WinHttpOpen function (winhttp.h)

The WinHttpOpen function initializes, for an application, the use of WinHTTP functions and returns a WinHTTP-session handle.


  [in, optional] LPCWSTR pszAgentW,
  [in]           DWORD   dwAccessType,
  [in]           LPCWSTR pszProxyW,
  [in]           LPCWSTR pszProxyBypassW,
  [in]           DWORD   dwFlags


[in, optional] pszAgentW

A pointer to a string variable that contains the name of the application or entity calling the WinHTTP functions. This name is used as the user agent in the HTTP protocol.

[in] dwAccessType

Type of access required. This can be one of the following values.

Value Meaning
Resolves all host names directly without a proxy.
Important  Use of this option is deprecated on Windows 8.1 and newer. Use WINHTTP_ACCESS_TYPE_AUTOMATIC_PROXY instead.
Retrieves the static proxy or direct configuration from the registry. WINHTTP_ACCESS_TYPE_DEFAULT_PROXY does not inherit browser proxy settings.

The WinHTTP proxy configuration is set by one of these mechanisms.

Passes requests to the proxy unless a proxy bypass list is supplied and the name to be resolved bypasses the proxy. In this case, this function uses the values passed for pwszProxyName and pwszProxyBypass.
Uses system and per-user proxy settings (including the Internet Explorer proxy configuration) to determine which proxy/proxies to use. Automatically attempts to handle failover between multiple proxies, different proxy configurations per interface, and authentication. Supported in Windows 8.1 and newer.

[in] pszProxyW

A pointer to a string variable that contains the name of the proxy server to use when proxy access is specified by setting dwAccessType to WINHTTP_ACCESS_TYPE_NAMED_PROXY. The WinHTTP functions recognize only CERN type proxies for HTTP. If dwAccessType is not set to WINHTTP_ACCESS_TYPE_NAMED_PROXY, this parameter must be set to WINHTTP_NO_PROXY_NAME.

[in] pszProxyBypassW

A pointer to a string variable that contains an optional semicolon delimited list of host names or IP addresses, or both, that should not be routed through the proxy when dwAccessType is set to WINHTTP_ACCESS_TYPE_NAMED_PROXY. The list can contain wildcard characters. Do not use an empty string, because the WinHttpOpen function uses it as the proxy bypass list. If this parameter specifies the "<local>" macro in the list as the only entry, this function bypasses any host name that does not contain a period. If dwAccessType is not set to WINHTTP_ACCESS_TYPE_NAMED_PROXY, this parameter must be set to WINHTTP_NO_PROXY_BYPASS.

[in] dwFlags

Unsigned long integer value that contains the flags that indicate various options affecting the behavior of this function. This parameter can have the following value.

Value Meaning
Use the WinHTTP functions asynchronously. By default, all WinHTTP functions that use the returned HINTERNET handle are performed synchronously. When this flag is set, the caller needs to specify a callback function through WinHttpSetStatusCallback.
When this flag is set, WinHttp will require use of TLS 1.2 or newer. If the caller attempts to enable older TLS versions by setting WINHTTP_OPTION_SECURE_PROTOCOLS, it will fail with ERROR_ACCESS_DENIED. Additionally, TLS fallback will be disabled. Note that setting this flag also sets flag WINHTTP_FLAG_ASYNC.

Return value

Returns a valid session handle if successful, or NULL otherwise. To retrieve extended error information, call GetLastError. Among the error codes returned are the following.

Error Code Description
An internal error has occurred.
Not enough memory was available to complete the requested operation. (Windows error code)


We strongly recommend that you use WinHTTP in asynchronous mode (that is, when WINHTTP_FLAG_ASYNC has been set in WinHttpOpen, so that usage of the returned HINTERNET become asynchronous). The return value indicates success or failure. To retrieve extended error information, call GetLastError.

The WinHttpOpen function is the first of the WinHTTP functions called by an application. It initializes internal WinHTTP data structures and prepares for future calls from the application. When the application finishes using the WinHTTP functions, it must call WinHttpCloseHandle to free the session handle and any associated resources.

The application can make any number of calls to WinHttpOpen, though a single call is normally sufficient. Each call to WinHttpOpen opens a new session context. Because user data is not shared between multiple session contexts, an application that makes requests on behalf of multiple users should create a separate session for each user, so as not to share user-specific cookies and authentication state. The application should define separate behaviors for each WinHttpOpen instance, such as different proxy servers configured for each.

After the calling application has finished using the HINTERNET handle returned by WinHttpOpen, it must be closed using the WinHttpCloseHandle function.

Note  For Windows XP and Windows 2000, see Run-Time Requirements.


The following example code shows how to retrieve the default connection time-out value.

    DWORD data;
    DWORD dwSize = sizeof(DWORD);

    // Use WinHttpOpen to obtain an HINTERNET handle.
    HINTERNET hSession = WinHttpOpen(L"A WinHTTP Example Program/1.0", 
                                    WINHTTP_NO_PROXY_BYPASS, 0);
    if (hSession)

        // Use WinHttpQueryOption to retrieve internet options.
        if (WinHttpQueryOption( hSession, 
                                &data, &dwSize))
            printf("Connection timeout: %u ms\n\n",data);
            printf( "Error %u in WinHttpQueryOption.\n", 
        // When finished, release the HINTERNET handle.
        printf("Error %u in WinHttpOpen.\n", GetLastError());


Requirement Value
Minimum supported client Windows XP, Windows 2000 Professional with SP3 [desktop apps only]
Minimum supported server Windows Server 2003, Windows 2000 Server with SP3 [desktop apps only]
Target Platform Windows
Header winhttp.h
Library Winhttp.lib
DLL Winhttp.dll
Redistributable WinHTTP 5.0 and Internet Explorer 5.01 or later on Windows XP and Windows 2000.

See also

About Microsoft Windows HTTP Services (WinHTTP)

WinHTTP Versions