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Posted 9 Nov 2000

Using WinInet HTTP functions in Full Asynchronous Mode

, 29 Jan 2001
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Using WinInet functions asynchronously is a nightmare since no samples exist. Here's one!


If you have ever dug 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 finally managed 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 (these 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 multi-proc NT server machines).


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 subtract this amount 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 occurs – 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 un-register the callback function.


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:

                          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, 

The callback will receive a lots of messages then. Here are their orders along with the dwInternetStatus value:

**At this point you can save the HINTERNET handle using code like this in your callback:
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

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.

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)

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

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 last 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 license of my shareware.



This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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About the Author

Benjamin Mayrargue
United States United States
No Biography provided

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Comments and Discussions

GeneralDont't use aync mode anymore! Pin
Member 841243830-Jan-03 19:34
memberMember 841243830-Jan-03 19:34 
QuestionWininet. Really works? Pin
Developpeur de mp3guest29-Dec-02 14:20
memberDeveloppeur de mp3guest29-Dec-02 14:20 
QuestionHow to implemente proxy authentication Pin
26-Mar-02 15:51
suss26-Mar-02 15:51 
GeneralInternetWriteFile Pin
Ed K28-Dec-01 12:06
memberEd K28-Dec-01 12:06 
GeneralRe: InternetWriteFile Pin
Chaitanya Ram31-Jan-02 19:04
memberChaitanya Ram31-Jan-02 19:04 
GeneralRe: InternetWriteFile Pin
tyounsi4-May-04 11:10
membertyounsi4-May-04 11:10 
GeneralRe: InternetWriteFile Pin
Anonymous7-Jun-05 4:13
memberAnonymous7-Jun-05 4:13 
GeneralRe: InternetWriteFile Pin
Member 342329118-May-09 2:36
memberMember 342329118-May-09 2:36 
I am not convinced that InternetWriteFile works in asynchronous mode. I have code similar to all the examples I have found, which loops attempting to send several quite small chunks of data with InternetWriteFile. If the data is sent as one chunk only, the code works fine. If the same data is sent as several smaller chunks then one of the InternetWriteFile's returns false, but the SAME code working on the SAME data when run in the debugger with a breakpoint after each chunk writes the data OK. That leads me to suspect there might be a timing issue, i.e. the function is really synchronous, but, depending on the speed of the transfers, you might get away with it. I wonder if anyone else has encountered a problem with asynchronous mode InternetWriteFile.

Sahlan Diver
Volodya Orlenko7-Dec-01 6:22
memberVolodya Orlenko7-Dec-01 6:22 
Volodya Orlenko12-Dec-01 4:26
memberVolodya Orlenko12-Dec-01 4:26 
Anonymous31-Mar-03 21:23
memberAnonymous31-Mar-03 21:23 
jweston25-Aug-04 11:43
memberjweston25-Aug-04 11:43 
Member 30455329-Oct-04 3:48
memberMember 30455329-Oct-04 3:48 
Lou Montulli6-Jul-05 17:25
sussLou Montulli6-Jul-05 17:25 
GeneralAsync Challenge Pin
3-Dec-01 23:20
suss3-Dec-01 23:20 
GeneralThe wonderful world of the WinInet Async API Pin
Rhea1-Dec-01 5:13
groupRhea1-Dec-01 5:13 
GeneralRe: The wonderful world of the WinInet Async API Pin
2-Dec-01 5:14
suss2-Dec-01 5:14 
GeneralInternetReadFileEx & Unicode Pin
22-Nov-01 21:17
suss22-Nov-01 21:17 
GeneralRe: InternetReadFileEx & Unicode Pin
30-Nov-01 3:30
suss30-Nov-01 3:30 
GeneralRe: InternetReadFileEx & Unicode Pin
1-Dec-01 4:46
suss1-Dec-01 4:46 
GeneralFinding out the file's size Pin
12-Sep-01 12:30
suss12-Sep-01 12:30 
GeneralRe: Finding out the file's size Pin
26-Oct-01 23:44
suss26-Oct-01 23:44 
GeneralI think that something is wrong... Pin
12-Sep-01 12:24
suss12-Sep-01 12:24 
GeneralNote Pin
Gilad Novik23-Jul-01 1:48
memberGilad Novik23-Jul-01 1:48 
GeneralRe: Note & WinHTTP Pin
9-Aug-01 5:10
suss9-Aug-01 5:10 

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