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CGI Programming on the World Wide Web

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3.4 The Content-length Header

As you've seen in previous examples, we are not limited to dealing just with HTML text (defined by the MIME type text/html) but we can also output documents formatted in numerous ways, like plain text, GIF or JPEG images, and even AIFF sound clips. Here is a program that returns a GIF image:

#!/usr/local/bin/perl
$gif_image = join ("/", $ENV{'DOCUMENT_ROOT'}, "icons/tiger.gif");
if (open (IMAGE, "<" . $gif_image)) {
        $no_bytes = (stat ($gif_image))[7];
        
        print "Content-type: image/gif", "\n";
        print "Content-length: $no_bytes", "\n\n";

The first thing to notice is that the content type is image/gif. This signals the browser that a GIF image will be sent, so the browser knows how to display it.

The next thing to notice is the Content-length header. The Content-length header notifies the server of the size of the data that you intend to send. This prevents unexpected end-of-data errors from the server when dealing with binary data, because the server will read the specified number of bytes from the data stream regardless of any spurious end-of-data characters.

To get the content length, we use the stat command, which returns a 13-element array containing the statistics for a given file, to determine the size of the file. The eighth element of this array (index number 7, because arrays are zero-based in Perl) represents the size of the file in bytes. The remainder of the script follows:

        print <IMAGE>;
} else {
        print "Content-type: text/plain", "\n\n";
        print "Sorry! I cannot open the file $gif_image!", "\n";
}
exit (0);

As is the case with binary files, one read on the file handle will retrieve the entire file. Compare that to text files where one read will return only a single line. As a result, this example is fine when dealing with small graphic files, but is not very efficient with larger files. Now, we'll look at an example that reads and displays the graphic file in small pieces:

    #!/usr/local/bin/perl
$gif_image = join ("/", $ENV{'DOCUMENT_ROOT'}, "icons/tiger.gif");
if (open (IMAGE, "<" . $gif_image)) {
        $no_bytes = (stat ($gif_image))[7];
        $piece_size = $no_bytes / 10;
        print "Content-type: image/gif", "\n";
        print "Content-length: $no_bytes", "\n\n";
        for ($loop=0; $loop <= $no_bytes; $loop += $piece_size) {
                read (IMAGE, $data, $piece_size);
            print $data;
        }
         close (IMAGE);
} else {
        print "Content-type: text/plain", "\n\n";
        print "Sorry! I cannot open the file $gif_image!", "\n";
}
exit (0);

The loop iterates through the file reading and displaying pieces of data that are one-tenth the size of the entire binary file.

As you will see in the following section, you can use server redirection to return existing files much more quickly and easily than with CGI programs like the ones described earlier.


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