Introduction

If you have ever dig into the MSDN for WinInet API, you noticed that it can be used asynchronously and that it is the recommended way to use it.
If then you decide to use it, you won’t find any explanation of how to use it asynchronously. And no samples are available anywhere on internet.
After a long research and lots of testing, I’ll finally manage to reconstruct a big part of the (voluntary?) missing documentation.

Why asynchronous is better ? Because it can handle timeouts correctly. Just what’s missing in Wininet under IE5.5.
If you try to use TerminateThread or CloseHandle functions to handle timeouts (theses methods are given in MSDN articles), you’ll fall into unrecoverable leaks of all kinds.

This has been tested successfully with: IE4.01SP3, IE5.0, IE5.01, IE5.5SP1 under WinNT4 on mono and multiprocessor machines, under a stressed environment (15 concurrent instances running non-stop for 12 hours on multiproc NT server machines).

Theory

To use WinInet functions in Full Asynchronous Mode, you must do things in the right order:

1 – Use INTERNET_FLAG_ASYNC to open the session
2 – Set a status callback using InternetSetStatusCallback
3 – Open the connection using InternetOpenUrl
4 – If InternetOpenUrl returned NULL and GetLastError is ERROR_IO_PENDING:
* wait for the INTERNET_STATUS_HANDLE_CREATED notification in the callback, and save the connection Handle.
* wait for the INTERNET_STATUS_REQUEST_COMPLETE notification in the callback before going further.
5 – Extract the content-length from the header and set up an INTERNET_BUFFERS structure:
dwStructSize  = sizeof(INTERNET_BUFFERS)
lpvBuffer = your allocated buffer
dwBufferLength = its length
6 – Use InternetReadFileEx with the IRF_ASYNC flag to read the remaining data asynchronously. Don’t use InternetReadFile since it is a synchronous function.
7 – If InternetReadFileEx returned False and GetLastError is ERROR_IO_PENDING:
* wait for the INTERNET_STATUS_REQUEST_COMPLETE notification in the callback before going further.
Warning: INTERNET_BUFFERS members are modified asynchronously (only the dwBufferLength member and the content of the buffer).
8 – If the dwBufferLength member is not 0, move the lpvBuffer pointer from this amount and substract this amout from the buffer length so dwBufferLength reflects the remaining size lpvBuffer points to, then loop to 6.
9 – Close the connection handle with InternetCloseHandle and wait for INTERNET_STATUS_HANDLE_CLOSING and the facultative INTERNET_STATUS_REQUEST_COMPLETE notification (sent only if an error occur – like a sudden closed connection -, you must test the cases).

At this state, you can either begin a new connection process or close the session handle. But before closing it, you should unregister the callback function.

 

Detail

After the Theory, let’s look at some code for some of the points:

1&2: Create the connection using INTERNET_FLAG_ASYNC and setup the callback func:

m_Session = InternetOpen(AGENT_NAME, INTERNET_OPEN_TYPE_PRECONFIG, NULL, NULL, INTERNET_FLAG_ASYNC);
InternetSetStatusCallback( m_Session, (INTERNET_STATUS_CALLBACK)InternetCallbackFunc );
 
3&4: Open the connection using InternetOpenUrl and wait for INTERNET_STATUS_REQUEST_COMPLETE

Use the lParam to send a session identifier to your callback. I always use the this pointer of my class for it. I assume you know how to handle callbacks.
InternetOpenUrl( m_Session, uurl, NULL, 0, INTERNET_FLAG_RELOAD | INTERNET_FLAG_PRAGMA_NOCACHE | INTERNET_FLAG_NO_CACHE_WRITE, (LPARAM)this );

The callback will receive a lots of messages then. Here their orders along with the "dwInternetStatus" value:
[openUrl] InternetStatus: 60 INTERNET_STATUS_HANDLE_CREATED
**At this point you can save the HINTERNET handle using code like this in your callback:
INTERNET_ASYNC_RESULT* res = (INTERNET_ASYNC_RESULT*)lpvStatusInformation;
m_hHttpFile = (HINTERNET)(res->dwResult);

