WinHttpAddRequestHeaders function (winhttp.h)

Article
07/27/2022

The WinHttpAddRequestHeaders function adds one or more HTTP request headers to the HTTP request handle.

Syntax

WINHTTPAPI BOOL WinHttpAddRequestHeaders(
  [in] HINTERNET hRequest,
  [in] LPCWSTR   lpszHeaders,
  [in] DWORD     dwHeadersLength,
  [in] DWORD     dwModifiers
);

Parameters

[in] hRequest

A HINTERNET handle returned by a call to the WinHttpOpenRequest function.

[in] lpszHeaders

A pointer to a string variable that contains the headers to append to the request. Each header except the last must be terminated by a carriage return/line feed (CR/LF).

[in] dwHeadersLength

An unsigned long integer value that contains the length, in characters, of pwszHeaders. If this parameter is -1L, the function assumes that pwszHeaders is zero-terminated (ASCIIZ), and the length is computed.

[in] dwModifiers

An unsigned long integer value that contains the flags used to modify the semantics of this function. Can be one or more of the following flags.

Value	Meaning
WINHTTP_ADDREQ_FLAG_ADD	Adds the header if it does not exist. Used with WINHTTP_ADDREQ_FLAG_REPLACE.
WINHTTP_ADDREQ_FLAG_ADD_IF_NEW	Adds the header only if it does not already exist; otherwise, an error is returned.
WINHTTP_ADDREQ_FLAG_COALESCE	Merges headers of the same name.
WINHTTP_ADDREQ_FLAG_COALESCE_WITH_COMMA	Merges headers of the same name using a comma. For example, adding "Accept: text/" followed by "Accept: audio/" with this flag results in a single header "Accept: text/, audio/". This causes the first header found to be merged. The calling application must to ensure a cohesive scheme with respect to merged and separate headers.
WINHTTP_ADDREQ_FLAG_COALESCE_WITH_SEMICOLON	Merges headers of the same name using a semicolon.
WINHTTP_ADDREQ_FLAG_REPLACE	Replaces or removes a header. If the header value is empty and the header is found, it is removed. If the value is not empty, it is replaced.

Return value

Returns TRUE if successful, or FALSE otherwise. For extended error information, call GetLastError. Among the error codes returned are the following.

Error Code	Description
ERROR_WINHTTP_INCORRECT_HANDLE_STATE	The requested operation cannot be performed because the handle supplied is not in the correct state.
ERROR_WINHTTP_INCORRECT_HANDLE_TYPE	The type of handle supplied is incorrect for this operation.
ERROR_WINHTTP_INTERNAL_ERROR	An internal error has occurred.
ERROR_NOT_ENOUGH_MEMORY	Not enough memory was available to complete the requested operation.

Remarks

Headers are transferred across redirects. This can be a security issue. To avoid having headers transferred when a redirect occurs, use the WINHTTP_STATUS_CALLBACK callback to correct the specific headers when a redirect occurs.

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.

The WinHttpAddRequestHeaders function appends additional free-format headers to the HTTP request handle and is intended for use by sophisticated clients that require detailed control over the exact request sent to the HTTP server.

The name and value of request headers added with this function are validated. Headers must be well formed. For more information about valid HTTP headers, see RFC 2616. If an invalid header is used, this function fails and GetLastError returns ERROR_INVALID_PARAMETER. The invalid header is not added.

If you are sending a Date: request header, you can use the WinHttpTimeFromSystemTime function to create structure for the header.

For basic WinHttpAddRequestHeaders, the application can pass in multiple headers in a single buffer.

An application can also use WinHttpSendRequest to add additional headers to the HTTP request handle before sending a request.

Note For more information, see Run-Time Requirements.

Examples

The following code example includes an If-Modified-Since header in a request. The response header is interpreted to determine whether the target document has been updated.


  DWORD dwSize = sizeof(DWORD);
  DWORD dwStatusCode = 0;
  BOOL  bResults = FALSE;
  HINTERNET hSession = NULL,
        hConnect = NULL,
        hRequest = NULL;

  // Use WinHttpOpen to obtain a session handle.
  hSession = WinHttpOpen( L"A WinHTTP Example Program/1.0", 
                          WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
                          WINHTTP_NO_PROXY_NAME, 
                          WINHTTP_NO_PROXY_BYPASS,
                          0 );

  // Specify an HTTP server.
  if( hSession )
    hConnect = WinHttpConnect( hSession,
                               L"www.microsoft.com",
                               INTERNET_DEFAULT_HTTP_PORT,
                               0 );

  // Create an HTTP Request handle.
  if( hConnect )
    hRequest = WinHttpOpenRequest( hConnect,
                                   L"GET",
                                   NULL, 
                                   NULL,
                                   WINHTTP_NO_REFERER, 
                                   WINHTTP_DEFAULT_ACCEPT_TYPES,
                                   0 );

  // Add a request header.
  if( hRequest )
    bResults = WinHttpAddRequestHeaders( hRequest, 
                 L"If-Modified-Since: Mon, 20 Nov 2000 20:00:00 GMT",
                                         (ULONG)-1L,
                                         WINHTTP_ADDREQ_FLAG_ADD );

  // Send a Request.
  if( bResults ) 
    bResults = WinHttpSendRequest( hRequest, 
                                   WINHTTP_NO_ADDITIONAL_HEADERS,
                                   0,
                                   WINHTTP_NO_REQUEST_DATA,
                                   0, 
                                   0,
                                   0 );

  // End the request.
  if( bResults )
    bResults = WinHttpReceiveResponse( hRequest, NULL);

  // Use WinHttpQueryHeaders to obtain the header buffer.
  if( bResults )
    bResults = WinHttpQueryHeaders( hRequest, 
                WINHTTP_QUERY_STATUS_CODE | WINHTTP_QUERY_FLAG_NUMBER,
                                    NULL, 
                                    &dwStatusCode,
                                    &dwSize,
                                    WINHTTP_NO_HEADER_INDEX );

  // Based on the status code, determine whether 
  // the document was recently updated.
  if( bResults )
  {
    if( dwStatusCode == 304 ) 
      printf( "Document has not been updated.\n" );
    else if( dwStatusCode == 200 ) 
      printf( "Document has been updated.\n" );
    else 
      printf( "Status code = %u.\n",dwStatusCode );
  }

  // Report any errors.
  if( !bResults )
    printf( "Error %d has occurred.\n", GetLastError( ) );

  // Close open handles.
  if( hRequest ) WinHttpCloseHandle( hRequest );
  if( hConnect ) WinHttpCloseHandle( hConnect );
  if( hSession ) WinHttpCloseHandle( hSession );

Requirements

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.

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