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Chapter 6
URLs

This chapter describes retrieving URLs and displaying them on specified target pages, posting data to an HTTP server, uploading files to an FTP server, and sending mail.

Uniform resource locator (URL) protocols provide a means for locating and accessing resources that are available on the Internet and on intranets. Plug-ins can request and receive the data associated with URLs of any type that the browser can handle, including HTTP, FTP, news, mailto, and gopher.

Table 6.1 summarizes URLs supported by the Netscape browser. In addition, Netscape may support URLs not listed on this table.

Table 6.1 Supported URL schemes
URL Scheme

Description

about 
Locates browser information or "fun" pages. Netscape proprietary.

file 
(Host-specific filenames) Locates files on a specific host computer rather than an Internet resource.

ftp 
(File Transfer Protocol) Locates files and directories on Internet hosts for file download.

gopher
(Gopher protocol) Locates specified items on a Gopher server.

http 
(Hypertext Transfer Protocol) Locates resources on the Internet.

javascript
Executes JavaScript code that follows the URL. Netscape-specific.

mailto 
(Electronic mail address) Locates the Internet mailing address of an individual or service.

nethelp
Displays a NetHelp topic in a NetHelp window. Communicator-specific.

news 
(USENET news) Locates USENET news groups or individual USENET articles.

nntp 
(USENET news using nntp access) Locates USENET news groups or individual USENET articles; alternate to news.

prospero 
(Prospero Directory Service) Locates a resource on a Prospero directory server.

telnet
(Reference to interactive sessions) Locates an interactive service.

wais
(Wide Area Information Servers) Locates WAIS databases and their documents.

wysiwyg
Placed before another URL; displays a page that JavaScript has updated using document.write.

For more information, see RFC 1738, "Uniform Resource Locators (URL).

For a reference entry that describes each function discussed in this chapter, see "URL Methods" in the API reference.

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Getting URLs

To retrieve a URL and display it on a specified target page, use the NPN_GetURL, NPN_GetURLNotify, and NPP_URLNotify functions. This section describes the methods and procedure used for getting the URL and displaying the page.

The plug-in uses the NPN_GetURL function to ask Communicator to display data retrieved from a URL in a specified target window or frame, or deliver it to the plug-in instance in a new stream. This is the way that plug-ins provide hyperlinks to other documents or retrieve data from the network.

If Communicator cannot locate the URL and retrieve the data, it does not create a stream for the instance; in this case, the plug-in receives notification of the result. To request a stream and receive notification of the result in all cases, use NPN_GetURLNotify.

For HTTP URLs, Communicator resolves NPN_GetURL as the HTTP server method GET, which requests URL objects.

Note that NPN_GetURL is typically asynchronous: it returns immediately and only later handles the request, such as displaying the URL or creating the stream for the instance and writing the data. For this reason as well, calling NPN_GetURLNotify may be more useful than NPN_GetURL; the plug-in is notified upon either successful or unsuccessful completion of the request.

NPError NPN_GetURL(NPP instance, const char *url, const char *target);
The instance parameter represents the current plug-in instance. The url parameter is the URL of the request, which can be of any type, including HTTP, FTP, news, mailto, or gopher.

The target parameter represents the destination where the URL will be displayed, a window or frame. If target refers to the window or frame containing the plug-in instance, it is destroyed and the plug-in may be unloaded. If the target parameter is set to null, the application creates a new stream and delivers the data to the plug-in instance, through calls to NPP_NewStream, NPP_WriteReady and NPP_Write, and NPP_DestroyStream.

In general, if a URL works in the location box of the Navigator, it works as a target for NPN_GetURL, except for the _self target.

Make sure that the target matches the URL type sent to it. For example, a null target does not make sense for some URL types (such as mailto). For some recommendations to help you with target parameter choice, see the reference entry for NPN_GetURL.

For complete information on named targets for this function (as well as for normal HTML links), see the Netscape document, "Targeting Windows."

The NPN_GetURLNotify method acts like NPN_GetURL. Both request the creation of a new stream with the contents of the specified URL, and, in addition, NPN_GetURLNotify notifies the plug-in of the successful or unsuccessful completion of the request. Communicator notifies the plug-in by calling the plug-in's NPP_URLNotify function and passing it the notifyData value, which may be used to track multiple requests.

NPN_GetURLNotify handles the URL request asynchronously. It returns immediately and only later handles the request and calls NPP_URLNotify. Without this notification, the plug-in cannot tell whether a request with a null target failed or a request with a non-null target was completed.

NPError NPN_GetURLNotify(NPP instance, const char* url, 
                        const char* target, void* notifyData);
The instance, url, and target parameters have the same definitions as those of NPN_GetURL. The notifyData parameter contains private plug-in data that can be used to associate the request with the subsequent NPP_URLNotify call (which returns this value) and/or to pass a pointer to some request-related payload.

If a request is not completed successfully (for example, because the URL is invalid or a HTTP server is down), Communicator should call NPP_URLNotify as soon as possible. If a request completes successfully, and the target is non-null, Communicator calls NPP_URLNotify after it has finished loading the URL. If the target is null, it calls NPP_URLNotify after calling NPP_DestroyStream to close the stream.

Both the NPN_GetURLNotify and NPN_PostURLNotify functions call the NPP_URLNotify method to notify the plug-in of the result of a request. Both functions pass the notifyData value to NPP_URLNotify, which tells the plug-in that the URL request was completed and the reason for completion.

void NPP_URLNotify(NPP instance, const char* url,
                   NPReason reason, void* notifyData);
The instance and url parameters have the same definitions as those of NPN_GetURL. The notifyData parameter contains the private plug-in data passed to the corresponding call to NPN_GetURLNotify and NPN_PostURLNotify.