[openUrl] InternetStatus: 10
[openUrl] InternetStatus: 11
[openUrl] InternetStatus: 20
[openUrl] InternetStatus: 21
[openUrl] InternetStatus: 30
[openUrl] InternetStatus: 31
[openUrl] InternetStatus: 40
[openUrl] InternetStatus: 41
[openUrl] InternetStatus: 100 INTERNET_STATUS_REQUEST_COMPLETE

5: Extract the content-length and set up the INTERNET_BUFFERS structure

Once you have the handle, try to call HttpQueryInfo with HTTP_QUERY_CONTENT_LENGTH to get the size of the data to retrieve. This function can fail if the content-length field is not in the HTTP header.

Set up the INTERNET_BUFFERS structure
INTERNET_BUFFERS ib = { sizeof(INTERNET_BUFFERS) };
ib.lpvBuffer = your allocated buffer
ib.dwBufferLength = its length
The dwBufferTotal is provided for your own use and is never modified by WinInet (as far as I know).
I use it to store the total size of the received data.

6&7&8 Read the remaining data in a loop

Use InternetReadFileEx with the IRF_ASYNC flag to read the remaining data asynchronously. Don’t use InternetReadFile since it is a synchronous function.
You must loop on InternetReadFileEx while the ib.dwBufferLength is not 0.
Before each iteration you must adjust the lpvBuffer pointer and reset the dwBufferLength members of ib: add the received length to the pointer and set dwBufferLength to your remaining buffer size.

//Start the pump
BOOL bOk = InternetReadFileEx( m_hHttpFile, &ib, IRF_ASYNC, (LPARAM)this );
if(!bOk && GetLastError()==ERROR_IO_PENDING)
  wait...

//Pump
while( bOk && ib.dwBufferLength!=0 )
{
  (adjust ib values)
  bOk = InternetReadFileEx( m_hHttpFile, &ib, IRF_ASYNC, (LPARAM)this );
  if(!bOk && GetLastError()==ERROR_IO_PENDING)
    wait...
}

Your callback should receive these notifications (maybe more than once):
[connect] InternetStatus: 40 (receiving response)
[connect] InternetStatus: 41 (response received)
[connect] InternetStatus: 50
[connect] InternetStatus: 51
and maybe [connect] InternetStatus: 100 INTERNET_STATUS_REQUEST_COMPLETE

The lattest is received only if GetLastError() returned ERROR_IO_PENDING.

If you stored the total data size (in bytes) in the dwBufferTotal member, use it to set the final “0” in your string buffer (if it’s a string).
buf[ib.dwBufferTotal] = 0;

9 Close the connection handle

InternetCloseHandle( m_httpFile );
The callback will receive this notification when the handle is closed:
[connect] InternetStatus: 70 INTERNET_STATUS_HANDLE_CLOSING

In most error cases, the connection is closed unexpectedly. If it happens you’ll receive a 70 followed by a 100 (INTERNET_STATUS_REQUEST_COMPLETE). This can happen anywhere during the process.

10 Before closing the m_Session handle:
you must deregister the callback:
InternetSetStatusCallback( m_Session, NULL );


This should help those who tried to go through asynchronous mode in WinInet !
Sorry, there are no attached files but you should be able to use the functions
and create nice classes now.
If you liked this article please add an entry in my guestbook and buy me a licence of my shareware J.
If you don't then I never wrote this article.

B.M.

Bibliography:
http://msdn.microsoft.com/workshop/networking/wininet/reference/functions/InternetReadFileEx.asp
Q176420: BUG: InternetSetOption Does Not Set Timeout Values
Q176176: FIX: InternetOpenUrl Does Not Work When Called Asynchronously
Q238425: INFO: WinInet Not Supported for Use in Services
Q190542: INFO: Using WinInet APIs in a System Service to access SSL sites
news://microsoft.public.inetsdk.programming.wininet