InternetOpenA function (wininet.h)

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Initializes an application's use of the WinINet functions.

Syntax

HINTERNET InternetOpenA(
  [in] LPCSTR lpszAgent,
  [in] DWORD  dwAccessType,
  [in] LPCSTR lpszProxy,
  [in] LPCSTR lpszProxyBypass,
  [in] DWORD  dwFlags
);

Parameters

[in] lpszAgent

Pointer to a null-terminated string that specifies the name of the application or entity calling the WinINet functions. This name is used as the user agent in the HTTP protocol.

[in] dwAccessType

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

Value Meaning
INTERNET_OPEN_TYPE_DIRECT
 Resolves all host names locally.
INTERNET_OPEN_TYPE_PRECONFIG
 Retrieves the proxy or direct configuration from the registry.
INTERNET_OPEN_TYPE_PRECONFIG_WITH_NO_AUTOPROXY
 Retrieves the proxy or direct configuration from the registry and prevents the use of a startup Microsoft JScript or Internet Setup (INS) file.
INTERNET_OPEN_TYPE_PROXY
 Passes requests to the proxy unless a proxy bypass list is supplied and the name to be resolved bypasses the proxy. In this case, the function uses INTERNET_OPEN_TYPE_DIRECT.

[in] lpszProxy

Pointer to a null-terminated string that specifies the name of the proxy server(s) to use when proxy access is specified by setting dwAccessType to INTERNET_OPEN_TYPE_PROXY. Do not use an empty string, because InternetOpen will use it as the proxy name. The WinINet functions recognize only CERN type proxies (HTTP only) and the TIS FTP gateway (FTP only). If Microsoft Internet Explorer is installed, these functions also support SOCKS proxies. FTP requests can be made through a CERN type proxy either by changing them to an HTTP request or by using InternetOpenUrl. If dwAccessType is not set to INTERNET_OPEN_TYPE_PROXY, this parameter is ignored and should be NULL. For more information about listing proxy servers, see the Listing Proxy Servers section of Enabling Internet Functionality.

[in] lpszProxyBypass

Pointer to a null-terminated string that specifies an optional list of host names or IP addresses, or both, that should not be routed through the proxy when dwAccessType is set to INTERNET_OPEN_TYPE_PROXY. The list can contain wildcards. Do not use an empty string, because InternetOpen will use it as the proxy bypass list. If this parameter specifies the "<local>" macro, the function bypasses the proxy for any host name that does not contain a period.

By default, WinINet will bypass the proxy for requests that use the host names "localhost", "loopback", "127.0.0.1", or "[::1]". This behavior exists because a remote proxy server typically will not resolve these addresses properly.Internet Explorer 9:  You can remove the local computer from the proxy bypass list using the "<-loopback>" macro.

If dwAccessType is not set to INTERNET_OPEN_TYPE_PROXY, this parameter is ignored and should be NULL.

[in] dwFlags

Options. This parameter can be a combination of the following values.

Value Meaning
INTERNET_FLAG_ASYNC
 Makes only asynchronous requests on handles descended from the handle returned from this function.
INTERNET_FLAG_FROM_CACHE
 Does not make network requests. All entities are returned from the cache. If the requested item is not in the cache, a suitable error, such as ERROR_FILE_NOT_FOUND, is returned.
INTERNET_FLAG_OFFLINE
 Identical to INTERNET_FLAG_FROM_CACHE. Does not make network requests. All entities are returned from the cache. If the requested item is not in the cache, a suitable error, such as ERROR_FILE_NOT_FOUND, is returned.

Return value

Returns a valid handle that the application passes to subsequent WinINet functions. If InternetOpen fails, it returns NULL. To retrieve a specific error message, call GetLastError.

Remarks

InternetOpen is the first WinINet function called by an application. It tells the Internet DLL to initialize internal data structures and prepare for future calls from the application. When the application finishes using the Internet functions, it should call InternetCloseHandle to free the handle and any associated resources.

The application can make any number of calls to InternetOpen, though a single call is normally sufficient. The application might need to define separate behaviors for each InternetOpen instance, such as different proxy servers configured for each.

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

Like all other aspects of the WinINet API, this function cannot be safely called from within DllMain or the constructors and destructors of global objects.

Note  WinINet does not support server implementations. In addition, it should not be used from a service. For server implementations or services use Microsoft Windows HTTP Services (WinHTTP).
 

Note

The wininet.h header defines InternetOpen as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.

Requirements

Requirement Value
Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header wininet.h
Library Wininet.lib
DLL Wininet.dll

See also

Enabling Internet Functionality

WinINet Functions