REFERER, 
                                       WINHTTP_DEFAULT_ACCEPT_TYPES, 
                                       WINHTTP_FLAG_SECURE);

    // Send a request.
    if (hRequest)
        bResults = WinHttpSendRequest( hRequest,
                                       WINHTTP_NO_ADDITIONAL_HEADERS,
                                       0, WINHTTP_NO_REQUEST_DATA, 0, 
                                       0, 0);

 
    // End the request.
    if (bResults)
        bResults = WinHttpReceiveResponse( hRequest, NULL);

    // Continue to verify data until there is nothing left.
    if (bResults)
        do 
        {

            // Verify available data.
            dwSize = 0;
            if (!WinHttpQueryDataAvailable( hRequest, &dwSize))
                printf( "Error %u in WinHttpQueryDataAvailable.\n",
                        GetLastError());

            // Allocate space for the buffer.
            pszOutBuffer = new char[dwSize+1];
            if (!pszOutBuffer)
            {
                printf("Out of memory\n");
                dwSize=0;
            }
            else
            {
                // Read the Data.
                ZeroMemory(pszOutBuffer, dwSize+1);

                if (!WinHttpReadData( hRequest, (LPVOID)pszOutBuffer, 
                                      dwSize, &dwDownloaded))
                    printf( "Error %u in WinHttpReadData.\n", GetLastError());
                else
                    printf( "%s\n", pszOutBuffer);
            
                // Free the memory allocated to the buffer.
                delete [] pszOutBuffer;
            }

        } while (dwSize > 0);


    // Report any errors.
    if (!bResults)
        printf( "Error %d has occurred.\n", GetLastError());

    // Close open handles.
    if (hRequest) WinHttpCloseHandle(hRequest);
    if (hConnect) WinHttpCloseHandle(hConnect);
    if (hSession) WinHttpCloseHandle(hSession);

Requirements

See also

About Microsoft Windows HTTP Services (WinHTTP)

WinHttpOpen

WinHttpOpenRequest

WinHttpSendRequest

WinHttpConnect

WinHttpReadData

WinHttpCloseHandle

WinHTTP Versions