Using WinInet HTTP functions in Full Asynchronous Mode

Detail

http://www.codeproject.com/Articles/822/Using-WinInet-HTTP-functions-in-Full-Asynchronous

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:

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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.


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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 are their orders along with the

dwInternetStatus

value:


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[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.


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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.


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//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):


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[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).


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buf[ib.dwBufferTotal] = 0;

9 Close the connection handle

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InternetCloseHandle( m_httpFile );

The callback will receive this notification when the handle is closed:


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[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:


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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 inmy guestbook and buy me a license of my shareware.