Downloading a File

- Alyce
by Alyce Watson

The API call URLDownloadToFileA can be used to download files from the internet and save them on the user's computer.

Valid URL?

We can download just about any type of file that is online, but before we attempt to download an HTML file, we'll check to see if the URL is valid. URL stands for Universal Resource Locator, and it is the address on the internet of the web page we want to download.

The function to test for a valid URL is called IsValidURL and it is part of "URLmon.DLL". The validation function is for HTML files, not for other types of files online. This DLL is not one of the standard DLLs that is recognized by Liberty BASIC, so we must open it and give it a handle in order to use it, and close it when it is no longer needed.

 open "URLmon" for dll as #url 
 'code to call on the DLL 
 close #url 

The API call is encapsulated in a Liberty BASIC function below. There are three arguments. The first and third arguments are ignored and must be set to 0. The second argument is a string containing the URL to check. The URL for the Liberty BASIC site is http://www.libertybasic.com/ URLs always begin with "http", which stands for HyperText Transfer Protocol." The return from the IsValidURL function is 0 if the URL is valid. Here is an extract from the demo program. To call the function:

 result = ValidURL("http://www.libertybasic.com/index.html") 

If the result is equal to 0, the URL is valid. The function itself:

 Function ValidURL(urlfile$) 
 open "URLmon" for dll as #url 
 calldll #url, "IsValidURL",_ 
 0 as long,_ 'ignored, must be 0 
 urlfile$ as ptr,_ 'urlfile to check 
 0 as ulong,_ 'ignored, must be 0 
 ValidURL as long 
 close #url 
 end function 

Doing the Download

Once we've determined that a URL is valid, we can download the file. We'll use another function from "URLmon.DLL" called URLDownloadToFileA. As before, the DLL must be opened, used, then closed.

 open "URLmon" for dll as #url 
 'code to call on the DLL 
 close #url 

The URLDownloadToFileA function is set up to work either with or without an ActiveX control. Liberty BASIC cannot use ActiveX controls natively, so three of the arguments for use with ActiveX controls must be set to null, or 0. There are two arguments we'll need that require strings as PTRs. The "urlfile$" argument is the URL, or web address of the file to download. The "localfile$" argument is the disk filename where the downloaded file will be saved. Downloaded files must be saved to disk.

Here is the API call, encapsulated in a Liberty BASIC function. The API call returns 0 if it is successful in downloading a file and saving it to disk. The demo calls the function like this:

 result = DownloadToFile("http://www.libertybasic.com/index.html", DefaultDir$ + "\test.htm") 

The function itself:

 Function DownloadToFile(urlfile$, localfile$) 
 open "URLmon" for dll as #url 
 calldll #url, "URLDownloadToFileA",_ 
 0 as long,_ 'null 
 urlfile$ as ptr,_ 'url to download 
 localfile$ as ptr,_ 'save file name 
 0 as long,_ 'reserved, must be 0 
 0 as long,_ 'callback address, can be 0 
 DownloadToFile as ulong '0=success 
 close #url 
 end function 

What do we do now?

The little demo below downloads an HTML file and saves it to disk. It then opens it, reads the contents into a string variable and displays the text in the mainwin. We can do anything with the downloaded files that we can do with a file on disk. If it is an image file, we can run a Paint program and pass the filename into the RUN command to display it.

 RUN "mspaint " + chr$(34) + filename$ + chr$(34) 

If it is an HTML file, we can use RUN to run Explorer with the file loaded.

 RUN "explorer " + chr$(34) + filename$ + chr$(34) 

We can open the file and parse the text to get information.

 open "test.htm" for input as #f 
 txt$ = input$(#f, LOF(#f)) 
 close #f 
 'parse the string of text now 

Downloading Other File Types

We can download other types of files from the internet. The demo program downloads the Liberty BASIC banner, which is an image file in JPEG format. We can download ZIP files, TEXT files, GIF image files, and so on.

Keep in mind that downloading the HTML file for a web page does not download the complete webpage. Most web pages include other types of files, such as images, style sheets (CSS files), music files, etc. If we want to get all files associated with a web page, we must open the HTML file that was saved to disk and read the text into a string variable. We must then parse this string to find the names of other files that are needed.

Parsing the HTML

Once you have the file on disk, you may want to read the contents to check for images, links, etc. Jerry Muelver has written a demo on parsing, here: Parsing Text .

DEMO

 'Download html from given 
 'URL to file on disk. 
 ' 
 'Minimum availability Internet Explorer 3.0 
 'Minimum operating systems Windows NT 4.0, Windows 95 
 ' 
 
 'check for valid URL for HTML page: 
 result = ValidURL("http://www.libertybasic.com/index.html") 
 
 if result = 0 then 
 'download html from libertybasic.com 
 result = DownloadToFile("http://www.libertybasic.com/index.html", DefaultDir$ + "\test.htm") 
 if result <> 0 then print "Error downloading libertybasic.com HTML file." 
 
 open DefaultDir$ + "\test.htm" for input as #f 
 test$ = input$(#f, LOF(#f)) 
 close #f 
 
 print test$ 
 print : print 
 print "The text above was downloaded from http://www.libertybasic.com/" 
 else 
 print "Invalid URL: http://www.libertybasic.com/index.html" 
 end if 
 
 'now attempt to download the banner: 
 result = DownloadToFile("http://www.libertybasic.com/lb3banner.jpg", DefaultDir$ + "\banner.jpg") 
 if result <> 0 then 
 print "Error downloading Liberty BASIC banner." 
 else 
 run "mspaint.exe " + chr$(34) + DefaultDir$ + "\banner.jpg" + chr$(34) 
 print : print : print "Done" 
 end if 
 
 wait 
 
 Function DownloadToFile(urlfile$, localfile$) 
 open "URLmon" for dll as #url 
 calldll #url, "URLDownloadToFileA",_ 
 0 as long,_ 'null 
 urlfile$ as ptr,_ 'url to download 
 localfile$ as ptr,_ 'save file name 
 0 as long,_ 'reserved, must be 0 
 0 as long,_ 'callback address, can be 0 
 DownloadToFile as ulong '0=success 
 close #url 
 end function 
 
 Function ValidURL(urlfile$) 
 open "URLmon" for dll as #url 
 calldll #url, "IsValidURL",_ 
 0 as long,_ 'ignored, must be 0 
 urlfile$ as ptr,_ 'urlfile to check 
 0 as ulong,_ 'ignored, must be 0 
 ValidURL as long 
 close #url 
 end function