[Top] [Getting URLs]


Getting the URL and Displaying the Page

To retrieve a URL and display it on a specified target page, you use the NPN_GetURL and NPN_GetURLNotify functions. The URL can be displayed in the same window or frame, a new window, or a different window or frame, depending on the value of the target parameter. Specify the display target with one of these special target names:

Be careful when you assign a target. If the target refers to the window or frame containing the instance or one of its parents/ancestors, the instance is destroyed and the plug-in may be unloaded.

Here's an example of getting a URL: A plug-in instance draws a button that acts like a link to another web page. When the user clicks the button, the plug-in calls NPN_GetURL to go to the page.

err = NPN_GetURL(instance, "http://home.netscape.com/", "_blank");
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Posting URLs

The plug-in calls NPN_PostURL to post data from a file or buffer to a URL. This function is the counterpart of NPN_GetURL.

For HTTP URLs only, Communicator resolves this method as the HTTP server method POST, which transmits data to the server.

You can use NPN_PostURL to post data to a URL from a memory buffer or file. The result from the server can also be sent to a particular Communicator window or frame for display, or delivered to the plug-in instance in a new stream. Plug-ins can use this capability to post form data to CGI scripts using HTTP or upload files to a remote server using FTP.

Communicator resolves this method as the HTTP server method POST, which transmits data to the server. The data to post can be contained either in a local temporary file or a new memory buffer. To post a file, set the flag file to true, the buffer buf to the path name string for a file, and len to the length of the path string. The file-type URL prefix "file://" is optional.

NPN_PostURL is typically asynchronous: it returns immediately and only later handles the request and calls NPP_Notify (which, in turn, calls NPP_URLNotify).

NPError NPN_PostURL(NPP instance, const char *url,
const char *target, uint32 len,
const char *buf, NPBool file);
The instance, url, and target parameters have the same definitions as those of NPN_GetURL.

The buf parameter identifies a local temporary file or data buffer that contains the data to post.

Windows and Mac OS
If a file is posted with any protocol other than FTP, the file must be text with Unix-style line breaks ('\n' separators only). § NPN_PostURL works identically with buffers and files. To post data from a memory buffer, set the flag file to false, the buffer buf to the data to post, and len to the length of the buffer.

Possible URL types include http (similar to an HTML form submission), mailto (sending mail), news (posting a news article), and ftp (uploading a file). For protocols in which the headers must be distinguished from the body, such as http, the buffer or file should contain the headers, followed by a blank line, then the body. If no custom headers are required, simply add a blank line ('\n') to the beginning of the file or buffer. For a list of URL schemes supported by Netscape, see Supported URL Schemes.

NOTE: You cannot use NPN_PostURL to specify headers (even a blank line) in a memory buffer. To do this, use NPN_PostURLNotify for this purpose. §
The NPN_PostURLNotify function has all the same capabilities and works like NPN_PostURL in most ways except that (1) it supports specifying headers when posting a memory buffer, and (2) it calls NPP_URLNotify upon successful or unsuccessful completion of the request. NPN_PostURLNotify is typically asynchronous: it returns immediately and only later handles the request and calls NPP_URLNotify.

NPError NPN_PostURLNotify(NPP instance, const char *url,
                     const char *target, uint32 len,
                     const char *buf, NPBool file, void* notifyData);
The parameters of this function have the same definitions as those of NPN_PostURL. The notifyData parameter contains plug-in-private data passed by NPP_URLNotify and may be used for tracking multiple posts.

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Posting Data to an HTTP Server

The following code posts two name-value pairs to a CGI script through HTTP. The response from the server is displayed in a new window.

char* myData = "Content Type:\tapplication/
               x-www-form-urlencoded\nContent
Length:\t25\n\nname1=value1&name2=value2\n";
uint32 myLength = strlen(myData) + 1;
err = NPN_PostURL(instance, "http://hoohoo.ncsa.uiuc.edu/
               cgi-bin/post-query","_blank", myLength, myData, FALSE);
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Uploading Files to an FTP Server

Plug-ins can use NPN_PostURL or NPN_PostURLNotify to upload files to a remote server using FTP. This example uploads a file from the root of the local file system to an FTP server and displays the response in a frame named response:

char* myData = "file:///c\/myDirectory/myFileName";
uint32 myLength = strlen(myData) + 1;
err = NPN_PostURL(instance, "ftp://fred@ftp.somewhere.com/pub/",
                  "response", myLength, myData, TRUE);
[Top] [Posting URLs]


Sending Mail

A plug-in can send an email message using NPN_PostURL or NPN_PostURLNotify. The following code sends a mail message with the default headers from the client machine.

char* myData = "\nHi Fred, this is a message from my plug-in!";
uint32 myLength = strlen(myData) + 1;
err = NPN_PostURLNotify(instance, "mailto:fred@somewhere.com",
NULL, myLength, myData, FALSE);
The example starts by defining the mail message, myData, and its length, myLength. It sends myData and myLength to the mailto URL mailto:fred@somewhere.com. The target window for displaying the message is null in the example. Normally, using a null target window causes the response to be delivered from the server to the plug-in instance in a new stream, but no response is expected for a mailto URL.

You cannot use either of these functions to set the body or attachments of an email message.

[Top] [Posting URLs]


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Last Updated: 01/15/97 16:36:54


Copyright 1997 Netscape Communications Corporation