WinHttpSetStatusCallback function (winhttp.h)

The WinHttpSetStatusCallback function sets up a callback function that WinHTTP can call as progress is made during an operation.

Syntax

WINHTTPAPI WINHTTP_STATUS_CALLBACK WinHttpSetStatusCallback(
  [in] HINTERNET               hInternet,
  [in] WINHTTP_STATUS_CALLBACK lpfnInternetCallback,
  [in] DWORD                   dwNotificationFlags,
  [in] DWORD_PTR               dwReserved
);

Parameters

[in] hInternet

HINTERNET handle for which the callback is to be set.

[in] lpfnInternetCallback

Pointer to the callback function to call when progress is made. Set this to NULL to remove the existing callback function. For more information about the callback function, see WINHTTP_STATUS_CALLBACK.

[in] dwNotificationFlags

Unsigned long integer value that specifies flags to indicate which events activate the callback function.

The possible values are as follows.

[in] dwReserved

This parameter is reserved and must be NULL.

Return value

If successful, returns a pointer to the previously defined status callback function or NULL if there was no previously defined status callback function. Returns WINHTTP_INVALID_STATUS_CALLBACK if the callback function could not be installed. For extended error information, call GetLastError. Among the error codes returned are the following.

Remarks

If you set the callback on the session handle before creating the request handle, the request handle inherits the callback function pointer from its parent session.

Even when WinHTTP is used in asynchronous mode (that is, when WINHTTP_FLAG_ASYNC has been set in WinHttpOpen), this function operates synchronously. The return value indicates success or failure. To get extended error information, call GetLastError.

Both synchronous and asynchronous functions use the callback function to indicate the progress of the request, such as resolving a name, connecting to a server, and so on. The callback function is required for an asynchronous operation.

A callback function can be set on any handle and is inherited by derived handles. A callback function can be changed using WinHttpSetStatusCallback, provided there are no pending requests that need to use the previous callback value. However, changing the callback function on a handle does not change the callbacks on derived handles, such as that returned by WinHttpConnect. You must change the callback function at each level.

Many WinHTTP functions perform several operations on the network. Each operation can take time to complete and each can fail.

After initiating the WinHttpSetStatusCallback function, the callback function can be accessed from within WinHTTP for monitoring time-intensive network operations.

At the end of asynchronous processing, the application may set the callback function to NULL. This prevents the client application from receiving additional notifications.

The following code snippet shows the recommended method for setting the callback function to NULL.

WinHttpSetStatusCallback( hOpen,
                          NULL,
                          WINHTTP_CALLBACK_FLAG_ALL_NOTIFICATIONS,
                          NULL );

Note, however, that WinHTTP does not synchronize WinHttpSetStatusCallback with worker threads. If a callback originating in another thread is in progress when an application calls WinHttpSetStatusCallback, the application still receives a callback notification even after WinHttpSetStatusCallback successfully sets the callback function to NULL and returns.

Examples

The following example shows how to install a callback function for asynchronous WinHTTP functions. The example assumes that a WINHTTP_STATUS_CALLBACK function named "AsyncCallback( )" has been previously implemented:

    // Use WinHttpOpen to obtain an HINTERNET handle.
    HINTERNET hSession = WinHttpOpen(L"A WinHTTP Example Program/1.0", 
                                    WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
                                    WINHTTP_NO_PROXY_NAME, 
                                    WINHTTP_NO_PROXY_BYPASS, 0);
    if (hSession)
    {
        // Install the status callback function.
        WINHTTP_STATUS_CALLBACK isCallback = WinHttpSetStatusCallback( hSession,
                                               (WINHTTP_STATUS_CALLBACK)AsyncCallback,
                                               WINHTTP_CALLBACK_FLAG_ALL_NOTIFICATIONS, 
                                               NULL);
                                               
        // Place additional code here.
    
        // When finished, release the HINTERNET handle.
        WinHttpCloseHandle(hSession);
    }
    else
    {
        printf("Error %u in WinHttpOpen.\n", GetLastError());
    }

Requirements

See also

About Microsoft Windows HTTP Services (WinHTTP)

WINHTTP_STATUS_CALLBACK

WinHTTP Versions

WinHttpConnect

WinHttpOpen